Merge branch 'bpf'

filter.txt: Linux Socket Filtering

Written by: Jay Schulist <jschlst@samba.org>

Linux Socket Filtering aka Berkeley Packet Filter (BPF)

=======================================================

Introduction

============

	Linux Socket Filtering is derived from the Berkeley

Packet Filter. There are some distinct differences between

the BSD and Linux Kernel Filtering.

Linux Socket Filtering (LSF) 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 the BSD Berkeley Packet Filter

(BPF), so referring to the BSD bpf.4 manpage is very helpful in

creating filters.

LSF is much simpler than BPF. 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.

Examples

========

Ioctls-

setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &Filter, sizeof(Filter));

setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &value, sizeof(value));

setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER, &value, sizeof(value));

See the BSD bpf.4 manpage and the BSD Packet Filter paper written by

Steven McCanne and Van Jacobson of Lawrence Berkeley Laboratory.

------------

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>

prefix = /usr

CC = gcc

LEX = flex

YACC = bison

all : bpf_jit_disasm

%.yacc.c: %.y

	$(YACC) -o $@ -d $<

%.lex.c: %.l

	$(LEX) -o $@ $<

all : bpf_jit_disasm bpf_dbg bpf_asm

bpf_jit_disasm : CFLAGS = -Wall -O2

bpf_jit_disasm : LDLIBS = -lopcodes -lbfd -ldl

bpf_jit_disasm : bpf_jit_disasm.o

bpf_dbg : CFLAGS = -Wall -O2

bpf_dbg : LDLIBS = -lreadline

bpf_dbg : bpf_dbg.o

bpf_asm : CFLAGS = -Wall -O2 -I.

bpf_asm : LDLIBS =

bpf_asm : bpf_asm.o bpf_exp.yacc.o bpf_exp.lex.o

bpf_exp.lex.o : bpf_exp.yacc.c

clean :

	rm -rf *.o bpf_jit_disasm

	rm -rf *.o bpf_jit_disasm bpf_dbg bpf_asm bpf_exp.yacc.* bpf_exp.lex.*

install :

	install bpf_jit_disasm $(prefix)/bin/bpf_jit_disasm

	install bpf_dbg $(prefix)/bin/bpf_dbg

	install bpf_asm $(prefix)/bin/bpf_asm

/*

 * Minimal BPF assembler

 *

 * Instead of libpcap high-level filter expressions, it can be quite

 * useful to define filters in low-level BPF assembler (that is kept

 * close to Steven McCanne and Van Jacobson's original BPF paper).

 * In particular for BPF JIT implementors, JIT security auditors, or

 * just for defining BPF expressions that contain extensions which are

 * not supported by compilers.

 *

 * How to get into it:

 *

 * 1) read Documentation/networking/filter.txt

 * 2) Run `bpf_asm [-c] <filter-prog file>` to translate into binary

 *    blob that is loadable with xt_bpf, cls_bpf et al. Note: -c will

 *    pretty print a C-like construct.

 *

 * Copyright 2013 Daniel Borkmann <borkmann@redhat.com>

 * Licensed under the GNU General Public License, version 2.0 (GPLv2)

 */

#include <stdbool.h>

#include <stdio.h>

#include <string.h>

extern void bpf_asm_compile(FILE *fp, bool cstyle);

int main(int argc, char **argv)

{

	FILE *fp = stdin;

	bool cstyle = false;

	int i;

	for (i = 1; i < argc; i++) {

		if (!strncmp("-c", argv[i], 2)) {

			cstyle = true;

			continue;

		}

		fp = fopen(argv[i], "r");

		if (!fp) {

			fp = stdin;

			continue;

		}

		break;

	}

	bpf_asm_compile(fp, cstyle);

	return 0;

}