bpfc man page

bpfc — a Berkeley Packet Filter assembler and compiler

Synopsis

bpfc { [options] | [source-file] }

Description

bpfc is a small Berkeley Packet Filter assembler and compiler which is able to translate BPF assembler-like mnemonics into a numerical or C-like format, that can be read by tools such as netsniff-ng, iptables (xt_bpf) and many others. BPF is the one and only upstream filtering construct that is used in combination with packet(7) sockets, but also seccomp-BPF for system call sandboxing.

The Linux kernel and also BSD kernels implement "virtual machine" like constructs and JIT compilers that mimic a small register-based machine in BPF architecture and execute filter code that is, for example, composed by bpfc on a data buffer that is given by network packets. The purpose of this is to shift computation in time, so that the kernel can drop or truncate incoming packets as early as possible without having to push them to user space for further analysis first. Meanwhile, BPF constructs also find application in other areas such as in the communication between user and kernel space like system call sand-boxing.

At the time of writing this man page, the only other available BPF compiler is part of the pcap(3) library and accessible through a high-level filter language that might be familiar to many people as tcpdump-like filters.

However, it is quite often useful to bypass that compiler and write optimized code that cannot be produced by the pcap(3) compiler, or is wrongly optimized, or is defective on purpose in order to debug test kernel code. Also, a reason to use bpfc could be to try out some new BPF extensions that are not supported by other compilers. Furthermore, bpfc can be useful to verify JIT compiler behavior or to find possible bugs that need to be fixed.

bpfc is implemented with the help of flex(1) and bison(1), tokenizes the source file in the first stage and parses its content into an AST. In two code generation stages it emits target opcodes. bpfc furthermore supports Linux kernel BPF extensions. More about that can be found in the syntax section.

The Linux kernel BPF JIT compiler is automatically turned on if detected by netsniff-ng. However, it can also be manually turned on through the command ''echo "1" > /proc/sys/net/core/bpf_jit_enable'' (normal working mode) or ''echo "2" > /proc/sys/net/core/bpf_jit_enable'' (debug mode where emitted opcodes of the image are printed to the kernel log). An architecture agnostic BPF JIT image disassembler can be found in the kernel source tree under ''tools/net/bpf_jit_disasm.c'' or within the netsniff-ng Git repository.

Options

-i <source-file/->, --input <source-file/->

Read BPF assembly instruction from an input file or from stdin.

-p, --cpp

Pass the bpf program through the C preprocessor before reading it in bpfc. This allows #define and #include directives (e.g. to include definitions from system headers) to be used in the bpf program.

-D <name>=<definition>, --define <name>=<definition>

Add macro definition for the C preprocessor to use it within bpf file. This option is used in combination with the -p,--cpp option.

-f <format>, --format <format>

Specify a different output format than the default that is netsniff-ng compatible. The <format> specifier can be: C, netsniff-ng, xt_bpf, tcpdump.

-b, --bypass

Bypass basic filter validation when emitting opcodes. This can be useful for explicitly creating malformed BPF expressions for injecting into the kernel, for example, for bug testing.

-V, --verbose

Be more verbose and display some bpfc debugging information.

-d, --dump

Dump all supported instructions to stdout.

-v, --version

Show version information and exit.

-h, --help

Show user help and exit.

Syntax

The BPF architecture resp. register machine consists of the following 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 bpfc into ''opcodes'' is an array that consists of the following elements:

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

The element o 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
''true'', one for condition ''false''. Last but not least the 32 bit wide element k contains a miscellaneous argument that can be interpreted in different ways depending on the given instruction resp. opcode.

The instruction set consists of load, store, branch, alu, miscellaneous and return instructions that are also represented in bpfc syntax. This table also includes bpfc's own extensions. All operations are based on unsigned data structures:

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 Copy A into M[]
stx 3 Copy 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

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 (see next table)

Extension (and alias) Description

#len, len, #pktlen, pktlen Length of packet (skb->len)
#pto, pto, #proto, proto Ethernet type field (skb->protocol)
#type, type Packet type (**) (skb->pkt_type)
#poff, poff Detected payload start offset
#ifx, ifx, #ifidx, ifidx Interface index (skb->dev->ifindex)
#nla, nla Netlink attribute of type X with offset A
#nlan, nlan Nested Netlink attribute of type X with offset A
#mark, mark Packet mark (skb->mark)
#que, que, #queue, queue, #Q, Q NIC queue index (skb->queue_mapping)
#hat, hat, #hatype, hatype NIC hardware type (**) (skb->dev->type)
#rxh, rxh, #rxhash, rxhash Receive hash (skb->rxhash)
#cpu, cpu Current CPU (raw_smp_processor_id())
#vlant, vlant, #vlan_tci, vlan_tci VLAN TCI value (vlan_tx_tag_get(skb))
#vlanp, vlanp VLAN present (vlan_tx_tag_present(skb))

Further extension details (**) Value

#type, type 0 - to us / host
1 - to all / broadcast
2 - to group / multicast
3 - to others (promiscuous mode)
4 - outgoing of any type

#hat, hat, #hatype, hatype 1 - Ethernet 10Mbps
8 - APPLEtalk
19 - ATM
24 - IEEE 1394 IPv4 - RFC 2734
32 - InfiniBand
768 - IPIP tunnel
769 - IP6IP6 tunnel
772 - Loopback device
778 - GRE over IP
783 - Linux-IrDA
801 - IEEE 802.11
802 - IEEE 802.11 + Prism2 header
803 - IEEE 802.11 + radiotap header
823 - GRE over IP6
824 - Netlink
[...] See include/uapi/linux/if_arp.h

Note that the majority of BPF extensions are available on Linux only.

There are two types of comments in bpfc source-files:

1. Multi-line C-style comments: /* put comment here */
2. Single-line ASM-style comments: ; put comment here

Used Abbreviations:

BHW: byte, half-word, or word

Source Examples

In this section, we give a couple of examples of bpfc source files, in other words, some small example filter programs:

Only return packet headers (truncate packets)

ld poff
ret a

Only allow ARP packets

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

Only allow IPv4 TCP packets

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

Only allow IPv4 TCP SSH traffic

ldh [12]
jne #0x800, drop
ldb [23]
jneq #6, drop
ldh [20]
jset #0x1fff, drop
ldxb 4 * ([14] & 0xf)
ldh [x + 14]
jeq #0x16, pass
ldh [x + 16]
jne #0x16, drop
pass: ret #-1
drop: ret #0

A loadable x86_64 seccomp-BPF filter to allow a given set of syscalls

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 */

Allow any (hardware accelerated) VLAN

ld vlanp
jeq #0, drop
ret #-1
drop: ret #0

Only allow traffic for (hardware accelerated) VLAN 10

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

More pedantic check for the above VLAN example

ld vlanp
jeq #0, drop
ld vlant
jneq #10, drop
ret #-1
drop: ret #0

Usage Example

bpfc fubar

Compile the source file ''fubar'' into BPF opcodes. Opcodes will be directed to stdout.

bpfc -f xt_bpf -b -p -i fubar, resp. iptables -A INPUT -m bpf --bytecode `bpfc -f xt_bpf -i fubar` -j LOG

Compile the source file ''fubar'' into BPF opcodes, bypass basic filter validation and emit opcodes in netfilter's xt_bpf readable format. Note that the source file ''fubar'' is first passed to the C preprocessor for textual replacements before handing over to the bpfc compiler.

bpfc -

Read bpfc instruction from stdin and emit opcodes to stdout.

bpfc foo > bar, resp. netsniff-ng -f bar ...

Compile filter instructions from file foo and redirect bpfc's output into the file bar, that can then be read by netsniff-ng(8) through option -f.

bpfc -f tcpdump -i fubar

Output opcodes from source file fubar in the same behavior as ''tcpdump -ddd''.

History

bpfc was originally written for the netsniff-ng toolkit by Daniel Borkmann. It is currently maintained by Tobias Klauser <tklauser@distanz.ch> and Daniel Borkmann <dborkma@tik.ee.ethz.ch>.

See Also

netsniff-ng(8), trafgen(8), mausezahn(8), ifpps(8), flowtop(8), astraceroute(8), curvetun(8)

Author

Manpage was written by Daniel Borkmann.

Colophon

This page is part of the Linux netsniff-ng toolkit project. A description of the project, and information about reporting bugs, can be found at http://netsniff-ng.org/.

Referenced By

astraceroute(8), curvetun(8), flowtop(8), ifpps(8), mausezahn(8), netsniff-ng(8), trafgen(8).

03 March 2013 Linux netsniff-ng toolkit