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

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/prctl/seccomp_filter.txt

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/net/ 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, 10       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, 10          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                 Jump on k == A
  jneq             8                    Jump on k != A
  jne              8                    Jump on k != A
  jlt              8                    Jump on k < A
  jle              8                    Jump on k <= A
  jgt              7, 8                 Jump on k > A
  jge              7, 8                 Jump on k >= A
  jset             7, 8                 Jump on k & A

  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              0, 4                 !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, 9                 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               #k,Lt                Jump to Lt if predicate is true
   9               a/%a                 Accumulator A
  10               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->rxhash
  cpu                                   raw_smp_processor_id()
  vlan_tci                              vlan_tx_tag_get(skb)
  vlan_pr                               vlan_tx_tag_present(skb)

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

** (Accelerated) VLAN w/ id 10:

  ld vlan_tci
  jneq #10, 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 */
  good: ret #0x7fff0000   /* SECCOMP_RET_ALLOW */

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/net/ 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 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

In the kernel source tree under tools/net/, 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.

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 <dborkman@redhat.com>