path: root/Documentation/networking/filter.rst

.. 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
------------------------------

The examples below use an assembly notation instead of less readable
opcodes. The notation 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 BPF program is an array of instructions with 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. This table lists the assembly notation and what
the corresponding opcodes defined in linux/filter.h do:

  ===========      ===================  =====================
  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 ARP filter shown above corresponds to the following sock_filter
entries::

    { 0x28,  0,  0, 0x0000000c },
    { 0x15,  0,  1, 0x00000806 },
    { 0x06,  0,  0, 0xffffffff },
    { 0x06,  0,  0, 0000000000 },

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

The Linux kernel has a built-in BPF JIT compiler for x86_64, SPARC,
PowerPC, ARM, ARM64, MIPS, RISC-V, s390, and ARC 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.

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, arc perform JIT compilation
from eBPF instruction set.

Testing
-------

The BPF selftests under tools/testing/selftests/bpf ship instruction
tests in test_progs, see jit_insns, which runs the eBPF
instructions over a range of operand magnitudes and signs, register
combinations, memory offsets and jump offsets against the BPF interpreter
or the JIT compiler, depending on the net.core.bpf_jit_enable setting.

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>