.. SPDX-License-Identifier: GPL-2.0

.. _networking-filter:

=======================================================
Linux Socket Filtering aka Berkeley Packet Filter (BPF)
=======================================================

Notice
------

This file used to document the eBPF format and mechanisms even when not
related to socket filtering.  The ../bpf/index.rst has more details
on eBPF.

Introduction
------------

Linux Socket Filtering (LSF) is derived from the Berkeley Packet Filter.
Though there are some distinct differences between the BSD and Linux
Kernel filtering, but when we speak of BPF or LSF in Linux context, we
mean the very same mechanism of filtering in the Linux kernel.

BPF allows a user-space program to attach a filter onto any socket and
allow or disallow certain types of data to come through the socket. LSF
follows exactly the same filter code structure as BSD's BPF, so referring
to the BSD bpf.4 manpage is very helpful in creating filters.

On Linux, BPF is much simpler than on BSD. One does not have to worry
about devices or anything like that. You simply create your filter code,
send it to the kernel via the SO_ATTACH_FILTER option and if your filter
code passes the kernel check on it, you then immediately begin filtering
data on that socket.

You can also detach filters from your socket via the SO_DETACH_FILTER
option. This will probably not be used much since when you close a socket
that has a filter on it the filter is automagically removed. The other
less common case may be adding a different filter on the same socket where
you had another filter that is still running: the kernel takes care of
removing the old one and placing your new one in its place, assuming your
filter has passed the checks, otherwise if it fails the old filter will
remain on that socket.

SO_LOCK_FILTER option allows to lock the filter attached to a socket. Once
set, a filter cannot be removed or changed. This allows one process to
setup a socket, attach a filter, lock it then drop privileges and be
assured that the filter will be kept until the socket is closed.

The biggest user of this construct might be libpcap. Issuing a high-level
filter command like `tcpdump -i em1 port 22` passes through the libpcap
internal compiler that generates a structure that can eventually be loaded
via SO_ATTACH_FILTER to the kernel. `tcpdump -i em1 port 22 -ddd`
displays what is being placed into this structure.

Although we were only speaking about sockets here, BPF in Linux is used
in many more places. There's xt_bpf for netfilter, cls_bpf in the kernel
qdisc layer, SECCOMP-BPF (SECure COMPuting [1]_), and lots of other places
such as team driver, PTP code, etc where BPF is being used.

.. [1] Documentation/userspace-api/seccomp_filter.rst

Original BPF paper:

Steven McCanne and Van Jacobson. 1993. The BSD packet filter: a new
architecture for user-level packet capture. In Proceedings of the
USENIX Winter 1993 Conference Proceedings on USENIX Winter 1993
Conference Proceedings (USENIX'93). USENIX Association, Berkeley,
CA, USA, 2-2. [http://www.tcpdump.org/papers/bpf-usenix93.pdf]

Structure
---------

User space applications include <linux/filter.h> which contains the
following relevant structures::

	struct sock_filter {	/* Filter block */
		__u16	code;   /* Actual filter code */
		__u8	jt;	/* Jump true */
		__u8	jf;	/* Jump false */
		__u32	k;      /* Generic multiuse field */
	};

Such a structure is assembled as an array of 4-tuples, that contains
a code, jt, jf and k value. jt and jf are jump offsets and k a generic
value to be used for a provided code::

	struct sock_fprog {			/* Required for SO_ATTACH_FILTER. */
		unsigned short		   len;	/* Number of filter blocks */
		struct sock_filter __user *filter;
	};

For socket filtering, a pointer to this structure (as shown in
follow-up example) is being passed to the kernel through setsockopt(2).

Example
-------

::

    #include <sys/socket.h>
    #include <sys/types.h>
    #include <arpa/inet.h>
    #include <linux/if_ether.h>
    /* ... */

    /* From the example above: tcpdump -i em1 port 22 -dd */
    struct sock_filter code[] = {
	    { 0x28,  0,  0, 0x0000000c },
	    { 0x15,  0,  8, 0x000086dd },
	    { 0x30,  0,  0, 0x00000014 },
	    { 0x15,  2,  0, 0x00000084 },
	    { 0x15,  1,  0, 0x00000006 },
	    { 0x15,  0, 17, 0x00000011 },
	    { 0x28,  0,  0, 0x00000036 },
	    { 0x15, 14,  0, 0x00000016 },
	    { 0x28,  0,  0, 0x00000038 },
	    { 0x15, 12, 13, 0x00000016 },
	    { 0x15,  0, 12, 0x00000800 },
	    { 0x30,  0,  0, 0x00000017 },
	    { 0x15,  2,  0, 0x00000084 },
	    { 0x15,  1,  0, 0x00000006 },
	    { 0x15,  0,  8, 0x00000011 },
	    { 0x28,  0,  0, 0x00000014 },
	    { 0x45,  6,  0, 0x00001fff },
	    { 0xb1,  0,  0, 0x0000000e },
	    { 0x48,  0,  0, 0x0000000e },
	    { 0x15,  2,  0, 0x00000016 },
	    { 0x48,  0,  0, 0x00000010 },
	    { 0x15,  0,  1, 0x00000016 },
	    { 0x06,  0,  0, 0x0000ffff },
	    { 0x06,  0,  0, 0x00000000 },
    };

    struct sock_fprog bpf = {
	    .len = ARRAY_SIZE(code),
	    .filter = code,
    };

    sock = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
    if (sock < 0)
	    /* ... bail out ... */

    ret = setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &bpf, sizeof(bpf));
    if (ret < 0)
	    /* ... bail out ... */

    /* ... */
    close(sock);

The above example code attaches a socket filter for a PF_PACKET socket
in order to let all IPv4/IPv6 packets with port 22 pass. The rest will
be dropped for this socket.

The setsockopt(2) call to SO_DETACH_FILTER doesn't need any arguments
and SO_LOCK_FILTER for preventing the filter to be detached, takes an
integer value with 0 or 1.

Note that socket filters are not restricted to PF_PACKET sockets only,
but can also be used on other socket families.

Summary of system calls:

 * setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &val, sizeof(val));
 * setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &val, sizeof(val));
 * setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER,   &val, sizeof(val));

Normally, most use cases for socket filtering on packet sockets will be
covered by libpcap in high-level syntax, so as an application developer
you should stick to that. libpcap wraps its own layer around all that.

Unless i) using/linking to libpcap is not an option, ii) the required BPF
filters use Linux extensions that are not supported by libpcap's compiler,
iii) a filter might be more complex and not cleanly implementable with
libpcap's compiler, or iv) particular filter codes should be optimized
differently than libpcap's internal compiler does; then in such cases
writing such a filter "by hand" can be of an alternative. For example,
xt_bpf and cls_bpf users might have requirements that could result in
more complex filter code, or one that cannot be expressed with libpcap
(e.g. different return codes for various code paths). Moreover, BPF JIT
implementors may wish to manually write test cases and thus need low-level
access to BPF code as well.

BPF engine and instruction set
------------------------------

Under tools/bpf/ there's a small helper tool called bpf_asm which can
be used to write low-level filters for example scenarios mentioned in the
previous section. Asm-like syntax mentioned here has been implemented in
bpf_asm and will be used for further explanations (instead of dealing with
less readable opcodes directly, principles are the same). The syntax is
closely modelled after Steven McCanne's and Van Jacobson's BPF paper.

The BPF architecture consists of the following basic elements:

  =======          ====================================================
  Element          Description
  =======          ====================================================
  A                32 bit wide accumulator
  X                32 bit wide X register
  M[]              16 x 32 bit wide misc registers aka "scratch memory
		   store", addressable from 0 to 15
  =======          ====================================================

A program, that is translated by bpf_asm into "opcodes" is an array that
consists of the following elements (as already mentioned)::

  op:16, jt:8, jf:8, k:32

The element op is a 16 bit wide opcode that has a particular instruction
encoded. jt and jf are two 8 bit wide jump targets, one for condition
"jump if true", the other one "jump if false". Eventually, element k
contains a miscellaneous argument that can be interpreted in different
ways depending on the given instruction in op.

The instruction set consists of load, store, branch, alu, miscellaneous
and return instructions that are also represented in bpf_asm syntax. This
table lists all bpf_asm instructions available resp. what their underlying
opcodes as defined in linux/filter.h stand for:

  ===========      ===================  =====================
  Instruction      Addressing mode      Description
  ===========      ===================  =====================
  ld               1, 2, 3, 4, 12       Load word into A
  ldi              4                    Load word into A
  ldh              1, 2                 Load half-word into A
  ldb              1, 2                 Load byte into A
  ldx              3, 4, 5, 12          Load word into X
  ldxi             4                    Load word into X
  ldxb             5                    Load byte into X

  st               3                    Store A into M[]
  stx              3                    Store X into M[]

  jmp              6                    Jump to label
  ja               6                    Jump to label
  jeq              7, 8, 9, 10          Jump on A == <x>
  jneq             9, 10                Jump on A != <x>
  jne              9, 10                Jump on A != <x>
  jlt              9, 10                Jump on A <  <x>
  jle              9, 10                Jump on A <= <x>
  jgt              7, 8, 9, 10          Jump on A >  <x>
  jge              7, 8, 9, 10          Jump on A >= <x>
  jset             7, 8, 9, 10          Jump on A &  <x>

  add              0, 4                 A + <x>
  sub              0, 4                 A - <x>
  mul              0, 4                 A * <x>
  div              0, 4                 A / <x>
  mod              0, 4                 A % <x>
  neg                                   !A
  and              0, 4                 A & <x>
  or               0, 4                 A | <x>
  xor              0, 4                 A ^ <x>
  lsh              0, 4                 A << <x>
  rsh              0, 4                 A >> <x>

  tax                                   Copy A into X
  txa                                   Copy X into A

  ret              4, 11                Return
  ===========      ===================  =====================

The next table shows addressing formats from the 2nd column:

  ===============  ===================  ===============================================
  Addressing mode  Syntax               Description
  ===============  ===================  ===============================================
   0               x/%x                 Register X
   1               [k]                  BHW at byte offset k in the packet
   2               [x + k]              BHW at the offset X + k in the packet
   3               M[k]                 Word at offset k in M[]
   4               #k                   Literal value stored in k
   5               4*([k]&0xf)          Lower nibble * 4 at byte offset k in the packet
   6               L                    Jump label L
   7               #k,Lt,Lf             Jump to Lt if true, otherwise jump to Lf
   8               x/%x,Lt,Lf           Jump to Lt if true, otherwise jump to Lf
   9               #k,Lt                Jump to Lt if predicate is true
  10               x/%x,Lt              Jump to Lt if predicate is true
  11               a/%a                 Accumulator A
  12               extension            BPF extension
  ===============  ===================  ===============================================

The Linux kernel also has a couple of BPF extensions that are used along
with the class of load instructions by "overloading" the k argument with
a negative offset + a particular extension offset. The result of such BPF
extensions are loaded into A.

Possible BPF extensions are shown in the following table:

  ===================================   =================================================
  Extension                             Description
  ===================================   =================================================
  len                                   skb->len
  proto                                 skb->protocol
  type                                  skb->pkt_type
  poff                                  Payload start offset
  ifidx                                 skb->dev->ifindex
  nla                                   Netlink attribute of type X with offset A
  nlan                                  Nested Netlink attribute of type X with offset A
  mark                                  skb->mark
  queue                                 skb->queue_mapping
  hatype                                skb->dev->type
  rxhash                                skb->hash
  cpu                                   raw_smp_processor_id()
  vlan_tci                              skb_vlan_tag_get(skb)
  vlan_avail                            skb_vlan_tag_present(skb)
  vlan_tpid                             skb->vlan_proto
  rand                                  get_random_u32()
  ===================================   =================================================

These extensions can also be prefixed with '#'.
Examples for low-level BPF:

**ARP packets**::

  ldh [12]
  jne #0x806, drop
  ret #-1
  drop: ret #0

**IPv4 TCP packets**::

  ldh [12]
  jne #0x800, drop
  ldb [23]
  jneq #6, drop
  ret #-1
  drop: ret #0

**icmp random packet sampling, 1 in 4**::

  ldh [12]
  jne #0x800, drop
  ldb [23]
  jneq #1, drop
  # get a random uint32 number
  ld rand
  mod #4
  jneq #1, drop
  ret #-1
  drop: ret #0

**SECCOMP filter example**::

  ld [4]                  /* offsetof(struct seccomp_data, arch) */
  jne #0xc000003e, bad    /* AUDIT_ARCH_X86_64 */
  ld [0]                  /* offsetof(struct seccomp_data, nr) */
  jeq #15, good           /* __NR_rt_sigreturn */
  jeq #231, good          /* __NR_exit_group */
  jeq #60, good           /* __NR_exit */
  jeq #0, good            /* __NR_read */
  jeq #1, good            /* __NR_write */
  jeq #5, good            /* __NR_fstat */
  jeq #9, good            /* __NR_mmap */
  jeq #14, good           /* __NR_rt_sigprocmask */
  jeq #13, good           /* __NR_rt_sigaction */
  jeq #35, good           /* __NR_nanosleep */
  bad: ret #0             /* SECCOMP_RET_KILL_THREAD */
  good: ret #0x7fff0000   /* SECCOMP_RET_ALLOW */

Examples for low-level BPF extension:

**Packet for interface index 13**::

  ld ifidx
  jneq #13, drop
  ret #-1
  drop: ret #0

**(Accelerated) VLAN w/ id 10**::

  ld vlan_tci
  jneq #10, drop
  ret #-1
  drop: ret #0

The above example code can be placed into a file (here called "foo"), and
then be passed to the bpf_asm tool for generating opcodes, output that xt_bpf
and cls_bpf understands and can directly be loaded with. Example with above
ARP code::

    $ ./bpf_asm foo
    4,40 0 0 12,21 0 1 2054,6 0 0 4294967295,6 0 0 0,

In copy and paste C-like output::

    $ ./bpf_asm -c foo
    { 0x28,  0,  0, 0x0000000c },
    { 0x15,  0,  1, 0x00000806 },
    { 0x06,  0,  0, 0xffffffff },
    { 0x06,  0,  0, 0000000000 },

In particular, as usage with xt_bpf or cls_bpf can result in more complex BPF
filters that might not be obvious at first, it's good to test filters before
attaching to a live system. For that purpose, there's a small tool called
bpf_dbg under tools/bpf/ in the kernel source directory. This debugger allows
for testing BPF filters against given pcap files, single stepping through the
BPF code on the pcap's packets and to do BPF machine register dumps.

Starting bpf_dbg is trivial and just requires issuing::

    # ./bpf_dbg

In case input and output do not equal stdin/stdout, bpf_dbg takes an
alternative stdin source as a first argument, and an alternative stdout
sink as a second one, e.g. `./bpf_dbg test_in.txt test_out.txt`.

Other than that, a particular libreadline configuration can be set via
file "~/.bpf_dbg_init" and the command history is stored in the file
"~/.bpf_dbg_history".

Interaction in bpf_dbg happens through a shell that also has auto-completion
support (follow-up example commands starting with '>' denote bpf_dbg shell).
The usual workflow would be to ...

* load bpf 6,40 0 0 12,21 0 3 2048,48 0 0 23,21 0 1 1,6 0 0 65535,6 0 0 0
  Loads a BPF filter from standard output of bpf_asm, or transformed via
  e.g. ``tcpdump -iem1 -ddd port 22 | tr '\n' ','``. Note that for JIT
  debugging (next section), this command creates a temporary socket and
  loads the BPF code into the kernel. Thus, this will also be useful for
  JIT developers.

* load pcap foo.pcap

  Loads standard tcpdump pcap file.

* run [<n>]

bpf passes:1 fails:9
  Runs through all packets from a pcap to account how many passes and fails
  the filter will generate. A limit of packets to traverse can be given.

* disassemble::

	l0:	ldh [12]
	l1:	jeq #0x800, l2, l5
	l2:	ldb [23]
	l3:	jeq #0x1, l4, l5
	l4:	ret #0xffff
	l5:	ret #0

  Prints out BPF code disassembly.

* dump::

	/* { op, jt, jf, k }, */
	{ 0x28,  0,  0, 0x0000000c },
	{ 0x15,  0,  3, 0x00000800 },
	{ 0x30,  0,  0, 0x00000017 },
	{ 0x15,  0,  1, 0x00000001 },
	{ 0x06,  0,  0, 0x0000ffff },
	{ 0x06,  0,  0, 0000000000 },

  Prints out C-style BPF code dump.

* breakpoint 0::

	breakpoint at: l0:	ldh [12]

* breakpoint 1::

	breakpoint at: l1:	jeq #0x800, l2, l5

  ...

  Sets breakpoints at particular BPF instructions. Issuing a `run` command
  will walk through the pcap file continuing from the current packet and
  break when a breakpoint is being hit (another `run` will continue from
  the currently active breakpoint executing next instructions):

  * run::

	-- register dump --
	pc:       [0]                       <-- program counter
	code:     [40] jt[0] jf[0] k[12]    <-- plain BPF code of current instruction
	curr:     l0:	ldh [12]              <-- disassembly of current instruction
	A:        [00000000][0]             <-- content of A (hex, decimal)
	X:        [00000000][0]             <-- content of X (hex, decimal)
	M[0,15]:  [00000000][0]             <-- folded content of M (hex, decimal)
	-- packet dump --                   <-- Current packet from pcap (hex)
	len: 42
	    0: 00 19 cb 55 55 a4 00 14 a4 43 78 69 08 06 00 01
	16: 08 00 06 04 00 01 00 14 a4 43 78 69 0a 3b 01 26
	32: 00 00 00 00 00 00 0a 3b 01 01
	(breakpoint)
	>

  * breakpoint::

	breakpoints: 0 1

    Prints currently set breakpoints.

* step [-<n>, +<n>]

  Performs single stepping through the BPF program from the current pc
  offset. Thus, on each step invocation, above register dump is issued.
  This can go forwards and backwards in time, a plain `step` will break
  on the next BPF instruction, thus +1. (No `run` needs to be issued here.)

* select <n>

  Selects a given packet from the pcap file to continue from. Thus, on
  the next `run` or `step`, the BPF program is being evaluated against
  the user pre-selected packet. Numbering starts just as in Wireshark
  with index 1.

* quit

  Exits bpf_dbg.

JIT compiler
------------

The Linux kernel has a built-in BPF JIT compiler for x86_64, SPARC,
PowerPC, ARM, ARM64, MIPS, RISC-V and s390 and can be enabled through
CONFIG_BPF_JIT. The JIT compiler is transparently invoked for each
attached filter from user space or for internal kernel users if it has
been previously enabled by root::

  echo 1 > /proc/sys/net/core/bpf_jit_enable

For JIT developers, doing audits etc, each compile run can output the generated
opcode image into the kernel log via::

  echo 2 > /proc/sys/net/core/bpf_jit_enable

Example output from dmesg::

    [ 3389.935842] flen=6 proglen=70 pass=3 image=ffffffffa0069c8f
    [ 3389.935847] JIT code: 00000000: 55 48 89 e5 48 83 ec 60 48 89 5d f8 44 8b 4f 68
    [ 3389.935849] JIT code: 00000010: 44 2b 4f 6c 4c 8b 87 d8 00 00 00 be 0c 00 00 00
    [ 3389.935850] JIT code: 00000020: e8 1d 94 ff e0 3d 00 08 00 00 75 16 be 17 00 00
    [ 3389.935851] JIT code: 00000030: 00 e8 28 94 ff e0 83 f8 01 75 07 b8 ff ff 00 00
    [ 3389.935852] JIT code: 00000040: eb 02 31 c0 c9 c3

When CONFIG_BPF_JIT_ALWAYS_ON is enabled, bpf_jit_enable is permanently set to 1 and
setting any other value than that will return in failure. This is even the case for
setting bpf_jit_enable to 2, since dumping the final JIT image into the kernel log
is discouraged and introspection through bpftool (under tools/bpf/bpftool/) is the
generally recommended approach instead.

In the kernel source tree under tools/bpf/, there's bpf_jit_disasm for
generating disassembly out of the kernel log's hexdump::

	# ./bpf_jit_disasm
	70 bytes emitted from JIT compiler (pass:3, flen:6)
	ffffffffa0069c8f + <x>:
	0:	push   %rbp
	1:	mov    %rsp,%rbp
	4:	sub    $0x60,%rsp
	8:	mov    %rbx,-0x8(%rbp)
	c:	mov    0x68(%rdi),%r9d
	10:	sub    0x6c(%rdi),%r9d
	14:	mov    0xd8(%rdi),%r8
	1b:	mov    $0xc,%esi
	20:	callq  0xffffffffe0ff9442
	25:	cmp    $0x800,%eax
	2a:	jne    0x0000000000000042
	2c:	mov    $0x17,%esi
	31:	callq  0xffffffffe0ff945e
	36:	cmp    $0x1,%eax
	39:	jne    0x0000000000000042
	3b:	mov    $0xffff,%eax
	40:	jmp    0x0000000000000044
	42:	xor    %eax,%eax
	44:	leaveq
	45:	retq

	Issuing option `-o` will "annotate" opcodes to resulting assembler
	instructions, which can be very useful for JIT developers:

	# ./bpf_jit_disasm -o
	70 bytes emitted from JIT compiler (pass:3, flen:6)
	ffffffffa0069c8f + <x>:
	0:	push   %rbp
		55
	1:	mov    %rsp,%rbp
		48 89 e5
	4:	sub    $0x60,%rsp
		48 83 ec 60
	8:	mov    %rbx,-0x8(%rbp)
		48 89 5d f8
	c:	mov    0x68(%rdi),%r9d
		44 8b 4f 68
	10:	sub    0x6c(%rdi),%r9d
		44 2b 4f 6c
	14:	mov    0xd8(%rdi),%r8
		4c 8b 87 d8 00 00 00
	1b:	mov    $0xc,%esi
		be 0c 00 00 00
	20:	callq  0xffffffffe0ff9442
		e8 1d 94 ff e0
	25:	cmp    $0x800,%eax
		3d 00 08 00 00
	2a:	jne    0x0000000000000042
		75 16
	2c:	mov    $0x17,%esi
		be 17 00 00 00
	31:	callq  0xffffffffe0ff945e
		e8 28 94 ff e0
	36:	cmp    $0x1,%eax
		83 f8 01
	39:	jne    0x0000000000000042
		75 07
	3b:	mov    $0xffff,%eax
		b8 ff ff 00 00
	40:	jmp    0x0000000000000044
		eb 02
	42:	xor    %eax,%eax
		31 c0
	44:	leaveq
		c9
	45:	retq
		c3

For BPF JIT developers, bpf_jit_disasm, bpf_asm and bpf_dbg provides a useful
toolchain for developing and testing the kernel's JIT compiler.

BPF kernel internals
--------------------
Internally, for the kernel interpreter, a different instruction set
format with similar underlying principles from BPF described in previous
paragraphs is being used. However, the instruction set format is modelled
closer to the underlying architecture to mimic native instruction sets, so
that a better performance can be achieved (more details later). This new
ISA is called eBPF.  See the ../bpf/index.rst for details.  (Note: eBPF which
originates from [e]xtended BPF is not the same as BPF extensions! While
eBPF is an ISA, BPF extensions date back to classic BPF's 'overloading'
of BPF_LD | BPF_{B,H,W} | BPF_ABS instruction.)

The new instruction set was originally designed with the possible goal in
mind to write programs in "restricted C" and compile into eBPF with a optional
GCC/LLVM backend, so that it can just-in-time map to modern 64-bit CPUs with
minimal performance overhead over two steps, that is, C -> eBPF -> native code.

Currently, the new format is being used for running user BPF programs, which
includes seccomp BPF, classic socket filters, cls_bpf traffic classifier,
team driver's classifier for its load-balancing mode, netfilter's xt_bpf
extension, PTP dissector/classifier, and much more. They are all internally
converted by the kernel into the new instruction set representation and run
in the eBPF interpreter. For in-kernel handlers, this all works transparently
by using bpf_prog_create() for setting up the filter, resp.
bpf_prog_destroy() for destroying it. The function
bpf_prog_run(filter, ctx) transparently invokes eBPF interpreter or JITed
code to run the filter. 'filter' is a pointer to struct bpf_prog that we
got from bpf_prog_create(), and 'ctx' the given context (e.g.
skb pointer). All constraints and restrictions from bpf_check_classic() apply
before a conversion to the new layout is being done behind the scenes!

Currently, the classic BPF format is being used for JITing on most
32-bit architectures, whereas x86-64, aarch64, s390x, powerpc64,
sparc64, arm32, riscv64, riscv32, loongarch64 perform JIT compilation
from eBPF instruction set.

Testing
-------

Next to the BPF toolchain, the kernel also ships a test module that contains
various test cases for classic and eBPF that can be executed against
the BPF interpreter and JIT compiler. It can be found in lib/test_bpf.c and
enabled via Kconfig::

  CONFIG_TEST_BPF=m

After the module has been built and installed, the test suite can be executed
via insmod or modprobe against 'test_bpf' module. Results of the test cases
including timings in nsec can be found in the kernel log (dmesg).

Misc
----

Also trinity, the Linux syscall fuzzer, has built-in support for BPF and
SECCOMP-BPF kernel fuzzing.

Written by
----------

The document was written in the hope that it is found useful and in order
to give potential BPF hackers or security auditors a better overview of
the underlying architecture.

- Jay Schulist <jschlst@samba.org>
- Daniel Borkmann <daniel@iogearbox.net>
- Alexei Starovoitov <ast@kernel.org>
