Provided by: iproute2_4.3.0-1ubuntu3.16.04.5_amd64 
NAME
BPF - BPF programmable classifier and actions for ingress/egress queueing disciplines
SYNOPSIS
eBPF classifier (filter) or action:
tc filter ... bpf [ object-file OBJ_FILE ] [ section CLS_NAME ] [ export UDS_FILE ] [
verbose ] [ police POLICE_SPEC ] [ action ACTION_SPEC ] [ classid CLASSID ]
tc action ... bpf [ object-file OBJ_FILE ] [ section CLS_NAME ] [ export UDS_FILE ] [
verbose ]
cBPF classifier (filter) or action:
tc filter ... bpf [ bytecode-file BPF_FILE | bytecode BPF_BYTECODE ] [ police POLICE_SPEC
] [ action ACTION_SPEC ] [ classid CLASSID ]
tc action ... bpf [ bytecode-file BPF_FILE | bytecode BPF_BYTECODE ]
DESCRIPTION
Extended Berkeley Packet Filter ( eBPF ) and classic Berkeley Packet Filter (originally
known as BPF, for better distinction referred to as cBPF here) are both available as a
fully programmable and highly efficient classifier and actions. They both offer a minimal
instruction set for implementing small programs which can safely be loaded into the kernel
and thus executed in a tiny virtual machine from kernel space. An in-kernel verifier
guarantees that a specified program always terminates and neither crashes nor leaks data
from the kernel.
In Linux, it's generally considered that eBPF is the successor of cBPF. The kernel
internally transforms cBPF expressions into eBPF expressions and executes the latter.
Execution of them can be performed in an interpreter or at setup time, they can be just-
in-time compiled (JIT'ed) to run as native machine code. Currently, x86_64, ARM64 and s390
architectures have eBPF JIT support, whereas PPC, SPARC, ARM and MIPS have cBPF, but did
not (yet) switch to eBPF JIT support.
eBPF's instruction set has similar underlying principles as the cBPF instruction set, it
however is modelled closer to the underlying architecture to better mimic native
instruction sets with the aim to achieve a better run-time performance. It is designed to
be JIT'ed with a one to one mapping, which can also open up the possibility for compilers
to generate optimized eBPF code through an eBPF backend that performs almost as fast as
natively compiled code. Given that LLVM provides such an eBPF backend, eBPF programs can
therefore easily be programmed in a subset of the C language. Other than that, eBPF
infrastructure also comes with a construct called "maps". eBPF maps are key/value stores
that are shared between multiple eBPF programs, but also between eBPF programs and user
space applications.
For the traffic control subsystem, classifier and actions that can be attached to ingress
and egress qdiscs can be written in eBPF or cBPF. The advantage over other classifier and
actions is that eBPF/cBPF provides the generic framework, while users can implement their
highly specialized use cases efficiently. This means that the classifier or action written
that way will not suffer from feature bloat, and can therefore execute its task highly
efficient. It allows for non-linear classification and even merging the action part into
the classification. Combined with efficient eBPF map data structures, user space can push
new policies like classids into the kernel without reloading a classifier, or it can
gather statistics that are pushed into one map and use another one for dynamically load
balancing traffic based on the determined load, just to provide a few examples.
PARAMETERS
object-file
points to an object file that has an executable and linkable format (ELF) and contains
eBPF opcodes and eBPF map definitions. The LLVM compiler infrastructure with clang(1) as a
C language front end is one project that supports emitting eBPF object files that can be
passed to the eBPF classifier (more details in the EXAMPLES section). This option is
mandatory when an eBPF classifier or action is to be loaded.
section
is the name of the ELF section from the object file, where the eBPF classifier or action
resides. By default the section name for the classifier is called "classifier", and for
the action "action". Given that a single object file can contain multiple classifier and
actions, the corresponding section name needs to be specified, if it differs from the
defaults.
export
points to a Unix domain socket file. In case the eBPF object file also contains a section
named "maps" with eBPF map specifications, then the map file descriptors can be handed off
via the Unix domain socket to an eBPF "agent" herding all descriptors after tc lifetime.
This can be some third party application implementing the IPC counterpart for the import,
that uses them for calling into bpf(2) system call to read out or update eBPF map data
from user space, for example, for monitoring purposes or to push down new policies.
verbose
if set, it will dump the eBPF verifier output, even if loading the eBPF program was
successful. By default, only on error, the verifier log is being emitted to the user.
police
is an optional parameter for an eBPF/cBPF classifier that specifies a police in tc(1)
which is attached to the classifier, for example, on an ingress qdisc.
action
is an optional parameter for an eBPF/cBPF classifier that specifies a subsequent action in
tc(1) which is attached to a classifier.
classid
flowid
provides the default traffic control class identifier for this eBPF/cBPF classifier. The
default class identifier can also be overwritten by the return code of the eBPF/cBPF
program. A default return code of -1 specifies the here provided default class identifier
to be used. A return code of the eBPF/cBPF program of 0 implies that no match took place,
and a return code other than these two will override the default classid. This allows for
efficient, non-linear classification with only a single eBPF/cBPF program as opposed to
having multiple individual programs for various class identifiers which would need to
reparse packet contents.
bytecode
is being used for loading cBPF classifier and actions only. The cBPF bytecode is directly
passed as a text string in the form of ´s,c t f k,c t f k,c t f k,...´ , where s denotes
the number of subsequent 4-tuples. One such 4-tuple consists of c t f k decimals, where c
represents the cBPF opcode, t the jump true offset target, f the jump false offset target
and k the immediate constant/literal. There are various tools that generate code in this
loadable format, for example, bpf_asm that ships with the Linux kernel source tree under
tools/net/ , so it is certainly not expected to hack this by hand. The bytecode or
bytecode-file option is mandatory when a cBPF classifier or action is to be loaded.
bytecode-file
also being used to load a cBPF classifier or action. It's effectively the same as bytecode
only that the cBPF bytecode is not passed directly via command line, but rather resides in
a text file.
EXAMPLES
eBPF TOOLING
A full blown example including eBPF agent code can be found inside the iproute2 source
package under: examples/bpf/
As prerequisites, the kernel needs to have the eBPF system call namely bpf(2) enabled and
ships with cls_bpf and act_bpf kernel modules for the traffic control subsystem. To enable
eBPF/eBPF JIT support, depending which of the two the given architecture supports:
echo 1 > /proc/sys/net/core/bpf_jit_enable
A given restricted C file can be compiled via LLVM as:
clang -O2 -emit-llvm -c bpf.c -o - | llc -march=bpf -filetype=obj -o bpf.o
The compiler invocation might still simplify in future, so for now, it's quite handy to
alias this construct in one way or another, for example:
__bcc() {
clang -O2 -emit-llvm -c $1 -o - | \
llc -march=bpf -filetype=obj -o "`basename $1 .c`.o"
}
alias bcc=__bcc
A minimal, stand-alone unit, which matches on all traffic with the default classid (return
code of -1) looks like:
#include <linux/bpf.h>
#ifndef __section
# define __section(x) __attribute__((section(x), used))
#endif
__section("classifier") int cls_main(struct __sk_buff *skb)
{
return -1;
}
char __license[] __section("license") = "GPL";
More examples can be found further below in subsection eBPF PROGRAMMING as focus here will
be on tooling.
There can be various other sections, for example, also for actions. Thus, an object file
in eBPF can contain multiple entrance points. Always a specific entrance point, however,
must be specified when configuring with tc. A license must be part of the restricted C
code and the license string syntax is the same as with Linux kernel modules. The kernel
reserves its right that some eBPF helper functions can be restricted to GPL compatible
licenses only, and thus may reject a program from loading into the kernel when such a
license mismatch occurs.
The resulting object file from the compilation can be inspected with the usual set of
tools that also operate on normal object files, for example objdump(1) for inspecting ELF
section headers:
objdump -h bpf.o
[...]
3 classifier 000007f8 0000000000000000 0000000000000000 00000040 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
4 action-mark 00000088 0000000000000000 0000000000000000 00000838 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
5 action-rand 00000098 0000000000000000 0000000000000000 000008c0 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
6 maps 00000030 0000000000000000 0000000000000000 00000958 2**2
CONTENTS, ALLOC, LOAD, DATA
7 license 00000004 0000000000000000 0000000000000000 00000988 2**0
CONTENTS, ALLOC, LOAD, DATA
[...]
Adding an eBPF classifier from an object file that contains a classifier in the default
ELF section is trivial (note that instead of "object-file" also shortcuts such as "obj"
can be used):
bcc bpf.c
tc filter add dev em1 parent 1: bpf obj bpf.o flowid 1:1
In case the classifier resides in ELF section "mycls", then that same command needs to be
invoked as:
tc filter add dev em1 parent 1: bpf obj bpf.o sec mycls flowid 1:1
Dumping the classifier configuration will tell the location of the classifier, in other
words that it's from object file "bpf.o" under section "mycls":
tc filter show dev em1
filter parent 1: protocol all pref 49152 bpf
filter parent 1: protocol all pref 49152 bpf handle 0x1 flowid 1:1 bpf.o:[mycls]
The same program can also be installed on ingress qdisc side as opposed to egress ...
tc qdisc add dev em1 handle ffff: ingress
tc filter add dev em1 parent ffff: bpf obj bpf.o sec mycls flowid ffff:1
... and again dumped from there:
tc filter show dev em1 parent ffff:
filter protocol all pref 49152 bpf
filter protocol all pref 49152 bpf handle 0x1 flowid ffff:1 bpf.o:[mycls]
Attaching a classifier and action on ingress has the restriction that it doesn't have an
actual underlying queueing discipline. What ingress can do is to classify, mangle,
redirect or drop packets. When queueing is required on ingress side, then ingress must
redirect packets to the ifb device, otherwise policing can be used. Moreover, ingress can
be used to have an early drop point of unwanted packets before they hit upper layers of
the networking stack, perform network accounting with eBPF maps that could be shared with
egress, or have an early mangle and/or redirection point to different networking devices.
Multiple eBPF actions and classifier can be placed into a single object file within
various sections. In that case, non-default section names must be provided, which is the
case for both actions in this example:
tc filter add dev em1 parent 1: bpf obj bpf.o flowid 1:1 \
action bpf obj bpf.o sec action-mark \
action bpf obj bpf.o sec action-rand ok
The advantage of this is that the classifier and the two actions can then share eBPF maps
with each other, if implemented in the programs.
In order to access eBPF maps from user space beyond tc(8) setup lifetime, the ownership
can be transferred to an eBPF agent via Unix domain sockets. There are two possibilities
for implementing this:
1) implementation of an own eBPF agent that takes care of setting up the Unix domain
socket and implementing the protocol that tc(8) dictates. A code example of this can be
found inside the iproute2 source package under: examples/bpf/
2) use tc exec for transferring the eBPF map file descriptors through a Unix domain
socket, and spawning an application such as sh(1) . This approach's advantage is that tc
will place the file descriptors into the environment and thus make them available just
like stdin, stdout, stderr file descriptors, meaning, in case user applications run from
within this fd-owner shell, they can terminate and restart without loosing eBPF maps file
descriptors. Example invocation with the previous classifier and action mixture:
tc exec bpf imp /tmp/bpf
tc filter add dev em1 parent 1: bpf obj bpf.o exp /tmp/bpf flowid 1:1 \
action bpf obj bpf.o sec action-mark \
action bpf obj bpf.o sec action-rand ok
Assuming that eBPF maps are shared with classifier and actions, it's enough to export them
once, for example, from within the classifier or action command. tc will setup all eBPF
map file descriptors at the time when the object file is first parsed.
When a shell has been spawned, the environment will have a couple of eBPF related
variables. BPF_NUM_MAPS provides the total number of maps that have been transferred over
the Unix domain socket. BPF_MAP<X>'s value is the file descriptor number that can be
accessed in eBPF agent applications, in other words, it can directly be used as the file
descriptor value for the bpf(2) system call to retrieve or alter eBPF map values. <X>
denotes the identifier of the eBPF map. It corresponds to the id member of struct
bpf_elf_map from the tc eBPF map specification.
The environment in this example looks as follows:
sh# env | grep BPF
BPF_NUM_MAPS=3
BPF_MAP1=6
BPF_MAP0=5
BPF_MAP2=7
sh# ls -la /proc/self/fd
[...]
lrwx------. 1 root root 64 Apr 14 16:46 5 -> anon_inode:bpf-map
lrwx------. 1 root root 64 Apr 14 16:46 6 -> anon_inode:bpf-map
lrwx------. 1 root root 64 Apr 14 16:46 7 -> anon_inode:bpf-map
sh# my_bpf_agent
eBPF agents are very useful in that they can prepopulate eBPF maps from user space,
monitor statistics via maps and based on that feedback, for example, rewrite classids in
eBPF map values during runtime. Given that eBPF agents are implemented as normal
applications, they can also dynamically receive traffic control policies from external
controllers and thus push them down into eBPF maps to dynamically adapt to network
conditions. Moreover, eBPF maps can also be shared with other eBPF program types (e.g.
tracing), thus very powerful combination can therefore be implemented.
eBPF PROGRAMMING
eBPF classifier and actions are being implemented in restricted C syntax (in future, there
could additionally be new language frontends supported).
The header file linux/bpf.h provides eBPF helper functions that can be called from an eBPF
program. This man page will only provide two minimal, stand-alone examples, have a look
at examples/bpf from the iproute2 source package for a fully fledged flow dissector
example to better demonstrate some of the possibilities with eBPF.
Supported 32 bit classifier return codes from the C program and their meanings:
0 , denotes a mismatch
-1 , denotes the default classid configured from the command line
else , everything else will override the default classid to provide a facility for
non-linear matching
Supported 32 bit action return codes from the C program and their meanings (
linux/pkt_cls.h ):
TC_ACT_OK (0) , will terminate the packet processing pipeline and allows the packet to
proceed
TC_ACT_SHOT (2) , will terminate the packet processing pipeline and drops the packet
TC_ACT_UNSPEC (-1) , will use the default action configured from tc (similarly as
returning -1 from a classifier)
TC_ACT_PIPE (3) , will iterate to the next action, if available
TC_ACT_RECLASSIFY (1) , will terminate the packet processing pipeline and start
classification from the beginning
else , everything else is an unspecified return code
Both classifier and action return codes are supported in eBPF and cBPF programs.
To demonstrate restricted C syntax, a minimal toy classifier example is provided, which
assumes that egress packets, for instance originating from a container, have previously
been marked in interval [0, 255]. The program keeps statistics on different marks for user
space and maps the classid to the root qdisc with the marking itself as the minor handle:
#include <stdint.h>
#include <asm/types.h>
#include <linux/bpf.h>
#include <linux/pkt_sched.h>
#include "helpers.h"
struct tuple {
long packets;
long bytes;
};
#define BPF_MAP_ID_STATS 1 /* agent's map identifier */
#define BPF_MAX_MARK 256
struct bpf_elf_map __section("maps") map_stats = {
.type = BPF_MAP_TYPE_ARRAY,
.id = BPF_MAP_ID_STATS,
.size_key = sizeof(uint32_t),
.size_value = sizeof(struct tuple),
.max_elem = BPF_MAX_MARK,
};
static inline void cls_update_stats(const struct __sk_buff *skb,
uint32_t mark)
{
struct tuple *tu;
tu = bpf_map_lookup_elem(&map_stats, &mark);
if (likely(tu)) {
__sync_fetch_and_add(&tu->packets, 1);
__sync_fetch_and_add(&tu->bytes, skb->len);
}
}
__section("cls") int cls_main(struct __sk_buff *skb)
{
uint32_t mark = skb->mark;
if (unlikely(mark >= BPF_MAX_MARK))
return 0;
cls_update_stats(skb, mark);
return TC_H_MAKE(TC_H_ROOT, mark);
}
char __license[] __section("license") = "GPL";
Another small example is a port redirector which demuxes destination port 80 into the
interval [8080, 8087] steered by RSS, that can then be attached to ingress qdisc. The
exercise of adding the egress counterpart and IPv6 support is left to the reader:
#include <asm/types.h>
#include <asm/byteorder.h>
#include <linux/bpf.h>
#include <linux/filter.h>
#include <linux/in.h>
#include <linux/if_ether.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include "helpers.h"
static inline void set_tcp_dport(struct __sk_buff *skb, int nh_off,
__u16 old_port, __u16 new_port)
{
bpf_l4_csum_replace(skb, nh_off + offsetof(struct tcphdr, check),
old_port, new_port, sizeof(new_port));
bpf_skb_store_bytes(skb, nh_off + offsetof(struct tcphdr, dest),
&new_port, sizeof(new_port), 0);
}
static inline int lb_do_ipv4(struct __sk_buff *skb, int nh_off)
{
__u16 dport, dport_new = 8080, off;
__u8 ip_proto, ip_vl;
ip_proto = load_byte(skb, nh_off +
offsetof(struct iphdr, protocol));
if (ip_proto != IPPROTO_TCP)
return 0;
ip_vl = load_byte(skb, nh_off);
if (likely(ip_vl == 0x45))
nh_off += sizeof(struct iphdr);
else
nh_off += (ip_vl & 0xF) << 2;
dport = load_half(skb, nh_off + offsetof(struct tcphdr, dest));
if (dport != 80)
return 0;
off = skb->queue_mapping & 7;
set_tcp_dport(skb, nh_off - BPF_LL_OFF, __constant_htons(80),
__cpu_to_be16(dport_new + off));
return -1;
}
__section("lb") int lb_main(struct __sk_buff *skb)
{
int ret = 0, nh_off = BPF_LL_OFF + ETH_HLEN;
if (likely(skb->protocol == __constant_htons(ETH_P_IP)))
ret = lb_do_ipv4(skb, nh_off);
return ret;
}
char __license[] __section("license") = "GPL";
The related helper header file helpers.h in both examples was:
/* Misc helper macros. */
#define __section(x) __attribute__((section(x), used))
#define offsetof(x, y) __builtin_offsetof(x, y)
#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)
/* Used map structure */
struct bpf_elf_map {
__u32 type;
__u32 size_key;
__u32 size_value;
__u32 max_elem;
__u32 id;
};
/* Some used BPF function calls. */
static int (*bpf_skb_store_bytes)(void *ctx, int off, void *from,
int len, int flags) =
(void *) BPF_FUNC_skb_store_bytes;
static int (*bpf_l4_csum_replace)(void *ctx, int off, int from,
int to, int flags) =
(void *) BPF_FUNC_l4_csum_replace;
static void *(*bpf_map_lookup_elem)(void *map, void *key) =
(void *) BPF_FUNC_map_lookup_elem;
/* Some used BPF intrinsics. */
unsigned long long load_byte(void *skb, unsigned long long off)
asm ("llvm.bpf.load.byte");
unsigned long long load_half(void *skb, unsigned long long off)
asm ("llvm.bpf.load.half");
Best practice, we recommend to only have a single eBPF classifier loaded in tc and perform
all necessary matching and mangling from there instead of a list of individual classifier
and separate actions. Just a single classifier tailored for a given use-case will be most
efficient to run.
eBPF DEBUGGING
Both tc filter and action commands for bpf support an optional verbose parameter that can
be used to inspect the eBPF verifier log. It is dumped by default in case of an error.
In case the eBPF/cBPF JIT compiler has been enabled, it can also be instructed to emit a
debug output of the resulting opcode image into the kernel log, which can be read via
dmesg(1) :
echo 2 > /proc/sys/net/core/bpf_jit_enable
The Linux kernel source tree ships additionally under tools/net/ a small helper called
bpf_jit_disasm that reads out the opcode image dump from the kernel log and dumps the
resulting disassembly:
bpf_jit_disasm -o
Other than that, the Linux kernel also contains an extensive eBPF/cBPF test suite module
called test_bpf . Upon ...
modprobe test_bpf
... it performs a diversity of test cases and dumps the results into the kernel log that
can be inspected with dmesg(1) . The results can differ depending on whether the JIT
compiler is enabled or not. In case of failed test cases, the module will fail to load. In
such cases, we urge you to file a bug report to the related JIT authors, Linux kernel and
networking mailing lists.
cBPF
Although we generally recommend switching to implementing eBPF classifier and actions, for
the sake of completeness, a few words on how to program in cBPF will be lost here.
Likewise, the bpf_jit_enable switch can be enabled as mentioned already. Tooling such as
bpf_jit_disasm is also independent whether eBPF or cBPF code is being loaded.
Unlike in eBPF, classifier and action are not implemented in restricted C, but rather in a
minimal assembler-like language or with the help of other tooling.
The raw interface with tc takes opcodes directly. For example, the most minimal classifier
matching on every packet resulting in the default classid of 1:1 looks like:
tc filter add dev em1 parent 1: bpf bytecode '1,6 0 0 4294967295,' flowid 1:1
The first decimal of the bytecode sequence denotes the number of subsequent 4-tuples of
cBPF opcodes. As mentioned, such a 4-tuple consists of c t f k decimals, where c
represents the cBPF opcode, t the jump true offset target, f the jump false offset target
and k the immediate constant/literal. Here, this denotes an unconditional return from the
program with immediate value of -1.
Thus, for egress classification, Willem de Bruijn implemented a minimal stand-alone helper
tool under the GNU General Public License version 2 for iptables(8) BPF extension, which
abuses the libpcap internal classic BPF compiler, his code derived here for usage with
tc(8) :
#include <pcap.h>
#include <stdio.h>
int main(int argc, char **argv)
{
struct bpf_program prog;
struct bpf_insn *ins;
int i, ret, dlt = DLT_RAW;
if (argc < 2 || argc > 3)
return 1;
if (argc == 3) {
dlt = pcap_datalink_name_to_val(argv[1]);
if (dlt == -1)
return 1;
}
ret = pcap_compile_nopcap(-1, dlt, &prog, argv[argc - 1],
1, PCAP_NETMASK_UNKNOWN);
if (ret)
return 1;
printf("%d,", prog.bf_len);
ins = prog.bf_insns;
for (i = 0; i < prog.bf_len - 1; ++ins, ++i)
printf("%u %u %u %u,", ins->code,
ins->jt, ins->jf, ins->k);
printf("%u %u %u %u",
ins->code, ins->jt, ins->jf, ins->k);
pcap_freecode(&prog);
return 0;
}
Given this small helper, any tcpdump(8) filter expression can be abused as a classifier
where a match will result in the default classid:
bpftool EN10MB 'tcp[tcpflags] & tcp-syn != 0' > /var/bpf/tcp-syn
tc filter add dev em1 parent 1: bpf bytecode-file /var/bpf/tcp-syn flowid 1:1
Basically, such a minimal generator is equivalent to:
tcpdump -iem1 -ddd 'tcp[tcpflags] & tcp-syn != 0' | tr '0 ',' > /var/bpf/tcp-syn
Since libpcap does not support all Linux' specific cBPF extensions in its compiler, the
Linux kernel also ships under tools/net/ a minimal BPF assembler called bpf_asm for
providing full control. For detailed syntax and semantics on implementing such programs by
hand, see references under FURTHER READING .
Trivial toy example in bpf_asm for classifying IPv4/TCP packets, saved in a text file
called foobar :
ldh [12]
jne #0x800, drop
ldb [23]
jneq #6, drop
ret #-1
drop: ret #0
Similarly, such a classifier can be loaded as:
bpf_asm foobar > /var/bpf/tcp-syn
tc filter add dev em1 parent 1: bpf bytecode-file /var/bpf/tcp-syn flowid 1:1
For BPF classifiers, the Linux kernel provides additionally under tools/net/ a small BPF
debugger called bpf_dbg , which can be used to test a classifier against pcap files,
single-step or add various breakpoints into the classifier program and dump register
contents during runtime.
Implementing an action in classic BPF is rather limited in the sense that packet mangling
is not supported. Therefore, it's generally recommended to make the switch to eBPF,
whenever possible.
FURTHER READING
Further and more technical details about the BPF architecture can be found in the Linux
kernel source tree under Documentation/networking/filter.txt .
Further details on eBPF tc(8) examples can be found in the iproute2 source tree under
examples/bpf/ .
SEE ALSO
tc(8), tc-ematch(8) bpf(2) bpf(4)
AUTHORS
Manpage written by Daniel Borkmann.
Please report corrections or improvements to the Linux kernel networking mailing list:
<netdev@vger.kernel.org>