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NAME
BPF-HELPERS - list of eBPF helper functions
DESCRIPTION
The extended Berkeley Packet Filter (eBPF) subsystem consists in programs written in a
pseudo-assembly language, then attached to one of the several kernel hooks and run in
reaction of specific events. This framework differs from the older, "classic" BPF (or
"cBPF") in several aspects, one of them being the ability to call special functions (or
"helpers") from within a program. These functions are restricted to a white-list of
helpers defined in the kernel.
These helpers are used by eBPF programs to interact with the system, or with the context
in which they work. For instance, they can be used to print debugging messages, to get the
time since the system was booted, to interact with eBPF maps, or to manipulate network
packets. Since there are several eBPF program types, and that they do not run in the same
context, each program type can only call a subset of those helpers.
Due to eBPF conventions, a helper can not have more than five arguments.
Internally, eBPF programs call directly into the compiled helper functions without
requiring any foreign-function interface. As a result, calling helpers introduces no
overhead, thus offering excellent performance.
This document is an attempt to list and document the helpers available to eBPF developers.
They are sorted by chronological order (the oldest helpers in the kernel at the top).
HELPERS
void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
Description
Perform a lookup in map for an entry associated to key.
Return Map value associated to key, or NULL if no entry was found.
long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64
flags)
Description
Add or update the value of the entry associated to key in map with value.
flags is one of:
BPF_NOEXIST
The entry for key must not exist in the map.
BPF_EXIST
The entry for key must already exist in the map.
BPF_ANY
No condition on the existence of the entry for key.
Flag value BPF_NOEXIST cannot be used for maps of types BPF_MAP_TYPE_ARRAY
or BPF_MAP_TYPE_PERCPU_ARRAY (all elements always exist), the helper would
return an error.
Return 0 on success, or a negative error in case of failure.
long bpf_map_delete_elem(struct bpf_map *map, const void *key)
Description
Delete entry with key from map.
Return 0 on success, or a negative error in case of failure.
long bpf_probe_read(void *dst, u32 size, const void *unsafe_ptr)
Description
For tracing programs, safely attempt to read size bytes from kernel space
address unsafe_ptr and store the data in dst.
Generally, use bpf_probe_read_user() or bpf_probe_read_kernel() instead.
Return 0 on success, or a negative error in case of failure.
u64 bpf_ktime_get_ns(void)
Description
Return the time elapsed since system boot, in nanoseconds. Does not include
time the system was suspended. See: clock_gettime(CLOCK_MONOTONIC)
Return Current ktime.
long bpf_trace_printk(const char *fmt, u32 fmt_size, ...)
Description
This helper is a "printk()-like" facility for debugging. It prints a message
defined by format fmt (of size fmt_size) to file
/sys/kernel/debug/tracing/trace from DebugFS, if available. It can take up
to three additional u64 arguments (as an eBPF helpers, the total number of
arguments is limited to five).
Each time the helper is called, it appends a line to the trace. Lines are
discarded while /sys/kernel/debug/tracing/trace is open, use
/sys/kernel/debug/tracing/trace_pipe to avoid this. The format of the trace
is customizable, and the exact output one will get depends on the options
set in /sys/kernel/debug/tracing/trace_options (see also the README file
under the same directory). However, it usually defaults to something like:
telnet-470 [001] .N.. 419421.045894: 0x00000001: <formatted msg>
In the above:
• telnet is the name of the current task.
• 470 is the PID of the current task.
• 001 is the CPU number on which the task is running.
• In .N.., each character refers to a set of options (whether irqs are
enabled, scheduling options, whether hard/softirqs are running, level
of preempt_disabled respectively). N means that TIF_NEED_RESCHED and
PREEMPT_NEED_RESCHED are set.
• 419421.045894 is a timestamp.
• 0x00000001 is a fake value used by BPF for the instruction pointer
register.
• <formatted msg> is the message formatted with fmt.
The conversion specifiers supported by fmt are similar, but more limited
than for printk(). They are %d, %i, %u, %x, %ld, %li, %lu, %lx, %lld, %lli,
%llu, %llx, %p, %s. No modifier (size of field, padding with zeroes, etc.)
is available, and the helper will return -EINVAL (but print nothing) if it
encounters an unknown specifier.
Also, note that bpf_trace_printk() is slow, and should only be used for
debugging purposes. For this reason, a notice block (spanning several lines)
is printed to kernel logs and states that the helper should not be used "for
production use" the first time this helper is used (or more precisely, when
trace_printk() buffers are allocated). For passing values to user space,
perf events should be preferred.
Return The number of bytes written to the buffer, or a negative error in case of
failure.
u32 bpf_get_prandom_u32(void)
Description
Get a pseudo-random number.
From a security point of view, this helper uses its own pseudo-random
internal state, and cannot be used to infer the seed of other random
functions in the kernel. However, it is essential to note that the generator
used by the helper is not cryptographically secure.
Return A random 32-bit unsigned value.
u32 bpf_get_smp_processor_id(void)
Description
Get the SMP (symmetric multiprocessing) processor id. Note that all programs
run with preemption disabled, which means that the SMP processor id is
stable during all the execution of the program.
Return The SMP id of the processor running the program.
long bpf_skb_store_bytes(struct sk_buff *skb, u32 offset, const void *from, u32 len, u64
flags)
Description
Store len bytes from address from into the packet associated to skb, at
offset. flags are a combination of BPF_F_RECOMPUTE_CSUM (automatically
recompute the checksum for the packet after storing the bytes) and
BPF_F_INVALIDATE_HASH (set skb->hash, skb->swhash and skb->l4hash to 0).
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_l3_csum_replace(struct sk_buff *skb, u32 offset, u64 from, u64 to, u64 size)
Description
Recompute the layer 3 (e.g. IP) checksum for the packet associated to skb.
Computation is incremental, so the helper must know the former value of the
header field that was modified (from), the new value of this field (to), and
the number of bytes (2 or 4) for this field, stored in size. Alternatively,
it is possible to store the difference between the previous and the new
values of the header field in to, by setting from and size to 0. For both
methods, offset indicates the location of the IP checksum within the packet.
This helper works in combination with bpf_csum_diff(), which does not update
the checksum in-place, but offers more flexibility and can handle sizes
larger than 2 or 4 for the checksum to update.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_l4_csum_replace(struct sk_buff *skb, u32 offset, u64 from, u64 to, u64 flags)
Description
Recompute the layer 4 (e.g. TCP, UDP, or ICMP) checksum for the packet
associated to skb. Computation is incremental, so the helper must know the
former value of the header field that was modified (from), the new value of
this field (to), and the number of bytes (2 or 4) for this field, stored on
the lowest four bits of flags. Alternatively, it is possible to store the
difference between the previous and the new values of the header field in
to, by setting from and the four lowest bits of flags to 0. For both
methods, offset indicates the location of the IP checksum within the packet.
In addition to the size of the field, flags can be added (bitwise OR) actual
flags. With BPF_F_MARK_MANGLED_0, a null checksum is left untouched (unless
BPF_F_MARK_ENFORCE is added as well), and for updates resulting in a null
checksum the value is set to CSUM_MANGLED_0 instead. Flag BPF_F_PSEUDO_HDR
indicates the checksum is to be computed against a pseudo-header.
This helper works in combination with bpf_csum_diff(), which does not update
the checksum in-place, but offers more flexibility and can handle sizes
larger than 2 or 4 for the checksum to update.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_tail_call(void *ctx, struct bpf_map *prog_array_map, u32 index)
Description
This special helper is used to trigger a "tail call", or in other words, to
jump into another eBPF program. The same stack frame is used (but values on
stack and in registers for the caller are not accessible to the callee).
This mechanism allows for program chaining, either for raising the maximum
number of available eBPF instructions, or to execute given programs in
conditional blocks. For security reasons, there is an upper limit to the
number of successive tail calls that can be performed.
Upon call of this helper, the program attempts to jump into a program
referenced at index index in prog_array_map, a special map of type
BPF_MAP_TYPE_PROG_ARRAY, and passes ctx, a pointer to the context.
If the call succeeds, the kernel immediately runs the first instruction of
the new program. This is not a function call, and it never returns to the
previous program. If the call fails, then the helper has no effect, and the
caller continues to run its subsequent instructions. A call can fail if the
destination program for the jump does not exist (i.e. index is superior to
the number of entries in prog_array_map), or if the maximum number of tail
calls has been reached for this chain of programs. This limit is defined in
the kernel by the macro MAX_TAIL_CALL_CNT (not accessible to user space),
which is currently set to 32.
Return 0 on success, or a negative error in case of failure.
long bpf_clone_redirect(struct sk_buff *skb, u32 ifindex, u64 flags)
Description
Clone and redirect the packet associated to skb to another net device of
index ifindex. Both ingress and egress interfaces can be used for
redirection. The BPF_F_INGRESS value in flags is used to make the
distinction (ingress path is selected if the flag is present, egress path
otherwise). This is the only flag supported for now.
In comparison with bpf_redirect() helper, bpf_clone_redirect() has the
associated cost of duplicating the packet buffer, but this can be executed
out of the eBPF program. Conversely, bpf_redirect() is more efficient, but
it is handled through an action code where the redirection happens only
after the eBPF program has returned.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
u64 bpf_get_current_pid_tgid(void)
Return A 64-bit integer containing the current tgid and pid, and created as such:
current_task->tgid << 32 | current_task->pid.
u64 bpf_get_current_uid_gid(void)
Return A 64-bit integer containing the current GID and UID, and created as such:
current_gid << 32 | current_uid.
long bpf_get_current_comm(void *buf, u32 size_of_buf)
Description
Copy the comm attribute of the current task into buf of size_of_buf. The
comm attribute contains the name of the executable (excluding the path) for
the current task. The size_of_buf must be strictly positive. On success, the
helper makes sure that the buf is NUL-terminated. On failure, it is filled
with zeroes.
Return 0 on success, or a negative error in case of failure.
u32 bpf_get_cgroup_classid(struct sk_buff *skb)
Description
Retrieve the classid for the current task, i.e. for the net_cls cgroup to
which skb belongs.
This helper can be used on TC egress path, but not on ingress.
The net_cls cgroup provides an interface to tag network packets based on a
user-provided identifier for all traffic coming from the tasks belonging to
the related cgroup. See also the related kernel documentation, available
from the Linux sources in file
Documentation/admin-guide/cgroup-v1/net_cls.rst.
The Linux kernel has two versions for cgroups: there are cgroups v1 and
cgroups v2. Both are available to users, who can use a mixture of them, but
note that the net_cls cgroup is for cgroup v1 only. This makes it
incompatible with BPF programs run on cgroups, which is a cgroup-v2-only
feature (a socket can only hold data for one version of cgroups at a time).
This helper is only available is the kernel was compiled with the
CONFIG_CGROUP_NET_CLASSID configuration option set to "y" or to "m".
Return The classid, or 0 for the default unconfigured classid.
long bpf_skb_vlan_push(struct sk_buff *skb, __be16 vlan_proto, u16 vlan_tci)
Description
Push a vlan_tci (VLAN tag control information) of protocol vlan_proto to the
packet associated to skb, then update the checksum. Note that if vlan_proto
is different from ETH_P_8021Q and ETH_P_8021AD, it is considered to be
ETH_P_8021Q.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_skb_vlan_pop(struct sk_buff *skb)
Description
Pop a VLAN header from the packet associated to skb.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_skb_get_tunnel_key(struct sk_buff *skb, struct bpf_tunnel_key *key, u32 size, u64
flags)
Description
Get tunnel metadata. This helper takes a pointer key to an empty struct
bpf_tunnel_key of size, that will be filled with tunnel metadata for the
packet associated to skb. The flags can be set to BPF_F_TUNINFO_IPV6, which
indicates that the tunnel is based on IPv6 protocol instead of IPv4.
The struct bpf_tunnel_key is an object that generalizes the principal
parameters used by various tunneling protocols into a single struct. This
way, it can be used to easily make a decision based on the contents of the
encapsulation header, "summarized" in this struct. In particular, it holds
the IP address of the remote end (IPv4 or IPv6, depending on the case) in
key->remote_ipv4 or key->remote_ipv6. Also, this struct exposes the
key->tunnel_id, which is generally mapped to a VNI (Virtual Network
Identifier), making it programmable together with the
bpf_skb_set_tunnel_key() helper.
Let's imagine that the following code is part of a program attached to the
TC ingress interface, on one end of a GRE tunnel, and is supposed to filter
out all messages coming from remote ends with IPv4 address other than
10.0.0.1:
int ret;
struct bpf_tunnel_key key = {};
ret = bpf_skb_get_tunnel_key(skb, &key, sizeof(key), 0);
if (ret < 0)
return TC_ACT_SHOT; // drop packet
if (key.remote_ipv4 != 0x0a000001)
return TC_ACT_SHOT; // drop packet
return TC_ACT_OK; // accept packet
This interface can also be used with all encapsulation devices that can
operate in "collect metadata" mode: instead of having one network device per
specific configuration, the "collect metadata" mode only requires a single
device where the configuration can be extracted from this helper.
This can be used together with various tunnels such as VXLan, Geneve, GRE,
or IP in IP (IPIP).
Return 0 on success, or a negative error in case of failure.
long bpf_skb_set_tunnel_key(struct sk_buff *skb, struct bpf_tunnel_key *key, u32 size, u64
flags)
Description
Populate tunnel metadata for packet associated to skb. The tunnel metadata
is set to the contents of key, of size. The flags can be set to a
combination of the following values:
BPF_F_TUNINFO_IPV6
Indicate that the tunnel is based on IPv6 protocol instead of IPv4.
BPF_F_ZERO_CSUM_TX
For IPv4 packets, add a flag to tunnel metadata indicating that
checksum computation should be skipped and checksum set to zeroes.
BPF_F_DONT_FRAGMENT
Add a flag to tunnel metadata indicating that the packet should not
be fragmented.
BPF_F_SEQ_NUMBER
Add a flag to tunnel metadata indicating that a sequence number
should be added to tunnel header before sending the packet. This flag
was added for GRE encapsulation, but might be used with other
protocols as well in the future.
Here is a typical usage on the transmit path:
struct bpf_tunnel_key key;
populate key ...
bpf_skb_set_tunnel_key(skb, &key, sizeof(key), 0);
bpf_clone_redirect(skb, vxlan_dev_ifindex, 0);
See also the description of the bpf_skb_get_tunnel_key() helper for
additional information.
Return 0 on success, or a negative error in case of failure.
u64 bpf_perf_event_read(struct bpf_map *map, u64 flags)
Description
Read the value of a perf event counter. This helper relies on a map of type
BPF_MAP_TYPE_PERF_EVENT_ARRAY. The nature of the perf event counter is
selected when map is updated with perf event file descriptors. The map is an
array whose size is the number of available CPUs, and each cell contains a
value relative to one CPU. The value to retrieve is indicated by flags, that
contains the index of the CPU to look up, masked with BPF_F_INDEX_MASK.
Alternatively, flags can be set to BPF_F_CURRENT_CPU to indicate that the
value for the current CPU should be retrieved.
Note that before Linux 4.13, only hardware perf event can be retrieved.
Also, be aware that the newer helper bpf_perf_event_read_value() is
recommended over bpf_perf_event_read() in general. The latter has some ABI
quirks where error and counter value are used as a return code (which is
wrong to do since ranges may overlap). This issue is fixed with
bpf_perf_event_read_value(), which at the same time provides more features
over the bpf_perf_event_read() interface. Please refer to the description of
bpf_perf_event_read_value() for details.
Return The value of the perf event counter read from the map, or a negative error
code in case of failure.
long bpf_redirect(u32 ifindex, u64 flags)
Description
Redirect the packet to another net device of index ifindex. This helper is
somewhat similar to bpf_clone_redirect(), except that the packet is not
cloned, which provides increased performance.
Except for XDP, both ingress and egress interfaces can be used for
redirection. The BPF_F_INGRESS value in flags is used to make the
distinction (ingress path is selected if the flag is present, egress path
otherwise). Currently, XDP only supports redirection to the egress
interface, and accepts no flag at all.
The same effect can also be attained with the more generic
bpf_redirect_map(), which uses a BPF map to store the redirect target
instead of providing it directly to the helper.
Return For XDP, the helper returns XDP_REDIRECT on success or XDP_ABORTED on error.
For other program types, the values are TC_ACT_REDIRECT on success or
TC_ACT_SHOT on error.
u32 bpf_get_route_realm(struct sk_buff *skb)
Description
Retrieve the realm or the route, that is to say the tclassid field of the
destination for the skb. The identifier retrieved is a user-provided tag,
similar to the one used with the net_cls cgroup (see description for
bpf_get_cgroup_classid() helper), but here this tag is held by a route (a
destination entry), not by a task.
Retrieving this identifier works with the clsact TC egress hook (see also
tc-bpf(8)), or alternatively on conventional classful egress qdiscs, but not
on TC ingress path. In case of clsact TC egress hook, this has the advantage
that, internally, the destination entry has not been dropped yet in the
transmit path. Therefore, the destination entry does not need to be
artificially held via netif_keep_dst() for a classful qdisc until the skb is
freed.
This helper is available only if the kernel was compiled with
CONFIG_IP_ROUTE_CLASSID configuration option.
Return The realm of the route for the packet associated to skb, or 0 if none was
found.
long bpf_perf_event_output(void *ctx, struct bpf_map *map, u64 flags, void *data, u64
size)
Description
Write raw data blob into a special BPF perf event held by map of type
BPF_MAP_TYPE_PERF_EVENT_ARRAY. This perf event must have the following
attributes: PERF_SAMPLE_RAW as sample_type, PERF_TYPE_SOFTWARE as type, and
PERF_COUNT_SW_BPF_OUTPUT as config.
The flags are used to indicate the index in map for which the value must be
put, masked with BPF_F_INDEX_MASK. Alternatively, flags can be set to
BPF_F_CURRENT_CPU to indicate that the index of the current CPU core should
be used.
The value to write, of size, is passed through eBPF stack and pointed by
data.
The context of the program ctx needs also be passed to the helper.
On user space, a program willing to read the values needs to call
perf_event_open() on the perf event (either for one or for all CPUs) and to
store the file descriptor into the map. This must be done before the eBPF
program can send data into it. An example is available in file
samples/bpf/trace_output_user.c in the Linux kernel source tree (the eBPF
program counterpart is in samples/bpf/trace_output_kern.c).
bpf_perf_event_output() achieves better performance than bpf_trace_printk()
for sharing data with user space, and is much better suitable for streaming
data from eBPF programs.
Note that this helper is not restricted to tracing use cases and can be used
with programs attached to TC or XDP as well, where it allows for passing
data to user space listeners. Data can be:
• Only custom structs,
• Only the packet payload, or
• A combination of both.
Return 0 on success, or a negative error in case of failure.
long bpf_skb_load_bytes(const void *skb, u32 offset, void *to, u32 len)
Description
This helper was provided as an easy way to load data from a packet. It can
be used to load len bytes from offset from the packet associated to skb,
into the buffer pointed by to.
Since Linux 4.7, usage of this helper has mostly been replaced by "direct
packet access", enabling packet data to be manipulated with skb->data and
skb->data_end pointing respectively to the first byte of packet data and to
the byte after the last byte of packet data. However, it remains useful if
one wishes to read large quantities of data at once from a packet into the
eBPF stack.
Return 0 on success, or a negative error in case of failure.
long bpf_get_stackid(void *ctx, struct bpf_map *map, u64 flags)
Description
Walk a user or a kernel stack and return its id. To achieve this, the helper
needs ctx, which is a pointer to the context on which the tracing program is
executed, and a pointer to a map of type BPF_MAP_TYPE_STACK_TRACE.
The last argument, flags, holds the number of stack frames to skip (from 0
to 255), masked with BPF_F_SKIP_FIELD_MASK. The next bits can be used to set
a combination of the following flags:
BPF_F_USER_STACK
Collect a user space stack instead of a kernel stack.
BPF_F_FAST_STACK_CMP
Compare stacks by hash only.
BPF_F_REUSE_STACKID
If two different stacks hash into the same stackid, discard the old
one.
The stack id retrieved is a 32 bit long integer handle which can be further
combined with other data (including other stack ids) and used as a key into
maps. This can be useful for generating a variety of graphs (such as flame
graphs or off-cpu graphs).
For walking a stack, this helper is an improvement over bpf_probe_read(),
which can be used with unrolled loops but is not efficient and consumes a
lot of eBPF instructions. Instead, bpf_get_stackid() can collect up to
PERF_MAX_STACK_DEPTH both kernel and user frames. Note that this limit can
be controlled with the sysctl program, and that it should be manually
increased in order to profile long user stacks (such as stacks for Java
programs). To do so, use:
# sysctl kernel.perf_event_max_stack=<new value>
Return The positive or null stack id on success, or a negative error in case of
failure.
s64 bpf_csum_diff(__be32 *from, u32 from_size, __be32 *to, u32 to_size, __wsum seed)
Description
Compute a checksum difference, from the raw buffer pointed by from, of
length from_size (that must be a multiple of 4), towards the raw buffer
pointed by to, of size to_size (same remark). An optional seed can be added
to the value (this can be cascaded, the seed may come from a previous call
to the helper).
This is flexible enough to be used in several ways:
• With from_size == 0, to_size > 0 and seed set to checksum, it can be used
when pushing new data.
• With from_size > 0, to_size == 0 and seed set to checksum, it can be used
when removing data from a packet.
• With from_size > 0, to_size > 0 and seed set to 0, it can be used to
compute a diff. Note that from_size and to_size do not need to be equal.
This helper can be used in combination with bpf_l3_csum_replace() and
bpf_l4_csum_replace(), to which one can feed in the difference computed with
bpf_csum_diff().
Return The checksum result, or a negative error code in case of failure.
long bpf_skb_get_tunnel_opt(struct sk_buff *skb, void *opt, u32 size)
Description
Retrieve tunnel options metadata for the packet associated to skb, and store
the raw tunnel option data to the buffer opt of size.
This helper can be used with encapsulation devices that can operate in
"collect metadata" mode (please refer to the related note in the description
of bpf_skb_get_tunnel_key() for more details). A particular example where
this can be used is in combination with the Geneve encapsulation protocol,
where it allows for pushing (with bpf_skb_get_tunnel_opt() helper) and
retrieving arbitrary TLVs (Type-Length-Value headers) from the eBPF program.
This allows for full customization of these headers.
Return The size of the option data retrieved.
long bpf_skb_set_tunnel_opt(struct sk_buff *skb, void *opt, u32 size)
Description
Set tunnel options metadata for the packet associated to skb to the option
data contained in the raw buffer opt of size.
See also the description of the bpf_skb_get_tunnel_opt() helper for
additional information.
Return 0 on success, or a negative error in case of failure.
long bpf_skb_change_proto(struct sk_buff *skb, __be16 proto, u64 flags)
Description
Change the protocol of the skb to proto. Currently supported are transition
from IPv4 to IPv6, and from IPv6 to IPv4. The helper takes care of the
groundwork for the transition, including resizing the socket buffer. The
eBPF program is expected to fill the new headers, if any, via
skb_store_bytes() and to recompute the checksums with bpf_l3_csum_replace()
and bpf_l4_csum_replace(). The main case for this helper is to perform NAT64
operations out of an eBPF program.
Internally, the GSO type is marked as dodgy so that headers are checked and
segments are recalculated by the GSO/GRO engine. The size for GSO target is
adapted as well.
All values for flags are reserved for future usage, and must be left at
zero.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_skb_change_type(struct sk_buff *skb, u32 type)
Description
Change the packet type for the packet associated to skb. This comes down to
setting skb->pkt_type to type, except the eBPF program does not have a write
access to skb->pkt_type beside this helper. Using a helper here allows for
graceful handling of errors.
The major use case is to change incoming skb*s to **PACKET_HOST* in a
programmatic way instead of having to recirculate via redirect(...,
BPF_F_INGRESS), for example.
Note that type only allows certain values. At this time, they are:
PACKET_HOST
Packet is for us.
PACKET_BROADCAST
Send packet to all.
PACKET_MULTICAST
Send packet to group.
PACKET_OTHERHOST
Send packet to someone else.
Return 0 on success, or a negative error in case of failure.
long bpf_skb_under_cgroup(struct sk_buff *skb, struct bpf_map *map, u32 index)
Description
Check whether skb is a descendant of the cgroup2 held by map of type
BPF_MAP_TYPE_CGROUP_ARRAY, at index.
Return The return value depends on the result of the test, and can be:
• 0, if the skb failed the cgroup2 descendant test.
• 1, if the skb succeeded the cgroup2 descendant test.
• A negative error code, if an error occurred.
u32 bpf_get_hash_recalc(struct sk_buff *skb)
Description
Retrieve the hash of the packet, skb->hash. If it is not set, in particular
if the hash was cleared due to mangling, recompute this hash. Later accesses
to the hash can be done directly with skb->hash.
Calling bpf_set_hash_invalid(), changing a packet prototype with
bpf_skb_change_proto(), or calling bpf_skb_store_bytes() with the
BPF_F_INVALIDATE_HASH are actions susceptible to clear the hash and to
trigger a new computation for the next call to bpf_get_hash_recalc().
Return The 32-bit hash.
u64 bpf_get_current_task(void)
Return A pointer to the current task struct.
long bpf_probe_write_user(void *dst, const void *src, u32 len)
Description
Attempt in a safe way to write len bytes from the buffer src to dst in
memory. It only works for threads that are in user context, and dst must be
a valid user space address.
This helper should not be used to implement any kind of security mechanism
because of TOC-TOU attacks, but rather to debug, divert, and manipulate
execution of semi-cooperative processes.
Keep in mind that this feature is meant for experiments, and it has a risk
of crashing the system and running programs. Therefore, when an eBPF
program using this helper is attached, a warning including PID and process
name is printed to kernel logs.
Return 0 on success, or a negative error in case of failure.
long bpf_current_task_under_cgroup(struct bpf_map *map, u32 index)
Description
Check whether the probe is being run is the context of a given subset of the
cgroup2 hierarchy. The cgroup2 to test is held by map of type
BPF_MAP_TYPE_CGROUP_ARRAY, at index.
Return The return value depends on the result of the test, and can be:
• 0, if the skb task belongs to the cgroup2.
• 1, if the skb task does not belong to the cgroup2.
• A negative error code, if an error occurred.
long bpf_skb_change_tail(struct sk_buff *skb, u32 len, u64 flags)
Description
Resize (trim or grow) the packet associated to skb to the new len. The flags
are reserved for future usage, and must be left at zero.
The basic idea is that the helper performs the needed work to change the
size of the packet, then the eBPF program rewrites the rest via helpers like
bpf_skb_store_bytes(), bpf_l3_csum_replace(), bpf_l3_csum_replace() and
others. This helper is a slow path utility intended for replies with control
messages. And because it is targeted for slow path, the helper itself can
afford to be slow: it implicitly linearizes, unclones and drops offloads
from the skb.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_skb_pull_data(struct sk_buff *skb, u32 len)
Description
Pull in non-linear data in case the skb is non-linear and not all of len are
part of the linear section. Make len bytes from skb readable and writable.
If a zero value is passed for len, then the whole length of the skb is
pulled.
This helper is only needed for reading and writing with direct packet
access.
For direct packet access, testing that offsets to access are within packet
boundaries (test on skb->data_end) is susceptible to fail if offsets are
invalid, or if the requested data is in non-linear parts of the skb. On
failure the program can just bail out, or in the case of a non-linear
buffer, use a helper to make the data available. The bpf_skb_load_bytes()
helper is a first solution to access the data. Another one consists in using
bpf_skb_pull_data to pull in once the non-linear parts, then retesting and
eventually access the data.
At the same time, this also makes sure the skb is uncloned, which is a
necessary condition for direct write. As this needs to be an invariant for
the write part only, the verifier detects writes and adds a prologue that is
calling bpf_skb_pull_data() to effectively unclone the skb from the very
beginning in case it is indeed cloned.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
s64 bpf_csum_update(struct sk_buff *skb, __wsum csum)
Description
Add the checksum csum into skb->csum in case the driver has supplied a
checksum for the entire packet into that field. Return an error otherwise.
This helper is intended to be used in combination with bpf_csum_diff(), in
particular when the checksum needs to be updated after data has been written
into the packet through direct packet access.
Return The checksum on success, or a negative error code in case of failure.
void bpf_set_hash_invalid(struct sk_buff *skb)
Description
Invalidate the current skb->hash. It can be used after mangling on headers
through direct packet access, in order to indicate that the hash is outdated
and to trigger a recalculation the next time the kernel tries to access this
hash or when the bpf_get_hash_recalc() helper is called.
long bpf_get_numa_node_id(void)
Description
Return the id of the current NUMA node. The primary use case for this helper
is the selection of sockets for the local NUMA node, when the program is
attached to sockets using the SO_ATTACH_REUSEPORT_EBPF option (see also
socket(7)), but the helper is also available to other eBPF program types,
similarly to bpf_get_smp_processor_id().
Return The id of current NUMA node.
long bpf_skb_change_head(struct sk_buff *skb, u32 len, u64 flags)
Description
Grows headroom of packet associated to skb and adjusts the offset of the MAC
header accordingly, adding len bytes of space. It automatically extends and
reallocates memory as required.
This helper can be used on a layer 3 skb to push a MAC header for
redirection into a layer 2 device.
All values for flags are reserved for future usage, and must be left at
zero.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_xdp_adjust_head(struct xdp_buff *xdp_md, int delta)
Description
Adjust (move) xdp_md->data by delta bytes. Note that it is possible to use a
negative value for delta. This helper can be used to prepare the packet for
pushing or popping headers.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_probe_read_str(void *dst, u32 size, const void *unsafe_ptr)
Description
Copy a NUL terminated string from an unsafe kernel address unsafe_ptr to
dst. See bpf_probe_read_kernel_str() for more details.
Generally, use bpf_probe_read_user_str() or bpf_probe_read_kernel_str()
instead.
Return On success, the strictly positive length of the string, including the
trailing NUL character. On error, a negative value.
u64 bpf_get_socket_cookie(struct sk_buff *skb)
Description
If the struct sk_buff pointed by skb has a known socket, retrieve the cookie
(generated by the kernel) of this socket. If no cookie has been set yet,
generate a new cookie. Once generated, the socket cookie remains stable for
the life of the socket. This helper can be useful for monitoring per socket
networking traffic statistics as it provides a global socket identifier that
can be assumed unique.
Return A 8-byte long non-decreasing number on success, or 0 if the socket field is
missing inside skb.
u64 bpf_get_socket_cookie(struct bpf_sock_addr *ctx)
Description
Equivalent to bpf_get_socket_cookie() helper that accepts skb, but gets
socket from struct bpf_sock_addr context.
Return A 8-byte long non-decreasing number.
u64 bpf_get_socket_cookie(struct bpf_sock_ops *ctx)
Description
Equivalent to bpf_get_socket_cookie() helper that accepts skb, but gets
socket from struct bpf_sock_ops context.
Return A 8-byte long non-decreasing number.
u32 bpf_get_socket_uid(struct sk_buff *skb)
Return The owner UID of the socket associated to skb. If the socket is NULL, or if
it is not a full socket (i.e. if it is a time-wait or a request socket
instead), overflowuid value is returned (note that overflowuid might also be
the actual UID value for the socket).
long bpf_set_hash(struct sk_buff *skb, u32 hash)
Description
Set the full hash for skb (set the field skb->hash) to value hash.
Return 0
long bpf_setsockopt(void *bpf_socket, int level, int optname, void *optval, int optlen)
Description
Emulate a call to setsockopt() on the socket associated to bpf_socket, which
must be a full socket. The level at which the option resides and the name
optname of the option must be specified, see setsockopt(2) for more
information. The option value of length optlen is pointed by optval.
bpf_socket should be one of the following:
• struct bpf_sock_ops for BPF_PROG_TYPE_SOCK_OPS.
• struct bpf_sock_addr for BPF_CGROUP_INET4_CONNECT and
BPF_CGROUP_INET6_CONNECT.
This helper actually implements a subset of setsockopt(). It supports the
following levels:
• SOL_SOCKET, which supports the following optnames: SO_RCVBUF, SO_SNDBUF,
SO_MAX_PACING_RATE, SO_PRIORITY, SO_RCVLOWAT, SO_MARK, SO_BINDTODEVICE,
SO_KEEPALIVE.
• IPPROTO_TCP, which supports the following optnames: TCP_CONGESTION,
TCP_BPF_IW, TCP_BPF_SNDCWND_CLAMP, TCP_SAVE_SYN, TCP_KEEPIDLE,
TCP_KEEPINTVL, TCP_KEEPCNT, TCP_SYNCNT, TCP_USER_TIMEOUT.
• IPPROTO_IP, which supports optname IP_TOS.
• IPPROTO_IPV6, which supports optname IPV6_TCLASS.
Return 0 on success, or a negative error in case of failure.
long bpf_skb_adjust_room(struct sk_buff *skb, s32 len_diff, u32 mode, u64 flags)
Description
Grow or shrink the room for data in the packet associated to skb by
len_diff, and according to the selected mode.
By default, the helper will reset any offloaded checksum indicator of the
skb to CHECKSUM_NONE. This can be avoided by the following flag:
• BPF_F_ADJ_ROOM_NO_CSUM_RESET: Do not reset offloaded checksum data of the
skb to CHECKSUM_NONE.
There are two supported modes at this time:
• BPF_ADJ_ROOM_MAC: Adjust room at the mac layer (room space is added or
removed below the layer 2 header).
• BPF_ADJ_ROOM_NET: Adjust room at the network layer (room space is added or
removed below the layer 3 header).
The following flags are supported at this time:
• BPF_F_ADJ_ROOM_FIXED_GSO: Do not adjust gso_size. Adjusting mss in this
way is not allowed for datagrams.
• BPF_F_ADJ_ROOM_ENCAP_L3_IPV4, BPF_F_ADJ_ROOM_ENCAP_L3_IPV6: Any new space
is reserved to hold a tunnel header. Configure skb offsets and other
fields accordingly.
• BPF_F_ADJ_ROOM_ENCAP_L4_GRE, BPF_F_ADJ_ROOM_ENCAP_L4_UDP: Use with
ENCAP_L3 flags to further specify the tunnel type.
• BPF_F_ADJ_ROOM_ENCAP_L2(len): Use with ENCAP_L3/L4 flags to further
specify the tunnel type; len is the length of the inner MAC header.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_redirect_map(struct bpf_map *map, u32 key, u64 flags)
Description
Redirect the packet to the endpoint referenced by map at index key.
Depending on its type, this map can contain references to net devices (for
forwarding packets through other ports), or to CPUs (for redirecting XDP
frames to another CPU; but this is only implemented for native XDP (with
driver support) as of this writing).
The lower two bits of flags are used as the return code if the map lookup
fails. This is so that the return value can be one of the XDP program return
codes up to XDP_TX, as chosen by the caller. Any higher bits in the flags
argument must be unset.
See also bpf_redirect(), which only supports redirecting to an ifindex, but
doesn't require a map to do so.
Return XDP_REDIRECT on success, or the value of the two lower bits of the flags
argument on error.
long bpf_sk_redirect_map(struct sk_buff *skb, struct bpf_map *map, u32 key, u64 flags)
Description
Redirect the packet to the socket referenced by map (of type
BPF_MAP_TYPE_SOCKMAP) at index key. Both ingress and egress interfaces can
be used for redirection. The BPF_F_INGRESS value in flags is used to make
the distinction (ingress path is selected if the flag is present, egress
path otherwise). This is the only flag supported for now.
Return SK_PASS on success, or SK_DROP on error.
long bpf_sock_map_update(struct bpf_sock_ops *skops, struct bpf_map *map, void *key, u64
flags)
Description
Add an entry to, or update a map referencing sockets. The skops is used as a
new value for the entry associated to key. flags is one of:
BPF_NOEXIST
The entry for key must not exist in the map.
BPF_EXIST
The entry for key must already exist in the map.
BPF_ANY
No condition on the existence of the entry for key.
If the map has eBPF programs (parser and verdict), those will be inherited
by the socket being added. If the socket is already attached to eBPF
programs, this results in an error.
Return 0 on success, or a negative error in case of failure.
long bpf_xdp_adjust_meta(struct xdp_buff *xdp_md, int delta)
Description
Adjust the address pointed by xdp_md->data_meta by delta (which can be
positive or negative). Note that this operation modifies the address stored
in xdp_md->data, so the latter must be loaded only after the helper has been
called.
The use of xdp_md->data_meta is optional and programs are not required to
use it. The rationale is that when the packet is processed with XDP (e.g. as
DoS filter), it is possible to push further meta data along with it before
passing to the stack, and to give the guarantee that an ingress eBPF program
attached as a TC classifier on the same device can pick this up for further
post-processing. Since TC works with socket buffers, it remains possible to
set from XDP the mark or priority pointers, or other pointers for the socket
buffer. Having this scratch space generic and programmable allows for more
flexibility as the user is free to store whatever meta data they need.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_perf_event_read_value(struct bpf_map *map, u64 flags, struct bpf_perf_event_value
*buf, u32 buf_size)
Description
Read the value of a perf event counter, and store it into buf of size
buf_size. This helper relies on a map of type BPF_MAP_TYPE_PERF_EVENT_ARRAY.
The nature of the perf event counter is selected when map is updated with
perf event file descriptors. The map is an array whose size is the number of
available CPUs, and each cell contains a value relative to one CPU. The
value to retrieve is indicated by flags, that contains the index of the CPU
to look up, masked with BPF_F_INDEX_MASK. Alternatively, flags can be set to
BPF_F_CURRENT_CPU to indicate that the value for the current CPU should be
retrieved.
This helper behaves in a way close to bpf_perf_event_read() helper, save
that instead of just returning the value observed, it fills the buf
structure. This allows for additional data to be retrieved: in particular,
the enabled and running times (in buf->enabled and buf->running,
respectively) are copied. In general, bpf_perf_event_read_value() is
recommended over bpf_perf_event_read(), which has some ABI issues and
provides fewer functionalities.
These values are interesting, because hardware PMU (Performance Monitoring
Unit) counters are limited resources. When there are more PMU based perf
events opened than available counters, kernel will multiplex these events so
each event gets certain percentage (but not all) of the PMU time. In case
that multiplexing happens, the number of samples or counter value will not
reflect the case compared to when no multiplexing occurs. This makes
comparison between different runs difficult. Typically, the counter value
should be normalized before comparing to other experiments. The usual
normalization is done as follows.
normalized_counter = counter * t_enabled / t_running
Where t_enabled is the time enabled for event and t_running is the time
running for event since last normalization. The enabled and running times
are accumulated since the perf event open. To achieve scaling factor between
two invocations of an eBPF program, users can use CPU id as the key (which
is typical for perf array usage model) to remember the previous value and do
the calculation inside the eBPF program.
Return 0 on success, or a negative error in case of failure.
long bpf_perf_prog_read_value(struct bpf_perf_event_data *ctx, struct bpf_perf_event_value
*buf, u32 buf_size)
Description
For en eBPF program attached to a perf event, retrieve the value of the
event counter associated to ctx and store it in the structure pointed by buf
and of size buf_size. Enabled and running times are also stored in the
structure (see description of helper bpf_perf_event_read_value() for more
details).
Return 0 on success, or a negative error in case of failure.
long bpf_getsockopt(void *bpf_socket, int level, int optname, void *optval, int optlen)
Description
Emulate a call to getsockopt() on the socket associated to bpf_socket, which
must be a full socket. The level at which the option resides and the name
optname of the option must be specified, see getsockopt(2) for more
information. The retrieved value is stored in the structure pointed by
opval and of length optlen.
bpf_socket should be one of the following:
• struct bpf_sock_ops for BPF_PROG_TYPE_SOCK_OPS.
• struct bpf_sock_addr for BPF_CGROUP_INET4_CONNECT and
BPF_CGROUP_INET6_CONNECT.
This helper actually implements a subset of getsockopt(). It supports the
following levels:
• IPPROTO_TCP, which supports optname TCP_CONGESTION.
• IPPROTO_IP, which supports optname IP_TOS.
• IPPROTO_IPV6, which supports optname IPV6_TCLASS.
Return 0 on success, or a negative error in case of failure.
long bpf_override_return(struct pt_regs *regs, u64 rc)
Description
Used for error injection, this helper uses kprobes to override the return
value of the probed function, and to set it to rc. The first argument is
the context regs on which the kprobe works.
This helper works by setting the PC (program counter) to an override
function which is run in place of the original probed function. This means
the probed function is not run at all. The replacement function just returns
with the required value.
This helper has security implications, and thus is subject to restrictions.
It is only available if the kernel was compiled with the
CONFIG_BPF_KPROBE_OVERRIDE configuration option, and in this case it only
works on functions tagged with ALLOW_ERROR_INJECTION in the kernel code.
Also, the helper is only available for the architectures having the
CONFIG_FUNCTION_ERROR_INJECTION option. As of this writing, x86 architecture
is the only one to support this feature.
Return 0
long bpf_sock_ops_cb_flags_set(struct bpf_sock_ops *bpf_sock, int argval)
Description
Attempt to set the value of the bpf_sock_ops_cb_flags field for the full TCP
socket associated to bpf_sock_ops to argval.
The primary use of this field is to determine if there should be calls to
eBPF programs of type BPF_PROG_TYPE_SOCK_OPS at various points in the TCP
code. A program of the same type can change its value, per connection and as
necessary, when the connection is established. This field is directly
accessible for reading, but this helper must be used for updates in order to
return an error if an eBPF program tries to set a callback that is not
supported in the current kernel.
argval is a flag array which can combine these flags:
• BPF_SOCK_OPS_RTO_CB_FLAG (retransmission time out)
• BPF_SOCK_OPS_RETRANS_CB_FLAG (retransmission)
• BPF_SOCK_OPS_STATE_CB_FLAG (TCP state change)
• BPF_SOCK_OPS_RTT_CB_FLAG (every RTT)
Therefore, this function can be used to clear a callback flag by setting the
appropriate bit to zero. e.g. to disable the RTO callback:
bpf_sock_ops_cb_flags_set(bpf_sock,
bpf_sock->bpf_sock_ops_cb_flags & ~BPF_SOCK_OPS_RTO_CB_FLAG)
Here are some examples of where one could call such eBPF program:
• When RTO fires.
• When a packet is retransmitted.
• When the connection terminates.
• When a packet is sent.
• When a packet is received.
Return Code -EINVAL if the socket is not a full TCP socket; otherwise, a positive
number containing the bits that could not be set is returned (which comes
down to 0 if all bits were set as required).
long bpf_msg_redirect_map(struct sk_msg_buff *msg, struct bpf_map *map, u32 key, u64
flags)
Description
This helper is used in programs implementing policies at the socket level.
If the message msg is allowed to pass (i.e. if the verdict eBPF program
returns SK_PASS), redirect it to the socket referenced by map (of type
BPF_MAP_TYPE_SOCKMAP) at index key. Both ingress and egress interfaces can
be used for redirection. The BPF_F_INGRESS value in flags is used to make
the distinction (ingress path is selected if the flag is present, egress
path otherwise). This is the only flag supported for now.
Return SK_PASS on success, or SK_DROP on error.
long bpf_msg_apply_bytes(struct sk_msg_buff *msg, u32 bytes)
Description
For socket policies, apply the verdict of the eBPF program to the next bytes
(number of bytes) of message msg.
For example, this helper can be used in the following cases:
• A single sendmsg() or sendfile() system call contains multiple logical
messages that the eBPF program is supposed to read and for which it should
apply a verdict.
• An eBPF program only cares to read the first bytes of a msg. If the
message has a large payload, then setting up and calling the eBPF program
repeatedly for all bytes, even though the verdict is already known, would
create unnecessary overhead.
When called from within an eBPF program, the helper sets a counter internal
to the BPF infrastructure, that is used to apply the last verdict to the
next bytes. If bytes is smaller than the current data being processed from a
sendmsg() or sendfile() system call, the first bytes will be sent and the
eBPF program will be re-run with the pointer for start of data pointing to
byte number bytes + 1. If bytes is larger than the current data being
processed, then the eBPF verdict will be applied to multiple sendmsg() or
sendfile() calls until bytes are consumed.
Note that if a socket closes with the internal counter holding a non-zero
value, this is not a problem because data is not being buffered for bytes
and is sent as it is received.
Return 0
long bpf_msg_cork_bytes(struct sk_msg_buff *msg, u32 bytes)
Description
For socket policies, prevent the execution of the verdict eBPF program for
message msg until bytes (byte number) have been accumulated.
This can be used when one needs a specific number of bytes before a verdict
can be assigned, even if the data spans multiple sendmsg() or sendfile()
calls. The extreme case would be a user calling sendmsg() repeatedly with
1-byte long message segments. Obviously, this is bad for performance, but it
is still valid. If the eBPF program needs bytes bytes to validate a header,
this helper can be used to prevent the eBPF program to be called again until
bytes have been accumulated.
Return 0
long bpf_msg_pull_data(struct sk_msg_buff *msg, u32 start, u32 end, u64 flags)
Description
For socket policies, pull in non-linear data from user space for msg and set
pointers msg->data and msg->data_end to start and end bytes offsets into
msg, respectively.
If a program of type BPF_PROG_TYPE_SK_MSG is run on a msg it can only parse
data that the (data, data_end) pointers have already consumed. For sendmsg()
hooks this is likely the first scatterlist element. But for calls relying on
the sendpage handler (e.g. sendfile()) this will be the range (0, 0) because
the data is shared with user space and by default the objective is to avoid
allowing user space to modify data while (or after) eBPF verdict is being
decided. This helper can be used to pull in data and to set the start and
end pointer to given values. Data will be copied if necessary (i.e. if data
was not linear and if start and end pointers do not point to the same
chunk).
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
All values for flags are reserved for future usage, and must be left at
zero.
Return 0 on success, or a negative error in case of failure.
long bpf_bind(struct bpf_sock_addr *ctx, struct sockaddr *addr, int addr_len)
Description
Bind the socket associated to ctx to the address pointed by addr, of length
addr_len. This allows for making outgoing connection from the desired IP
address, which can be useful for example when all processes inside a cgroup
should use one single IP address on a host that has multiple IP configured.
This helper works for IPv4 and IPv6, TCP and UDP sockets. The domain
(addr->sa_family) must be AF_INET (or AF_INET6). It's advised to pass zero
port (sin_port or sin6_port) which triggers IP_BIND_ADDRESS_NO_PORT-like
behavior and lets the kernel efficiently pick up an unused port as long as
4-tuple is unique. Passing non-zero port might lead to degraded performance.
Return 0 on success, or a negative error in case of failure.
long bpf_xdp_adjust_tail(struct xdp_buff *xdp_md, int delta)
Description
Adjust (move) xdp_md->data_end by delta bytes. It is possible to both shrink
and grow the packet tail. Shrink done via delta being a negative integer.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_skb_get_xfrm_state(struct sk_buff *skb, u32 index, struct bpf_xfrm_state
*xfrm_state, u32 size, u64 flags)
Description
Retrieve the XFRM state (IP transform framework, see also ip-xfrm(8)) at
index in XFRM "security path" for skb.
The retrieved value is stored in the struct bpf_xfrm_state pointed by
xfrm_state and of length size.
All values for flags are reserved for future usage, and must be left at
zero.
This helper is available only if the kernel was compiled with CONFIG_XFRM
configuration option.
Return 0 on success, or a negative error in case of failure.
long bpf_get_stack(void *ctx, void *buf, u32 size, u64 flags)
Description
Return a user or a kernel stack in bpf program provided buffer. To achieve
this, the helper needs ctx, which is a pointer to the context on which the
tracing program is executed. To store the stacktrace, the bpf program
provides buf with a nonnegative size.
The last argument, flags, holds the number of stack frames to skip (from 0
to 255), masked with BPF_F_SKIP_FIELD_MASK. The next bits can be used to set
the following flags:
BPF_F_USER_STACK
Collect a user space stack instead of a kernel stack.
BPF_F_USER_BUILD_ID
Collect buildid+offset instead of ips for user stack, only valid if
BPF_F_USER_STACK is also specified.
bpf_get_stack() can collect up to PERF_MAX_STACK_DEPTH both kernel and user
frames, subject to sufficient large buffer size. Note that this limit can be
controlled with the sysctl program, and that it should be manually increased
in order to profile long user stacks (such as stacks for Java programs). To
do so, use:
# sysctl kernel.perf_event_max_stack=<new value>
Return A non-negative value equal to or less than size on success, or a negative
error in case of failure.
long bpf_skb_load_bytes_relative(const void *skb, u32 offset, void *to, u32 len, u32
start_header)
Description
This helper is similar to bpf_skb_load_bytes() in that it provides an easy
way to load len bytes from offset from the packet associated to skb, into
the buffer pointed by to. The difference to bpf_skb_load_bytes() is that a
fifth argument start_header exists in order to select a base offset to start
from. start_header can be one of:
BPF_HDR_START_MAC
Base offset to load data from is skb's mac header.
BPF_HDR_START_NET
Base offset to load data from is skb's network header.
In general, "direct packet access" is the preferred method to access packet
data, however, this helper is in particular useful in socket filters where
skb->data does not always point to the start of the mac header and where
"direct packet access" is not available.
Return 0 on success, or a negative error in case of failure.
long bpf_fib_lookup(void *ctx, struct bpf_fib_lookup *params, int plen, u32 flags)
Description
Do FIB lookup in kernel tables using parameters in params. If lookup is
successful and result shows packet is to be forwarded, the neighbor tables
are searched for the nexthop. If successful (ie., FIB lookup shows
forwarding and nexthop is resolved), the nexthop address is returned in
ipv4_dst or ipv6_dst based on family, smac is set to mac address of egress
device, dmac is set to nexthop mac address, rt_metric is set to metric from
route (IPv4/IPv6 only), and ifindex is set to the device index of the
nexthop from the FIB lookup.
plen argument is the size of the passed in struct. flags argument can be a
combination of one or more of the following values:
BPF_FIB_LOOKUP_DIRECT
Do a direct table lookup vs full lookup using FIB rules.
BPF_FIB_LOOKUP_OUTPUT
Perform lookup from an egress perspective (default is ingress).
ctx is either struct xdp_md for XDP programs or struct sk_buff tc cls_act
programs.
Return
• < 0 if any input argument is invalid
• 0 on success (packet is forwarded, nexthop neighbor exists)
• > 0 one of BPF_FIB_LKUP_RET_ codes explaining why the packet is not
forwarded or needs assist from full stack
long bpf_sock_hash_update(struct bpf_sock_ops *skops, struct bpf_map *map, void *key, u64
flags)
Description
Add an entry to, or update a sockhash map referencing sockets. The skops is
used as a new value for the entry associated to key. flags is one of:
BPF_NOEXIST
The entry for key must not exist in the map.
BPF_EXIST
The entry for key must already exist in the map.
BPF_ANY
No condition on the existence of the entry for key.
If the map has eBPF programs (parser and verdict), those will be inherited
by the socket being added. If the socket is already attached to eBPF
programs, this results in an error.
Return 0 on success, or a negative error in case of failure.
long bpf_msg_redirect_hash(struct sk_msg_buff *msg, struct bpf_map *map, void *key, u64
flags)
Description
This helper is used in programs implementing policies at the socket level.
If the message msg is allowed to pass (i.e. if the verdict eBPF program
returns SK_PASS), redirect it to the socket referenced by map (of type
BPF_MAP_TYPE_SOCKHASH) using hash key. Both ingress and egress interfaces
can be used for redirection. The BPF_F_INGRESS value in flags is used to
make the distinction (ingress path is selected if the flag is present,
egress path otherwise). This is the only flag supported for now.
Return SK_PASS on success, or SK_DROP on error.
long bpf_sk_redirect_hash(struct sk_buff *skb, struct bpf_map *map, void *key, u64 flags)
Description
This helper is used in programs implementing policies at the skb socket
level. If the sk_buff skb is allowed to pass (i.e. if the verdict eBPF
program returns SK_PASS), redirect it to the socket referenced by map (of
type BPF_MAP_TYPE_SOCKHASH) using hash key. Both ingress and egress
interfaces can be used for redirection. The BPF_F_INGRESS value in flags is
used to make the distinction (ingress path is selected if the flag is
present, egress otherwise). This is the only flag supported for now.
Return SK_PASS on success, or SK_DROP on error.
long bpf_lwt_push_encap(struct sk_buff *skb, u32 type, void *hdr, u32 len)
Description
Encapsulate the packet associated to skb within a Layer 3 protocol header.
This header is provided in the buffer at address hdr, with len its size in
bytes. type indicates the protocol of the header and can be one of:
BPF_LWT_ENCAP_SEG6
IPv6 encapsulation with Segment Routing Header (struct ipv6_sr_hdr).
hdr only contains the SRH, the IPv6 header is computed by the kernel.
BPF_LWT_ENCAP_SEG6_INLINE
Only works if skb contains an IPv6 packet. Insert a Segment Routing
Header (struct ipv6_sr_hdr) inside the IPv6 header.
BPF_LWT_ENCAP_IP
IP encapsulation (GRE/GUE/IPIP/etc). The outer header must be IPv4 or
IPv6, followed by zero or more additional headers, up to
LWT_BPF_MAX_HEADROOM total bytes in all prepended headers. Please
note that if skb_is_gso(skb) is true, no more than two headers can be
prepended, and the inner header, if present, should be either GRE or
UDP/GUE.
BPF_LWT_ENCAP_SEG6* types can be called by BPF programs of type
BPF_PROG_TYPE_LWT_IN; BPF_LWT_ENCAP_IP type can be called by bpf programs of
types BPF_PROG_TYPE_LWT_IN and BPF_PROG_TYPE_LWT_XMIT.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_lwt_seg6_store_bytes(struct sk_buff *skb, u32 offset, const void *from, u32 len)
Description
Store len bytes from address from into the packet associated to skb, at
offset. Only the flags, tag and TLVs inside the outermost IPv6 Segment
Routing Header can be modified through this helper.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_lwt_seg6_adjust_srh(struct sk_buff *skb, u32 offset, s32 delta)
Description
Adjust the size allocated to TLVs in the outermost IPv6 Segment Routing
Header contained in the packet associated to skb, at position offset by
delta bytes. Only offsets after the segments are accepted. delta can be as
well positive (growing) as negative (shrinking).
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_lwt_seg6_action(struct sk_buff *skb, u32 action, void *param, u32 param_len)
Description
Apply an IPv6 Segment Routing action of type action to the packet associated
to skb. Each action takes a parameter contained at address param, and of
length param_len bytes. action can be one of:
SEG6_LOCAL_ACTION_END_X
End.X action: Endpoint with Layer-3 cross-connect. Type of param:
struct in6_addr.
SEG6_LOCAL_ACTION_END_T
End.T action: Endpoint with specific IPv6 table lookup. Type of
param: int.
SEG6_LOCAL_ACTION_END_B6
End.B6 action: Endpoint bound to an SRv6 policy. Type of param:
struct ipv6_sr_hdr.
SEG6_LOCAL_ACTION_END_B6_ENCAP
End.B6.Encap action: Endpoint bound to an SRv6 encapsulation policy.
Type of param: struct ipv6_sr_hdr.
A call to this helper is susceptible to change the underlying packet buffer.
Therefore, at load time, all checks on pointers previously done by the
verifier are invalidated and must be performed again, if the helper is used
in combination with direct packet access.
Return 0 on success, or a negative error in case of failure.
long bpf_rc_repeat(void *ctx)
Description
This helper is used in programs implementing IR decoding, to report a
successfully decoded repeat key message. This delays the generation of a key
up event for previously generated key down event.
Some IR protocols like NEC have a special IR message for repeating last
button, for when a button is held down.
The ctx should point to the lirc sample as passed into the program.
This helper is only available is the kernel was compiled with the
CONFIG_BPF_LIRC_MODE2 configuration option set to "y".
Return 0
long bpf_rc_keydown(void *ctx, u32 protocol, u64 scancode, u32 toggle)
Description
This helper is used in programs implementing IR decoding, to report a
successfully decoded key press with scancode, toggle value in the given
protocol. The scancode will be translated to a keycode using the rc keymap,
and reported as an input key down event. After a period a key up event is
generated. This period can be extended by calling either bpf_rc_keydown()
again with the same values, or calling bpf_rc_repeat().
Some protocols include a toggle bit, in case the button was released and
pressed again between consecutive scancodes.
The ctx should point to the lirc sample as passed into the program.
The protocol is the decoded protocol number (see enum rc_proto for some
predefined values).
This helper is only available is the kernel was compiled with the
CONFIG_BPF_LIRC_MODE2 configuration option set to "y".
Return 0
u64 bpf_skb_cgroup_id(struct sk_buff *skb)
Description
Return the cgroup v2 id of the socket associated with the skb. This is
roughly similar to the bpf_get_cgroup_classid() helper for cgroup v1 by
providing a tag resp. identifier that can be matched on or used for map
lookups e.g. to implement policy. The cgroup v2 id of a given path in the
hierarchy is exposed in user space through the f_handle API in order to get
to the same 64-bit id.
This helper can be used on TC egress path, but not on ingress, and is
available only if the kernel was compiled with the CONFIG_SOCK_CGROUP_DATA
configuration option.
Return The id is returned or 0 in case the id could not be retrieved.
u64 bpf_get_current_cgroup_id(void)
Return A 64-bit integer containing the current cgroup id based on the cgroup within
which the current task is running.
void *bpf_get_local_storage(void *map, u64 flags)
Description
Get the pointer to the local storage area. The type and the size of the
local storage is defined by the map argument. The flags meaning is specific
for each map type, and has to be 0 for cgroup local storage.
Depending on the BPF program type, a local storage area can be shared
between multiple instances of the BPF program, running simultaneously.
A user should care about the synchronization by himself. For example, by
using the BPF_STX_XADD instruction to alter the shared data.
Return A pointer to the local storage area.
long bpf_sk_select_reuseport(struct sk_reuseport_md *reuse, struct bpf_map *map, void
*key, u64 flags)
Description
Select a SO_REUSEPORT socket from a BPF_MAP_TYPE_REUSEPORT_ARRAY map. It
checks the selected socket is matching the incoming request in the socket
buffer.
Return 0 on success, or a negative error in case of failure.
u64 bpf_skb_ancestor_cgroup_id(struct sk_buff *skb, int ancestor_level)
Description
Return id of cgroup v2 that is ancestor of cgroup associated with the skb at
the ancestor_level. The root cgroup is at ancestor_level zero and each step
down the hierarchy increments the level. If ancestor_level == level of
cgroup associated with skb, then return value will be same as that of
bpf_skb_cgroup_id().
The helper is useful to implement policies based on cgroups that are upper
in hierarchy than immediate cgroup associated with skb.
The format of returned id and helper limitations are same as in
bpf_skb_cgroup_id().
Return The id is returned or 0 in case the id could not be retrieved.
struct bpf_sock *bpf_sk_lookup_tcp(void *ctx, struct bpf_sock_tuple *tuple, u32
tuple_size, u64 netns, u64 flags)
Description
Look for TCP socket matching tuple, optionally in a child network namespace
netns. The return value must be checked, and if non-NULL, released via
bpf_sk_release().
The ctx should point to the context of the program, such as the skb or
socket (depending on the hook in use). This is used to determine the base
network namespace for the lookup.
tuple_size must be one of:
sizeof(tuple->ipv4)
Look for an IPv4 socket.
sizeof(tuple->ipv6)
Look for an IPv6 socket.
If the netns is a negative signed 32-bit integer, then the socket lookup
table in the netns associated with the ctx will be used. For the TC hooks,
this is the netns of the device in the skb. For socket hooks, this is the
netns of the socket. If netns is any other signed 32-bit value greater than
or equal to zero then it specifies the ID of the netns relative to the netns
associated with the ctx. netns values beyond the range of 32-bit integers
are reserved for future use.
All values for flags are reserved for future usage, and must be left at
zero.
This helper is available only if the kernel was compiled with CONFIG_NET
configuration option.
Return Pointer to struct bpf_sock, or NULL in case of failure. For sockets with
reuseport option, the struct bpf_sock result is from reuse->socks[] using
the hash of the tuple.
struct bpf_sock *bpf_sk_lookup_udp(void *ctx, struct bpf_sock_tuple *tuple, u32
tuple_size, u64 netns, u64 flags)
Description
Look for UDP socket matching tuple, optionally in a child network namespace
netns. The return value must be checked, and if non-NULL, released via
bpf_sk_release().
The ctx should point to the context of the program, such as the skb or
socket (depending on the hook in use). This is used to determine the base
network namespace for the lookup.
tuple_size must be one of:
sizeof(tuple->ipv4)
Look for an IPv4 socket.
sizeof(tuple->ipv6)
Look for an IPv6 socket.
If the netns is a negative signed 32-bit integer, then the socket lookup
table in the netns associated with the ctx will be used. For the TC hooks,
this is the netns of the device in the skb. For socket hooks, this is the
netns of the socket. If netns is any other signed 32-bit value greater than
or equal to zero then it specifies the ID of the netns relative to the netns
associated with the ctx. netns values beyond the range of 32-bit integers
are reserved for future use.
All values for flags are reserved for future usage, and must be left at
zero.
This helper is available only if the kernel was compiled with CONFIG_NET
configuration option.
Return Pointer to struct bpf_sock, or NULL in case of failure. For sockets with
reuseport option, the struct bpf_sock result is from reuse->socks[] using
the hash of the tuple.
long bpf_sk_release(struct bpf_sock *sock)
Description
Release the reference held by sock. sock must be a non-NULL pointer that was
returned from bpf_sk_lookup_xxx().
Return 0 on success, or a negative error in case of failure.
long bpf_map_push_elem(struct bpf_map *map, const void *value, u64 flags)
Description
Push an element value in map. flags is one of:
BPF_EXIST
If the queue/stack is full, the oldest element is removed to make
room for this.
Return 0 on success, or a negative error in case of failure.
long bpf_map_pop_elem(struct bpf_map *map, void *value)
Description
Pop an element from map.
Return 0 on success, or a negative error in case of failure.
long bpf_map_peek_elem(struct bpf_map *map, void *value)
Description
Get an element from map without removing it.
Return 0 on success, or a negative error in case of failure.
long bpf_msg_push_data(struct sk_msg_buff *msg, u32 start, u32 len, u64 flags)
Description
For socket policies, insert len bytes into msg at offset start.
If a program of type BPF_PROG_TYPE_SK_MSG is run on a msg it may want to
insert metadata or options into the msg. This can later be read and used by
any of the lower layer BPF hooks.
This helper may fail if under memory pressure (a malloc fails) in these
cases BPF programs will get an appropriate error and BPF programs will need
to handle them.
Return 0 on success, or a negative error in case of failure.
long bpf_msg_pop_data(struct sk_msg_buff *msg, u32 start, u32 len, u64 flags)
Description
Will remove len bytes from a msg starting at byte start. This may result in
ENOMEM errors under certain situations if an allocation and copy are
required due to a full ring buffer. However, the helper will try to avoid
doing the allocation if possible. Other errors can occur if input parameters
are invalid either due to start byte not being valid part of msg payload
and/or pop value being to large.
Return 0 on success, or a negative error in case of failure.
long bpf_rc_pointer_rel(void *ctx, s32 rel_x, s32 rel_y)
Description
This helper is used in programs implementing IR decoding, to report a
successfully decoded pointer movement.
The ctx should point to the lirc sample as passed into the program.
This helper is only available is the kernel was compiled with the
CONFIG_BPF_LIRC_MODE2 configuration option set to "y".
Return 0
long bpf_spin_lock(struct bpf_spin_lock *lock)
Description
Acquire a spinlock represented by the pointer lock, which is stored as part
of a value of a map. Taking the lock allows to safely update the rest of the
fields in that value. The spinlock can (and must) later be released with a
call to bpf_spin_unlock(lock).
Spinlocks in BPF programs come with a number of restrictions and
constraints:
• bpf_spin_lock objects are only allowed inside maps of types
BPF_MAP_TYPE_HASH and BPF_MAP_TYPE_ARRAY (this list could be extended in
the future).
• BTF description of the map is mandatory.
• The BPF program can take ONE lock at a time, since taking two or more
could cause dead locks.
• Only one struct bpf_spin_lock is allowed per map element.
• When the lock is taken, calls (either BPF to BPF or helpers) are not
allowed.
• The BPF_LD_ABS and BPF_LD_IND instructions are not allowed inside a
spinlock-ed region.
• The BPF program MUST call bpf_spin_unlock() to release the lock, on all
execution paths, before it returns.
• The BPF program can access struct bpf_spin_lock only via the
bpf_spin_lock() and bpf_spin_unlock() helpers. Loading or storing data
into the struct bpf_spin_lock lock; field of a map is not allowed.
• To use the bpf_spin_lock() helper, the BTF description of the map value
must be a struct and have struct bpf_spin_lock anyname; field at the top
level. Nested lock inside another struct is not allowed.
• The struct bpf_spin_lock lock field in a map value must be aligned on a
multiple of 4 bytes in that value.
• Syscall with command BPF_MAP_LOOKUP_ELEM does not copy the bpf_spin_lock
field to user space.
• Syscall with command BPF_MAP_UPDATE_ELEM, or update from a BPF program, do
not update the bpf_spin_lock field.
• bpf_spin_lock cannot be on the stack or inside a networking packet (it can
only be inside of a map values).
• bpf_spin_lock is available to root only.
• Tracing programs and socket filter programs cannot use bpf_spin_lock() due
to insufficient preemption checks (but this may change in the future).
• bpf_spin_lock is not allowed in inner maps of map-in-map.
Return 0
long bpf_spin_unlock(struct bpf_spin_lock *lock)
Description
Release the lock previously locked by a call to bpf_spin_lock(lock).
Return 0
struct bpf_sock *bpf_sk_fullsock(struct bpf_sock *sk)
Description
This helper gets a struct bpf_sock pointer such that all the fields in this
bpf_sock can be accessed.
Return A struct bpf_sock pointer on success, or NULL in case of failure.
struct bpf_tcp_sock *bpf_tcp_sock(struct bpf_sock *sk)
Description
This helper gets a struct bpf_tcp_sock pointer from a struct bpf_sock
pointer.
Return A struct bpf_tcp_sock pointer on success, or NULL in case of failure.
long bpf_skb_ecn_set_ce(struct sk_buff *skb)
Description
Set ECN (Explicit Congestion Notification) field of IP header to CE
(Congestion Encountered) if current value is ECT (ECN Capable Transport).
Otherwise, do nothing. Works with IPv6 and IPv4.
Return 1 if the CE flag is set (either by the current helper call or because it was
already present), 0 if it is not set.
struct bpf_sock *bpf_get_listener_sock(struct bpf_sock *sk)
Description
Return a struct bpf_sock pointer in TCP_LISTEN state. bpf_sk_release() is
unnecessary and not allowed.
Return A struct bpf_sock pointer on success, or NULL in case of failure.
struct bpf_sock *bpf_skc_lookup_tcp(void *ctx, struct bpf_sock_tuple *tuple, u32
tuple_size, u64 netns, u64 flags)
Description
Look for TCP socket matching tuple, optionally in a child network namespace
netns. The return value must be checked, and if non-NULL, released via
bpf_sk_release().
This function is identical to bpf_sk_lookup_tcp(), except that it also
returns timewait or request sockets. Use bpf_sk_fullsock() or bpf_tcp_sock()
to access the full structure.
This helper is available only if the kernel was compiled with CONFIG_NET
configuration option.
Return Pointer to struct bpf_sock, or NULL in case of failure. For sockets with
reuseport option, the struct bpf_sock result is from reuse->socks[] using
the hash of the tuple.
long bpf_tcp_check_syncookie(struct bpf_sock *sk, void *iph, u32 iph_len, struct tcphdr
*th, u32 th_len)
Description
Check whether iph and th contain a valid SYN cookie ACK for the listening
socket in sk.
iph points to the start of the IPv4 or IPv6 header, while iph_len contains
sizeof(struct iphdr) or sizeof(struct ip6hdr).
th points to the start of the TCP header, while th_len contains
sizeof(struct tcphdr).
Return 0 if iph and th are a valid SYN cookie ACK, or a negative error otherwise.
long bpf_sysctl_get_name(struct bpf_sysctl *ctx, char *buf, size_t buf_len, u64 flags)
Description
Get name of sysctl in /proc/sys/ and copy it into provided by program buffer
buf of size buf_len.
The buffer is always NUL terminated, unless it's zero-sized.
If flags is zero, full name (e.g. "net/ipv4/tcp_mem") is copied. Use
BPF_F_SYSCTL_BASE_NAME flag to copy base name only (e.g. "tcp_mem").
Return Number of character copied (not including the trailing NUL).
-E2BIG if the buffer wasn't big enough (buf will contain truncated name in
this case).
long bpf_sysctl_get_current_value(struct bpf_sysctl *ctx, char *buf, size_t buf_len)
Description
Get current value of sysctl as it is presented in /proc/sys (incl. newline,
etc), and copy it as a string into provided by program buffer buf of size
buf_len.
The whole value is copied, no matter what file position user space issued
e.g. sys_read at.
The buffer is always NUL terminated, unless it's zero-sized.
Return Number of character copied (not including the trailing NUL).
-E2BIG if the buffer wasn't big enough (buf will contain truncated name in
this case).
-EINVAL if current value was unavailable, e.g. because sysctl is
uninitialized and read returns -EIO for it.
long bpf_sysctl_get_new_value(struct bpf_sysctl *ctx, char *buf, size_t buf_len)
Description
Get new value being written by user space to sysctl (before the actual write
happens) and copy it as a string into provided by program buffer buf of size
buf_len.
User space may write new value at file position > 0.
The buffer is always NUL terminated, unless it's zero-sized.
Return Number of character copied (not including the trailing NUL).
-E2BIG if the buffer wasn't big enough (buf will contain truncated name in
this case).
-EINVAL if sysctl is being read.
long bpf_sysctl_set_new_value(struct bpf_sysctl *ctx, const char *buf, size_t buf_len)
Description
Override new value being written by user space to sysctl with value provided
by program in buffer buf of size buf_len.
buf should contain a string in same form as provided by user space on sysctl
write.
User space may write new value at file position > 0. To override the whole
sysctl value file position should be set to zero.
Return 0 on success.
-E2BIG if the buf_len is too big.
-EINVAL if sysctl is being read.
long bpf_strtol(const char *buf, size_t buf_len, u64 flags, long *res)
Description
Convert the initial part of the string from buffer buf of size buf_len to a
long integer according to the given base and save the result in res.
The string may begin with an arbitrary amount of white space (as determined
by isspace(3)) followed by a single optional '-' sign.
Five least significant bits of flags encode base, other bits are currently
unused.
Base must be either 8, 10, 16, or 0 to detect it automatically similar to
user space strtol(3).
Return Number of characters consumed on success. Must be positive but no more than
buf_len.
-EINVAL if no valid digits were found or unsupported base was provided.
-ERANGE if resulting value was out of range.
long bpf_strtoul(const char *buf, size_t buf_len, u64 flags, unsigned long *res)
Description
Convert the initial part of the string from buffer buf of size buf_len to an
unsigned long integer according to the given base and save the result in
res.
The string may begin with an arbitrary amount of white space (as determined
by isspace(3)).
Five least significant bits of flags encode base, other bits are currently
unused.
Base must be either 8, 10, 16, or 0 to detect it automatically similar to
user space strtoul(3).
Return Number of characters consumed on success. Must be positive but no more than
buf_len.
-EINVAL if no valid digits were found or unsupported base was provided.
-ERANGE if resulting value was out of range.
void *bpf_sk_storage_get(struct bpf_map *map, struct bpf_sock *sk, void *value, u64 flags)
Description
Get a bpf-local-storage from a sk.
Logically, it could be thought of getting the value from a map with sk as
the key. From this perspective, the usage is not much different from
bpf_map_lookup_elem(map, &sk) except this helper enforces the key must be a
full socket and the map must be a BPF_MAP_TYPE_SK_STORAGE also.
Underneath, the value is stored locally at sk instead of the map. The map
is used as the bpf-local-storage "type". The bpf-local-storage "type" (i.e.
the map) is searched against all bpf-local-storages residing at sk.
An optional flags (BPF_SK_STORAGE_GET_F_CREATE) can be used such that a new
bpf-local-storage will be created if one does not exist. value can be used
together with BPF_SK_STORAGE_GET_F_CREATE to specify the initial value of a
bpf-local-storage. If value is NULL, the new bpf-local-storage will be zero
initialized.
Return A bpf-local-storage pointer is returned on success.
NULL if not found or there was an error in adding a new bpf-local-storage.
long bpf_sk_storage_delete(struct bpf_map *map, struct bpf_sock *sk)
Description
Delete a bpf-local-storage from a sk.
Return 0 on success.
-ENOENT if the bpf-local-storage cannot be found.
long bpf_send_signal(u32 sig)
Description
Send signal sig to the process of the current task. The signal may be
delivered to any of this process's threads.
Return 0 on success or successfully queued.
-EBUSY if work queue under nmi is full.
-EINVAL if sig is invalid.
-EPERM if no permission to send the sig.
-EAGAIN if bpf program can try again.
s64 bpf_tcp_gen_syncookie(struct bpf_sock *sk, void *iph, u32 iph_len, struct tcphdr *th,
u32 th_len)
Description
Try to issue a SYN cookie for the packet with corresponding IP/TCP headers,
iph and th, on the listening socket in sk.
iph points to the start of the IPv4 or IPv6 header, while iph_len contains
sizeof(struct iphdr) or sizeof(struct ip6hdr).
th points to the start of the TCP header, while th_len contains the length
of the TCP header.
Return On success, lower 32 bits hold the generated SYN cookie in followed by 16
bits which hold the MSS value for that cookie, and the top 16 bits are
unused.
On failure, the returned value is one of the following:
-EINVAL SYN cookie cannot be issued due to error
-ENOENT SYN cookie should not be issued (no SYN flood)
-EOPNOTSUPP kernel configuration does not enable SYN cookies
-EPROTONOSUPPORT IP packet version is not 4 or 6
long bpf_skb_output(void *ctx, struct bpf_map *map, u64 flags, void *data, u64 size)
Description
Write raw data blob into a special BPF perf event held by map of type
BPF_MAP_TYPE_PERF_EVENT_ARRAY. This perf event must have the following
attributes: PERF_SAMPLE_RAW as sample_type, PERF_TYPE_SOFTWARE as type, and
PERF_COUNT_SW_BPF_OUTPUT as config.
The flags are used to indicate the index in map for which the value must be
put, masked with BPF_F_INDEX_MASK. Alternatively, flags can be set to
BPF_F_CURRENT_CPU to indicate that the index of the current CPU core should
be used.
The value to write, of size, is passed through eBPF stack and pointed by
data.
ctx is a pointer to in-kernel struct sk_buff.
This helper is similar to bpf_perf_event_output() but restricted to
raw_tracepoint bpf programs.
Return 0 on success, or a negative error in case of failure.
long bpf_probe_read_user(void *dst, u32 size, const void *unsafe_ptr)
Description
Safely attempt to read size bytes from user space address unsafe_ptr and
store the data in dst.
Return 0 on success, or a negative error in case of failure.
long bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
Description
Safely attempt to read size bytes from kernel space address unsafe_ptr and
store the data in dst.
Return 0 on success, or a negative error in case of failure.
long bpf_probe_read_user_str(void *dst, u32 size, const void *unsafe_ptr)
Description
Copy a NUL terminated string from an unsafe user address unsafe_ptr to dst.
The size should include the terminating NUL byte. In case the string length
is smaller than size, the target is not padded with further NUL bytes. If
the string length is larger than size, just size-1 bytes are copied and the
last byte is set to NUL.
On success, the length of the copied string is returned. This makes this
helper useful in tracing programs for reading strings, and more importantly
to get its length at runtime. See the following snippet:
SEC("kprobe/sys_open")
void bpf_sys_open(struct pt_regs *ctx)
{
char buf[PATHLEN]; // PATHLEN is defined to 256
int res = bpf_probe_read_user_str(buf, sizeof(buf),
ctx->di);
// Consume buf, for example push it to
// user space via bpf_perf_event_output(); we
// can use res (the string length) as event
// size, after checking its boundaries.
}
In comparison, using bpf_probe_read_user() helper here instead to read the
string would require to estimate the length at compile time, and would often
result in copying more memory than necessary.
Another useful use case is when parsing individual process arguments or
individual environment variables navigating current->mm->arg_start and
current->mm->env_start: using this helper and the return value, one can
quickly iterate at the right offset of the memory area.
Return On success, the strictly positive length of the string, including the
trailing NUL character. On error, a negative value.
long bpf_probe_read_kernel_str(void *dst, u32 size, const void *unsafe_ptr)
Description
Copy a NUL terminated string from an unsafe kernel address unsafe_ptr to
dst. Same semantics as with bpf_probe_read_user_str() apply.
Return On success, the strictly positive length of the string, including the
trailing NUL character. On error, a negative value.
long bpf_tcp_send_ack(void *tp, u32 rcv_nxt)
Description
Send out a tcp-ack. tp is the in-kernel struct tcp_sock. rcv_nxt is the
ack_seq to be sent out.
Return 0 on success, or a negative error in case of failure.
long bpf_send_signal_thread(u32 sig)
Description
Send signal sig to the thread corresponding to the current task.
Return 0 on success or successfully queued.
-EBUSY if work queue under nmi is full.
-EINVAL if sig is invalid.
-EPERM if no permission to send the sig.
-EAGAIN if bpf program can try again.
u64 bpf_jiffies64(void)
Description
Obtain the 64bit jiffies
Return The 64 bit jiffies
long bpf_read_branch_records(struct bpf_perf_event_data *ctx, void *buf, u32 size, u64
flags)
Description
For an eBPF program attached to a perf event, retrieve the branch records
(struct perf_branch_entry) associated to ctx and store it in the buffer
pointed by buf up to size size bytes.
Return On success, number of bytes written to buf. On error, a negative value.
The flags can be set to BPF_F_GET_BRANCH_RECORDS_SIZE to instead return the
number of bytes required to store all the branch entries. If this flag is
set, buf may be NULL.
-EINVAL if arguments invalid or size not a multiple of sizeof(struct
perf_branch_entry).
-ENOENT if architecture does not support branch records.
long bpf_get_ns_current_pid_tgid(u64 dev, u64 ino, struct bpf_pidns_info *nsdata, u32
size)
Description
Returns 0 on success, values for pid and tgid as seen from the current
namespace will be returned in nsdata.
Return 0 on success, or one of the following in case of failure:
-EINVAL if dev and inum supplied don't match dev_t and inode number with
nsfs of current task, or if dev conversion to dev_t lost high bits.
-ENOENT if pidns does not exists for the current task.
long bpf_xdp_output(void *ctx, struct bpf_map *map, u64 flags, void *data, u64 size)
Description
Write raw data blob into a special BPF perf event held by map of type
BPF_MAP_TYPE_PERF_EVENT_ARRAY. This perf event must have the following
attributes: PERF_SAMPLE_RAW as sample_type, PERF_TYPE_SOFTWARE as type, and
PERF_COUNT_SW_BPF_OUTPUT as config.
The flags are used to indicate the index in map for which the value must be
put, masked with BPF_F_INDEX_MASK. Alternatively, flags can be set to
BPF_F_CURRENT_CPU to indicate that the index of the current CPU core should
be used.
The value to write, of size, is passed through eBPF stack and pointed by
data.
ctx is a pointer to in-kernel struct xdp_buff.
This helper is similar to bpf_perf_eventoutput() but restricted to
raw_tracepoint bpf programs.
Return 0 on success, or a negative error in case of failure.
u64 bpf_get_netns_cookie(void *ctx)
Description
Retrieve the cookie (generated by the kernel) of the network namespace the
input ctx is associated with. The network namespace cookie remains stable
for its lifetime and provides a global identifier that can be assumed
unique. If ctx is NULL, then the helper returns the cookie for the initial
network namespace. The cookie itself is very similar to that of
bpf_get_socket_cookie() helper, but for network namespaces instead of
sockets.
Return A 8-byte long opaque number.
u64 bpf_get_current_ancestor_cgroup_id(int ancestor_level)
Description
Return id of cgroup v2 that is ancestor of the cgroup associated with the
current task at the ancestor_level. The root cgroup is at ancestor_level
zero and each step down the hierarchy increments the level. If
ancestor_level == level of cgroup associated with the current task, then
return value will be the same as that of bpf_get_current_cgroup_id().
The helper is useful to implement policies based on cgroups that are upper
in hierarchy than immediate cgroup associated with the current task.
The format of returned id and helper limitations are same as in
bpf_get_current_cgroup_id().
Return The id is returned or 0 in case the id could not be retrieved.
long bpf_sk_assign(struct sk_buff *skb, struct bpf_sock *sk, u64 flags)
Description
Helper is overloaded depending on BPF program type. This description applies
to BPF_PROG_TYPE_SCHED_CLS and BPF_PROG_TYPE_SCHED_ACT programs.
Assign the sk to the skb. When combined with appropriate routing
configuration to receive the packet towards the socket, will cause skb to be
delivered to the specified socket. Subsequent redirection of skb via
bpf_redirect(), bpf_clone_redirect() or other methods outside of BPF may
interfere with successful delivery to the socket.
This operation is only valid from TC ingress path.
The flags argument must be zero.
Return 0 on success, or a negative error in case of failure:
-EINVAL if specified flags are not supported.
-ENOENT if the socket is unavailable for assignment.
-ENETUNREACH if the socket is unreachable (wrong netns).
-EOPNOTSUPP if the operation is not supported, for example a call from
outside of TC ingress.
-ESOCKTNOSUPPORT if the socket type is not supported (reuseport).
long bpf_sk_assign(struct bpf_sk_lookup *ctx, struct bpf_sock *sk, u64 flags)
Description
Helper is overloaded depending on BPF program type. This description applies
to BPF_PROG_TYPE_SK_LOOKUP programs.
Select the sk as a result of a socket lookup.
For the operation to succeed passed socket must be compatible with the
packet description provided by the ctx object.
L4 protocol (IPPROTO_TCP or IPPROTO_UDP) must be an exact match. While IP
family (AF_INET or AF_INET6) must be compatible, that is IPv6 sockets that
are not v6-only can be selected for IPv4 packets.
Only TCP listeners and UDP unconnected sockets can be selected. sk can also
be NULL to reset any previous selection.
flags argument can combination of following values:
• BPF_SK_LOOKUP_F_REPLACE to override the previous socket selection,
potentially done by a BPF program that ran before us.
• BPF_SK_LOOKUP_F_NO_REUSEPORT to skip load-balancing within reuseport group
for the socket being selected.
On success ctx->sk will point to the selected socket.
Return 0 on success, or a negative errno in case of failure.
• -EAFNOSUPPORT if socket family (sk->family) is not compatible with packet
family (ctx->family).
• -EEXIST if socket has been already selected, potentially by another
program, and BPF_SK_LOOKUP_F_REPLACE flag was not specified.
• -EINVAL if unsupported flags were specified.
• -EPROTOTYPE if socket L4 protocol (sk->protocol) doesn't match packet
protocol (ctx->protocol).
• -ESOCKTNOSUPPORT if socket is not in allowed state (TCP listening or UDP
unconnected).
u64 bpf_ktime_get_boot_ns(void)
Description
Return the time elapsed since system boot, in nanoseconds. Does include the
time the system was suspended. See: clock_gettime(CLOCK_BOOTTIME)
Return Current ktime.
long bpf_seq_printf(struct seq_file *m, const char *fmt, u32 fmt_size, const void *data,
u32 data_len)
Description
bpf_seq_printf() uses seq_file seq_printf() to print out the format string.
The m represents the seq_file. The fmt and fmt_size are for the format
string itself. The data and data_len are format string arguments. The data
are a u64 array and corresponding format string values are stored in the
array. For strings and pointers where pointees are accessed, only the
pointer values are stored in the data array. The data_len is the size of
data in bytes.
Formats %s, %p{i,I}{4,6} requires to read kernel memory. Reading kernel
memory may fail due to either invalid address or valid address but requiring
a major memory fault. If reading kernel memory fails, the string for %s will
be an empty string, and the ip address for %p{i,I}{4,6} will be 0. Not
returning error to bpf program is consistent with what bpf_trace_printk()
does for now.
Return 0 on success, or a negative error in case of failure:
-EBUSY if per-CPU memory copy buffer is busy, can try again by returning 1
from bpf program.
-EINVAL if arguments are invalid, or if fmt is invalid/unsupported.
-E2BIG if fmt contains too many format specifiers.
-EOVERFLOW if an overflow happened: The same object will be tried again.
long bpf_seq_write(struct seq_file *m, const void *data, u32 len)
Description
bpf_seq_write() uses seq_file seq_write() to write the data. The m
represents the seq_file. The data and len represent the data to write in
bytes.
Return 0 on success, or a negative error in case of failure:
-EOVERFLOW if an overflow happened: The same object will be tried again.
u64 bpf_sk_cgroup_id(struct bpf_sock *sk)
Description
Return the cgroup v2 id of the socket sk.
sk must be a non-NULL pointer to a full socket, e.g. one returned from
bpf_sk_lookup_xxx(), bpf_sk_fullsock(), etc. The format of returned id is
same as in bpf_skb_cgroup_id().
This helper is available only if the kernel was compiled with the
CONFIG_SOCK_CGROUP_DATA configuration option.
Return The id is returned or 0 in case the id could not be retrieved.
u64 bpf_sk_ancestor_cgroup_id(struct bpf_sock *sk, int ancestor_level)
Description
Return id of cgroup v2 that is ancestor of cgroup associated with the sk at
the ancestor_level. The root cgroup is at ancestor_level zero and each step
down the hierarchy increments the level. If ancestor_level == level of
cgroup associated with sk, then return value will be same as that of
bpf_sk_cgroup_id().
The helper is useful to implement policies based on cgroups that are upper
in hierarchy than immediate cgroup associated with sk.
The format of returned id and helper limitations are same as in
bpf_sk_cgroup_id().
Return The id is returned or 0 in case the id could not be retrieved.
long bpf_ringbuf_output(void *ringbuf, void *data, u64 size, u64 flags)
Description
Copy size bytes from data into a ring buffer ringbuf. If BPF_RB_NO_WAKEUP
is specified in flags, no notification of new data availability is sent. If
BPF_RB_FORCE_WAKEUP is specified in flags, notification of new data
availability is sent unconditionally.
Return 0 on success, or a negative error in case of failure.
void *bpf_ringbuf_reserve(void *ringbuf, u64 size, u64 flags)
Description
Reserve size bytes of payload in a ring buffer ringbuf.
Return Valid pointer with size bytes of memory available; NULL, otherwise.
void bpf_ringbuf_submit(void *data, u64 flags)
Description
Submit reserved ring buffer sample, pointed to by data. If BPF_RB_NO_WAKEUP
is specified in flags, no notification of new data availability is sent. If
BPF_RB_FORCE_WAKEUP is specified in flags, notification of new data
availability is sent unconditionally.
Return Nothing. Always succeeds.
void bpf_ringbuf_discard(void *data, u64 flags)
Description
Discard reserved ring buffer sample, pointed to by data. If
BPF_RB_NO_WAKEUP is specified in flags, no notification of new data
availability is sent. If BPF_RB_FORCE_WAKEUP is specified in flags,
notification of new data availability is sent unconditionally.
Return Nothing. Always succeeds.
u64 bpf_ringbuf_query(void *ringbuf, u64 flags)
Description
Query various characteristics of provided ring buffer. What exactly is
queries is determined by flags:
• BPF_RB_AVAIL_DATA: Amount of data not yet consumed.
• BPF_RB_RING_SIZE: The size of ring buffer.
• BPF_RB_CONS_POS: Consumer position (can wrap around).
• BPF_RB_PROD_POS: Producer(s) position (can wrap around).
Data returned is just a momentary snapshot of actual values and could be
inaccurate, so this facility should be used to power heuristics and for
reporting, not to make 100% correct calculation.
Return Requested value, or 0, if flags are not recognized.
long bpf_csum_level(struct sk_buff *skb, u64 level)
Description
Change the skbs checksum level by one layer up or down, or reset it entirely
to none in order to have the stack perform checksum validation. The level is
applicable to the following protocols: TCP, UDP, GRE, SCTP, FCOE. For
example, a decap of | ETH | IP | UDP | GUE | IP | TCP | into | ETH | IP |
TCP | through bpf_skb_adjust_room() helper with passing in
BPF_F_ADJ_ROOM_NO_CSUM_RESET flag would require one call to bpf_csum_level()
with BPF_CSUM_LEVEL_DEC since the UDP header is removed. Similarly, an encap
of the latter into the former could be accompanied by a helper call to
bpf_csum_level() with BPF_CSUM_LEVEL_INC if the skb is still intended to be
processed in higher layers of the stack instead of just egressing at tc.
There are three supported level settings at this time:
• BPF_CSUM_LEVEL_INC: Increases skb->csum_level for skbs with
CHECKSUM_UNNECESSARY.
• BPF_CSUM_LEVEL_DEC: Decreases skb->csum_level for skbs with
CHECKSUM_UNNECESSARY.
• BPF_CSUM_LEVEL_RESET: Resets skb->csum_level to 0 and sets CHECKSUM_NONE
to force checksum validation by the stack.
• BPF_CSUM_LEVEL_QUERY: No-op, returns the current skb->csum_level.
Return 0 on success, or a negative error in case of failure. In the case of
BPF_CSUM_LEVEL_QUERY, the current skb->csum_level is returned or the error
code -EACCES in case the skb is not subject to CHECKSUM_UNNECESSARY.
struct tcp6_sock *bpf_skc_to_tcp6_sock(void *sk)
Description
Dynamically cast a sk pointer to a tcp6_sock pointer.
Return sk if casting is valid, or NULL otherwise.
struct tcp_sock *bpf_skc_to_tcp_sock(void *sk)
Description
Dynamically cast a sk pointer to a tcp_sock pointer.
Return sk if casting is valid, or NULL otherwise.
struct tcp_timewait_sock *bpf_skc_to_tcp_timewait_sock(void *sk)
Description
Dynamically cast a sk pointer to a tcp_timewait_sock pointer.
Return sk if casting is valid, or NULL otherwise.
struct tcp_request_sock *bpf_skc_to_tcp_request_sock(void *sk)
Description
Dynamically cast a sk pointer to a tcp_request_sock pointer.
Return sk if casting is valid, or NULL otherwise.
struct udp6_sock *bpf_skc_to_udp6_sock(void *sk)
Description
Dynamically cast a sk pointer to a udp6_sock pointer.
Return sk if casting is valid, or NULL otherwise.
long bpf_get_task_stack(struct task_struct *task, void *buf, u32 size, u64 flags)
Description
Return a user or a kernel stack in bpf program provided buffer. To achieve
this, the helper needs task, which is a valid pointer to struct task_struct.
To store the stacktrace, the bpf program provides buf with a nonnegative
size.
The last argument, flags, holds the number of stack frames to skip (from 0
to 255), masked with BPF_F_SKIP_FIELD_MASK. The next bits can be used to set
the following flags:
BPF_F_USER_STACK
Collect a user space stack instead of a kernel stack.
BPF_F_USER_BUILD_ID
Collect buildid+offset instead of ips for user stack, only valid if
BPF_F_USER_STACK is also specified.
bpf_get_task_stack() can collect up to PERF_MAX_STACK_DEPTH both kernel and
user frames, subject to sufficient large buffer size. Note that this limit
can be controlled with the sysctl program, and that it should be manually
increased in order to profile long user stacks (such as stacks for Java
programs). To do so, use:
# sysctl kernel.perf_event_max_stack=<new value>
Return A non-negative value equal to or less than size on success, or a negative
error in case of failure.
EXAMPLES
Example usage for most of the eBPF helpers listed in this manual page are available within
the Linux kernel sources, at the following locations:
• samples/bpf/
• tools/testing/selftests/bpf/
LICENSE
eBPF programs can have an associated license, passed along with the bytecode instructions
to the kernel when the programs are loaded. The format for that string is identical to the
one in use for kernel modules (Dual licenses, such as "Dual BSD/GPL", may be used). Some
helper functions are only accessible to programs that are compatible with the GNU Privacy
License (GPL).
In order to use such helpers, the eBPF program must be loaded with the correct license
string passed (via attr) to the bpf() system call, and this generally translates into the
C source code of the program containing a line similar to the following:
char ____license[] __attribute__((section("license"), used)) = "GPL";
IMPLEMENTATION
This manual page is an effort to document the existing eBPF helper functions. But as of
this writing, the BPF sub-system is under heavy development. New eBPF program or map types
are added, along with new helper functions. Some helpers are occasionally made available
for additional program types. So in spite of the efforts of the community, this page might
not be up-to-date. If you want to check by yourself what helper functions exist in your
kernel, or what types of programs they can support, here are some files among the kernel
tree that you may be interested in:
• include/uapi/linux/bpf.h is the main BPF header. It contains the full list of all helper
functions, as well as many other BPF definitions including most of the flags, structs or
constants used by the helpers.
• net/core/filter.c contains the definition of most network-related helper functions, and
the list of program types from which they can be used.
• kernel/trace/bpf_trace.c is the equivalent for most tracing program-related helpers.
• kernel/bpf/verifier.c contains the functions used to check that valid types of eBPF maps
are used with a given helper function.
• kernel/bpf/ directory contains other files in which additional helpers are defined (for
cgroups, sockmaps, etc.).
• The bpftool utility can be used to probe the availability of helper functions on the
system (as well as supported program and map types, and a number of other parameters).
To do so, run bpftool feature probe (see bpftool-feature(8) for details). Add the
unprivileged keyword to list features available to unprivileged users.
Compatibility between helper functions and program types can generally be found in the
files where helper functions are defined. Look for the struct bpf_func_proto objects and
for functions returning them: these functions contain a list of helpers that a given
program type can call. Note that the default: label of the switch ... case used to filter
helpers can call other functions, themselves allowing access to additional helpers. The
requirement for GPL license is also in those struct bpf_func_proto.
Compatibility between helper functions and map types can be found in the
check_map_func_compatibility() function in file kernel/bpf/verifier.c.
Helper functions that invalidate the checks on data and data_end pointers for network
processing are listed in function bpf_helper_changes_pkt_data() in file net/core/filter.c.
SEE ALSO
bpf(2), bpftool(8), cgroups(7), ip(8), perf_event_open(2), sendmsg(2), socket(7),
tc-bpf(8)
COLOPHON
This page is part of release 5.13 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.
BPF-HELPERS(7)