bpfc (8) - Linux Manuals
bpfc: a Berkeley Packet Filter assembler and compiler
NAME
bpfc - a Berkeley Packet Filter assembler and compilerSYNOPSIS
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 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.-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:
A program, that is translated by bpfc into ''opcodes'' is an array that consists of the following elements:
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 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
ld
ldi
ldh
ldb
ldx
ldxi
ldxb
st
stx
jmp
ja
jeq
jneq
jne
jlt
jle
jgt
jge
jset
add
sub
mul
div
mod
neg
and
or
xor
lsh
rsh
tax
txa
ret
Addressing mode
10
Extension
#len, len,
#pto, pto,
#type, type
#poff, poff
#ifx, ifx,
#nla, nla
#nlan, nlan
#mark, mark
#que, que,
#hat, hat,
#rxh, rxh,
#cpu, cpu
#vlant, vlant,
#vlanp, vlanp
Further extension details
#type, type
#hat, hat,
Note that the majority of BPF extensions are available on Linux only.
There are two types of comments in bpfc source-files:
Used Abbreviations:
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):
Only allow ARP packets:
Only allow IPv4 TCP packets:
Only allow IPv4 TCP SSH traffic:
A loadable x86_64 seccomp-BPF filter to allow a given set of syscalls:
Allow any (hardware accelerated) VLAN:
Only allow traffic for (hardware accelerated) VLAN 10:
More pedantic check for the above VLAN example:
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''.LEGAL
bpfc is licensed under the GNU GPL version 2.0.HISTORY
bpfc was originally written for the netsniff-ng toolkit by Daniel Borkmann. It is currently maintained by Tobias Klauser <tklauser [at] distanz.ch> and Daniel Borkmann <dborkma [at] tik.ee.ethz.ch>.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/.
