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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | SYNTAX | SOURCE EXAMPLES | USAGE EXAMPLE | LEGAL | HISTORY | SEE ALSO | AUTHOR | COLOPHON | COLOPHON |
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BPFC(8) netsniff-ng toolkit BPFC(8)
bpfc - a Berkeley Packet Filter assembler and compiler
bpfc { [options] | [source-file] }
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.
-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.
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
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
Filter rtnetlink messages:
ldh #proto /* A = skb->protocol */
jneq #0, skip /* check for NETLINK_ROUTE */
ldb [4] /* A = nlmsg_type */
jneq #0x10, skip /* check type == RTNL_NEWLINK */
ldx #16 /* X = offset(ifinfomsg) */
ldb [x + 4] /* offset(ifi_index) */
jneq #0x3, skip /* check ifindex == 3 */
ld #32 /* A = len(nlmsghdr) + len(ifinfomsg), payload
offset */
ldx #16 /* X = IFLA_OPERSTATE */
ld #nla /* A = offset(IFLA_OPERSTATE) */
jeq #0, skip
tax
ldb [x + 4] /* A = value(IFLA_OPERSTATE) */
jneq #0x6, skip /* check oper state is UP */
ret #-1
skip: ret #0
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.
cat fubar | bpfc -
Read bpfc instruction from stdin and emit opcodes to
stdout.
bpfc foo > bar && 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''.
bpfc is licensed under the GNU GPL version 2.0.
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>.
netsniff-ng(8), trafgen(8), mausezahn(8), ifpps(8), flowtop(8),
astraceroute(8), curvetun(8)
Manpage was written by Daniel Borkmann.
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/.
This page is part of the netsniff-ng (a free Linux networking
toolkit) project. Information about the project can be found at
⟨http://netsniff-ng.org/⟩. If you have a bug report for this
manual page, send it to netsniff-ng@googlegroups.com. This page
was obtained from the project's upstream Git repository
⟨https://github.com/netsniff-ng/netsniff-ng⟩ on 2024-06-14. (At
that time, the date of the most recent commit that was found in
the repository was 2023-02-01.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
Linux 03 March 2013 BPFC(8)
Pages that refer to this page: astraceroute(8), curvetun(8), flowtop(8), ifpps(8), mausezahn(8), netsniff-ng(8), trafgen(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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