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NAME
bpf - perform a command on an extended BPF map or program
SYNOPSIS
#include <linux/bpf.h>
int bpf(int cmd, union bpf_attr *attr, unsigned int size);
DESCRIPTION
The bpf() system call performs a range of operations related to extended Berkeley Packet
Filters. Extended BPF (or eBPF) is similar to the original ("classic") BPF (cBPF) used to
filter network packets. For both cBPF and eBPF programs, the kernel statically analyzes
the programs before loading them, in order to ensure that they cannot harm the running
system.
eBPF extends cBPF in multiple ways, including the ability to call a fixed set of in-kernel
helper functions (via the BPF_CALL opcode extension provided by eBPF) and access shared
data structures such as eBPF maps.
Extended BPF Design/Architecture
eBPF maps are a generic data structure for storage of different data types. Data types
are generally treated as binary blobs, so a user just specifies the size of the key and
the size of the value at map-creation time. In other words, a key/value for a given map
can have an arbitrary structure.
A user process can create multiple maps (with key/value-pairs being opaque bytes of data)
and access them via file descriptors. Different eBPF programs can access the same maps in
parallel. It's up to the user process and eBPF program to decide what they store inside
maps.
There's one special map type, called a program array. This type of map stores file
descriptors referring to other eBPF programs. When a lookup in the map is performed, the
program flow is redirected in-place to the beginning of another eBPF program and does not
return back to the calling program. The level of nesting has a fixed limit of 32, so that
infinite loops cannot be crafted. At runtime, the program file descriptors stored in the
map can be modified, so program functionality can be altered based on specific
requirements. All programs referred to in a program-array map must have been previously
loaded into the kernel via bpf(). If a map lookup fails, the current program continues
its execution. See BPF_MAP_TYPE_PROG_ARRAY below for further details.
Generally, eBPF programs are loaded by the user process and automatically unloaded when
the process exits. In some cases, for example, tc-bpf(8), the program will continue to
stay alive inside the kernel even after the process that loaded the program exits. In
that case, the tc subsystem holds a reference to the eBPF program after the file
descriptor has been closed by the user-space program. Thus, whether a specific program
continues to live inside the kernel depends on how it is further attached to a given
kernel subsystem after it was loaded via bpf().
Each eBPF program is a set of instructions that is safe to run until its completion. An
in-kernel verifier statically determines that the eBPF program terminates and is safe to
execute. During verification, the kernel increments reference counts for each of the maps
that the eBPF program uses, so that the attached maps can't be removed until the program
is unloaded.
eBPF programs can be attached to different events. These events can be the arrival of
network packets, tracing events, classification events by network queueing disciplines
(for eBPF programs attached to a tc(8) classifier), and other types that may be added in
the future. A new event triggers execution of the eBPF program, which may store
information about the event in eBPF maps. Beyond storing data, eBPF programs may call a
fixed set of in-kernel helper functions.
The same eBPF program can be attached to multiple events and different eBPF programs can
access the same map:
tracing tracing tracing packet packet packet
event A event B event C on eth0 on eth1 on eth2
| | | | | ^
| | | | v |
--> tracing <-- tracing socket tc ingress tc egress
prog_1 prog_2 prog_3 classifier action
| | | | prog_4 prog_5
|--- -----| |------| map_3 | |
map_1 map_2 --| map_4 |--
Arguments
The operation to be performed by the bpf() system call is determined by the cmd argument.
Each operation takes an accompanying argument, provided via attr, which is a pointer to a
union of type bpf_attr (see below). The size argument is the size of the union pointed to
by attr.
The value provided in cmd is one of the following:
BPF_MAP_CREATE
Create a map and return a file descriptor that refers to the map.
BPF_MAP_LOOKUP_ELEM
Look up an element by key in a specified map and return its value.
BPF_MAP_UPDATE_ELEM
Create or update an element (key/value pair) in a specified map.
BPF_MAP_DELETE_ELEM
Look up and delete an element by key in a specified map.
BPF_MAP_GET_NEXT_KEY
Look up an element by key in a specified map and return the key of the next
element.
BPF_PROG_LOAD
Verify and load an eBPF program, returning a new file descriptor associated with
the program.
The bpf_attr union consists of various anonymous structures that are used by different
bpf() commands:
union bpf_attr {
struct { /* Used by BPF_MAP_CREATE */
__u32 map_type;
__u32 key_size; /* size of key in bytes */
__u32 value_size; /* size of value in bytes */
__u32 max_entries; /* maximum number of entries
in a map */
};
struct { /* Used by BPF_MAP_*_ELEM and BPF_MAP_GET_NEXT_KEY
commands */
__u32 map_fd;
__aligned_u64 key;
union {
__aligned_u64 value;
__aligned_u64 next_key;
};
__u64 flags;
};
struct { /* Used by BPF_PROG_LOAD */
__u32 prog_type;
__u32 insn_cnt;
__aligned_u64 insns; /* 'const struct bpf_insn *' */
__aligned_u64 license; /* 'const char *' */
__u32 log_level; /* verbosity level of verifier */
__u32 log_size; /* size of user buffer */
__aligned_u64 log_buf; /* user supplied 'char *'
buffer */
__u32 kern_version;
/* checked when prog_type=kprobe
(since Linux 4.1) */
};
} __attribute__((aligned(8)));
eBPF maps
Maps are a generic data structure for storage of different types of data. They allow
sharing of data between eBPF kernel programs, and also between kernel and user-space
applications.
Each map type has the following attributes:
* type
* maximum number of elements
* key size in bytes
* value size in bytes
The following wrapper functions demonstrate how various bpf() commands can be used to
access the maps. The functions use the cmd argument to invoke different operations.
BPF_MAP_CREATE
The BPF_MAP_CREATE command creates a new map, returning a new file descriptor that
refers to the map.
int
bpf_create_map(enum bpf_map_type map_type,
unsigned int key_size,
unsigned int value_size,
unsigned int max_entries)
{
union bpf_attr attr = {
.map_type = map_type,
.key_size = key_size,
.value_size = value_size,
.max_entries = max_entries
};
return bpf(BPF_MAP_CREATE, &attr, sizeof(attr));
}
The new map has the type specified by map_type, and attributes as specified in
key_size, value_size, and max_entries. On success, this operation returns a file
descriptor. On error, -1 is returned and errno is set to EINVAL, EPERM, or ENOMEM.
The key_size and value_size attributes will be used by the verifier during program
loading to check that the program is calling bpf_map_*_elem() helper functions with
a correctly initialized key and to check that the program doesn't access the map
element value beyond the specified value_size. For example, when a map is created
with a key_size of 8 and the eBPF program calls
bpf_map_lookup_elem(map_fd, fp - 4)
the program will be rejected, since the in-kernel helper function
bpf_map_lookup_elem(map_fd, void *key)
expects to read 8 bytes from the location pointed to by key, but the fp - 4 (where
fp is the top of the stack) starting address will cause out-of-bounds stack access.
Similarly, when a map is created with a value_size of 1 and the eBPF program
contains
value = bpf_map_lookup_elem(...);
*(u32 *) value = 1;
the program will be rejected, since it accesses the value pointer beyond the
specified 1 byte value_size limit.
Currently, the following values are supported for map_type:
enum bpf_map_type {
BPF_MAP_TYPE_UNSPEC, /* Reserve 0 as invalid map type */
BPF_MAP_TYPE_HASH,
BPF_MAP_TYPE_ARRAY,
BPF_MAP_TYPE_PROG_ARRAY,
};
map_type selects one of the available map implementations in the kernel. For all
map types, eBPF programs access maps with the same bpf_map_lookup_elem() and
bpf_map_update_elem() helper functions. Further details of the various map types
are given below.
BPF_MAP_LOOKUP_ELEM
The BPF_MAP_LOOKUP_ELEM command looks up an element with a given key in the map
referred to by the file descriptor fd.
int
bpf_lookup_elem(int fd, const void *key, void *value)
{
union bpf_attr attr = {
.map_fd = fd,
.key = ptr_to_u64(key),
.value = ptr_to_u64(value),
};
return bpf(BPF_MAP_LOOKUP_ELEM, &attr, sizeof(attr));
}
If an element is found, the operation returns zero and stores the element's value
into value, which must point to a buffer of value_size bytes.
If no element is found, the operation returns -1 and sets errno to ENOENT.
BPF_MAP_UPDATE_ELEM
The BPF_MAP_UPDATE_ELEM command creates or updates an element with a given
key/value in the map referred to by the file descriptor fd.
int
bpf_update_elem(int fd, const void *key, const void *value,
uint64_t flags)
{
union bpf_attr attr = {
.map_fd = fd,
.key = ptr_to_u64(key),
.value = ptr_to_u64(value),
.flags = flags,
};
return bpf(BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr));
}
The flags argument should be specified as one of the following:
BPF_ANY
Create a new element or update an existing element.
BPF_NOEXIST
Create a new element only if it did not exist.
BPF_EXIST
Update an existing element.
On success, the operation returns zero. On error, -1 is returned and errno is set
to EINVAL, EPERM, ENOMEM, or E2BIG. E2BIG indicates that the number of elements in
the map reached the max_entries limit specified at map creation time. EEXIST will
be returned if flags specifies BPF_NOEXIST and the element with key already exists
in the map. ENOENT will be returned if flags specifies BPF_EXIST and the element
with key doesn't exist in the map.
BPF_MAP_DELETE_ELEM
The BPF_MAP_DELETE_ELEM command deleted the element whose key is key from the map
referred to by the file descriptor fd.
int
bpf_delete_elem(int fd, const void *key)
{
union bpf_attr attr = {
.map_fd = fd,
.key = ptr_to_u64(key),
};
return bpf(BPF_MAP_DELETE_ELEM, &attr, sizeof(attr));
}
On success, zero is returned. If the element is not found, -1 is returned and
errno is set to ENOENT.
BPF_MAP_GET_NEXT_KEY
The BPF_MAP_GET_NEXT_KEY command looks up an element by key in the map referred to
by the file descriptor fd and sets the next_key pointer to the key of the next
element.
int
bpf_get_next_key(int fd, const void *key, void *next_key)
{
union bpf_attr attr = {
.map_fd = fd,
.key = ptr_to_u64(key),
.next_key = ptr_to_u64(next_key),
};
return bpf(BPF_MAP_GET_NEXT_KEY, &attr, sizeof(attr));
}
If key is found, the operation returns zero and sets the next_key pointer to the
key of the next element. If key is not found, the operation returns zero and sets
the next_key pointer to the key of the first element. If key is the last element,
-1 is returned and errno is set to ENOENT. Other possible errno values are ENOMEM,
EFAULT, EPERM, and EINVAL. This method can be used to iterate over all elements in
the map.
close(map_fd)
Delete the map referred to by the file descriptor map_fd. When the user-space
program that created a map exits, all maps will be deleted automatically (but see
NOTES).
eBPF map types
The following map types are supported:
BPF_MAP_TYPE_HASH
Hash-table maps have the following characteristics:
* Maps are created and destroyed by user-space programs. Both user-space and eBPF
programs can perform lookup, update, and delete operations.
* The kernel takes care of allocating and freeing key/value pairs.
* The map_update_elem() helper with fail to insert new element when the
max_entries limit is reached. (This ensures that eBPF programs cannot exhaust
memory.)
* map_update_elem() replaces existing elements atomically.
Hash-table maps are optimized for speed of lookup.
BPF_MAP_TYPE_ARRAY
Array maps have the following characteristics:
* Optimized for fastest possible lookup. In the future the verifier/JIT compiler
may recognize lookup() operations that employ a constant key and optimize it
into constant pointer. It is possible to optimize a non-constant key into
direct pointer arithmetic as well, since pointers and value_size are constant
for the life of the eBPF program. In other words, array_map_lookup_elem() may
be 'inlined' by the verifier/JIT compiler while preserving concurrent access to
this map from user space.
* All array elements pre-allocated and zero initialized at init time
* The key is an array index, and must be exactly four bytes.
* map_delete_elem() fails with the error EINVAL, since elements cannot be deleted.
* map_update_elem() replaces elements in a nonatomic fashion; for atomic updates,
a hash-table map should be used instead. There is however one special case that
can also be used with arrays: the atomic built-in __sync_fetch_and_add() can be
used on 32 and 64 bit atomic counters. For example, it can be applied on the
whole value itself if it represents a single counter, or in case of a structure
containing multiple counters, it could be used on individual counters. This is
quite often useful for aggregation and accounting of events.
Among the uses for array maps are the following:
* As "global" eBPF variables: an array of 1 element whose key is (index) 0 and
where the value is a collection of 'global' variables which eBPF programs can
use to keep state between events.
* Aggregation of tracing events into a fixed set of buckets.
* Accounting of networking events, for example, number of packets and packet
sizes.
BPF_MAP_TYPE_PROG_ARRAY (since Linux 4.2)
A program array map is a special kind of array map whose map values contain only
file descriptors referring to other eBPF programs. Thus, both the key_size and
value_size must be exactly four bytes. This map is used in conjunction with the
bpf_tail_call() helper.
This means that an eBPF program with a program array map attached to it can call
from kernel side into
void bpf_tail_call(void *context, void *prog_map, unsigned int index);
and therefore replace its own program flow with the one from the program at the
given program array slot, if present. This can be regarded as kind of a jump table
to a different eBPF program. The invoked program will then reuse the same stack.
When a jump into the new program has been performed, it won't return to the old
program anymore.
If no eBPF program is found at the given index of the program array (because the
map slot doesn't contain a valid program file descriptor, the specified lookup
index/key is out of bounds, or the limit of 32 nested calls has been exceed),
execution continues with the current eBPF program. This can be used as a fall-
through for default cases.
A program array map is useful, for example, in tracing or networking, to handle
individual system calls or protocols in their own subprograms and use their
identifiers as an individual map index. This approach may result in performance
benefits, and also makes it possible to overcome the maximum instruction limit of a
single eBPF program. In dynamic environments, a user-space daemon might atomically
replace individual subprograms at run-time with newer versions to alter overall
program behavior, for instance, if global policies change.
eBPF programs
The BPF_PROG_LOAD command is used to load an eBPF program into the kernel. The return
value for this command is a new file descriptor associated with this eBPF program.
char bpf_log_buf[LOG_BUF_SIZE];
int
bpf_prog_load(enum bpf_prog_type type,
const struct bpf_insn *insns, int insn_cnt,
const char *license)
{
union bpf_attr attr = {
.prog_type = type,
.insns = ptr_to_u64(insns),
.insn_cnt = insn_cnt,
.license = ptr_to_u64(license),
.log_buf = ptr_to_u64(bpf_log_buf),
.log_size = LOG_BUF_SIZE,
.log_level = 1,
};
return bpf(BPF_PROG_LOAD, &attr, sizeof(attr));
}
prog_type is one of the available program types:
enum bpf_prog_type {
BPF_PROG_TYPE_UNSPEC, /* Reserve 0 as invalid
program type */
BPF_PROG_TYPE_SOCKET_FILTER,
BPF_PROG_TYPE_KPROBE,
BPF_PROG_TYPE_SCHED_CLS,
BPF_PROG_TYPE_SCHED_ACT,
};
For further details of eBPF program types, see below.
The remaining fields of bpf_attr are set as follows:
* insns is an array of struct bpf_insn instructions.
* insn_cnt is the number of instructions in the program referred to by insns.
* license is a license string, which must be GPL compatible to call helper functions
marked gpl_only. (The licensing rules are the same as for kernel modules, so that also
dual licenses, such as "Dual BSD/GPL", may be used.)
* log_buf is a pointer to a caller-allocated buffer in which the in-kernel verifier can
store the verification log. This log is a multi-line string that can be checked by the
program author in order to understand how the verifier came to the conclusion that the
eBPF program is unsafe. The format of the output can change at any time as the
verifier evolves.
* log_size size of the buffer pointed to by log_bug. If the size of the buffer is not
large enough to store all verifier messages, -1 is returned and errno is set to ENOSPC.
* log_level verbosity level of the verifier. A value of zero means that the verifier
will not provide a log; in this case, log_buf must be a NULL pointer, and log_size must
be zero.
Applying close(2) to the file descriptor returned by BPF_PROG_LOAD will unload the eBPF
program (but see NOTES).
Maps are accessible from eBPF programs and are used to exchange data between eBPF programs
and between eBPF programs and user-space programs. For example, eBPF programs can process
various events (like kprobe, packets) and store their data into a map, and user-space
programs can then fetch data from the map. Conversely, user-space programs can use a map
as a configuration mechanism, populating the map with values checked by the eBPF program,
which then modifies its behavior on the fly according to those values.
eBPF program types
The eBPF program type (prog_type) determines the subset of kernel helper functions that
the program may call. The program type also determines the program input (context)—the
format of struct bpf_context (which is the data blob passed into the eBPF program as the
first argument).
For example, a tracing program does not have the exact same subset of helper functions as
a socket filter program (though they may have some helpers in common). Similarly, the
input (context) for a tracing program is a set of register values, while for a socket
filter it is a network packet.
The set of functions available to eBPF programs of a given type may increase in the
future.
The following program types are supported:
BPF_PROG_TYPE_SOCKET_FILTER (since Linux 3.19)
Currently, the set of functions for BPF_PROG_TYPE_SOCKET_FILTER is:
bpf_map_lookup_elem(map_fd, void *key)
/* look up key in a map_fd */
bpf_map_update_elem(map_fd, void *key, void *value)
/* update key/value */
bpf_map_delete_elem(map_fd, void *key)
/* delete key in a map_fd */
The bpf_context argument is a pointer to a struct __sk_buff.
BPF_PROG_TYPE_KPROBE (since Linux 4.1)
[To be documented]
BPF_PROG_TYPE_SCHED_CLS (since Linux 4.1)
[To be documented]
BPF_PROG_TYPE_SCHED_ACT (since Linux 4.1)
[To be documented]
Events
Once a program is loaded, it can be attached to an event. Various kernel subsystems have
different ways to do so.
Since Linux 3.19, the following call will attach the program prog_fd to the socket sockfd,
which was created by an earlier call to socket(2):
setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_BPF,
&prog_fd, sizeof(prog_fd));
Since Linux 4.1, the following call may be used to attach the eBPF program referred to by
the file descriptor prog_fd to a perf event file descriptor, event_fd, that was created by
a previous call to perf_event_open(2):
ioctl(event_fd, PERF_EVENT_IOC_SET_BPF, prog_fd);
EXAMPLES
/* bpf+sockets example:
* 1. create array map of 256 elements
* 2. load program that counts number of packets received
* r0 = skb->data[ETH_HLEN + offsetof(struct iphdr, protocol)]
* map[r0]++
* 3. attach prog_fd to raw socket via setsockopt()
* 4. print number of received TCP/UDP packets every second
*/
int
main(int argc, char **argv)
{
int sock, map_fd, prog_fd, key;
long long value = 0, tcp_cnt, udp_cnt;
map_fd = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(key),
sizeof(value), 256);
if (map_fd < 0) {
printf("failed to create map '%s'\n", strerror(errno));
/* likely not run as root */
return 1;
}
struct bpf_insn prog[] = {
BPF_MOV64_REG(BPF_REG_6, BPF_REG_1), /* r6 = r1 */
BPF_LD_ABS(BPF_B, ETH_HLEN + offsetof(struct iphdr, protocol)),
/* r0 = ip->proto */
BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_0, -4),
/* *(u32 *)(fp - 4) = r0 */
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), /* r2 = fp */
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -4), /* r2 = r2 - 4 */
BPF_LD_MAP_FD(BPF_REG_1, map_fd), /* r1 = map_fd */
BPF_CALL_FUNC(BPF_FUNC_map_lookup_elem),
/* r0 = map_lookup(r1, r2) */
BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2),
/* if (r0 == 0) goto pc+2 */
BPF_MOV64_IMM(BPF_REG_1, 1), /* r1 = 1 */
BPF_XADD(BPF_DW, BPF_REG_0, BPF_REG_1, 0, 0),
/* lock *(u64 *) r0 += r1 */
BPF_MOV64_IMM(BPF_REG_0, 0), /* r0 = 0 */
BPF_EXIT_INSN(), /* return r0 */
};
prog_fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, prog,
sizeof(prog), "GPL");
sock = open_raw_sock("lo");
assert(setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, &prog_fd,
sizeof(prog_fd)) == 0);
for (;;) {
key = IPPROTO_TCP;
assert(bpf_lookup_elem(map_fd, &key, &tcp_cnt) == 0);
key = IPPROTO_UDP
assert(bpf_lookup_elem(map_fd, &key, &udp_cnt) == 0);
printf("TCP %lld UDP %lld packets0, tcp_cnt, udp_cnt);
sleep(1);
}
return 0;
}
Some complete working code can be found in the samples/bpf directory in the kernel source
tree.
RETURN VALUE
For a successful call, the return value depends on the operation:
BPF_MAP_CREATE
The new file descriptor associated with the eBPF map.
BPF_PROG_LOAD
The new file descriptor associated with the eBPF program.
All other commands
Zero.
On error, -1 is returned, and errno is set appropriately.
ERRORS
EPERM The call was made without sufficient privilege (without the CAP_SYS_ADMIN
capability).
ENOMEM Cannot allocate sufficient memory.
EBADF fd is not an open file descriptor.
EFAULT One of the pointers (key or value or log_buf or insns) is outside the accessible
address space.
EINVAL The value specified in cmd is not recognized by this kernel.
EINVAL For BPF_MAP_CREATE, either map_type or attributes are invalid.
EINVAL For BPF_MAP_*_ELEM commands, some of the fields of union bpf_attr that are not used
by this command are not set to zero.
EINVAL For BPF_PROG_LOAD, indicates an attempt to load an invalid program. eBPF programs
can be deemed invalid due to unrecognized instructions, the use of reserved fields,
jumps out of range, infinite loops or calls of unknown functions.
EACCES For BPF_PROG_LOAD, even though all program instructions are valid, the program has
been rejected because it was deemed unsafe. This may be because it may have
accessed a disallowed memory region or an uninitialized stack/register or because
the function constraints don't match the actual types or because there was a
misaligned memory access. In this case, it is recommended to call bpf() again with
log_level = 1 and examine log_buf for the specific reason provided by the verifier.
ENOENT For BPF_MAP_LOOKUP_ELEM or BPF_MAP_DELETE_ELEM, indicates that the element with the
given key was not found.
E2BIG The eBPF program is too large or a map reached the max_entries limit (maximum
number of elements).
VERSIONS
The bpf() system call first appeared in Linux 3.18.
CONFORMING TO
The bpf() system call is Linux-specific.
NOTES
In the current implementation, all bpf() commands require the caller to have the
CAP_SYS_ADMIN capability.
eBPF objects (maps and programs) can be shared between processes. For example, after
fork(2), the child inherits file descriptors referring to the same eBPF objects. In
addition, file descriptors referring to eBPF objects can be transferred over UNIX domain
sockets. File descriptors referring to eBPF objects can be duplicated in the usual way,
using dup(2) and similar calls. An eBPF object is deallocated only after all file
descriptors referring to the object have been closed.
eBPF programs can be written in a restricted C that is compiled (using the clang compiler)
into eBPF bytecode. Various features are omitted from this restricted C, such as loops,
global variables, variadic functions, floating-point numbers, and passing structures as
function arguments. Some examples can be found in the samples/bpf/*_kern.c files in the
kernel source tree.
The kernel contains a just-in-time (JIT) compiler that translates eBPF bytecode into
native machine code for better performance. The JIT compiler is disabled by default, but
its operation can be controlled by writing one of the following integer strings to the
file /proc/sys/net/core/bpf_jit_enable:
0 Disable JIT compilation (default).
1 Normal compilation.
2 Debugging mode. The generated opcodes are dumped in hexadecimal into the kernel log.
These opcodes can then be disassembled using the program tools/net/bpf_jit_disasm.c
provided in the kernel source tree.
JIT compiler for eBPF is currently available for the x86-64, arm64, and s390
architectures.
SEE ALSO
seccomp(2), socket(7), tc(8), tc-bpf(8)
Both classic and extended BPF are explained in the kernel source file
Documentation/networking/filter.txt.
COLOPHON
This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.