Linux Socket Filtering aka Berkeley Packet Filter (BPF)¶
Notice¶
This file used to document the eBPF format and mechanisms even when notrelated to socket filtering. TheBPF Documentation has more detailson eBPF.
Introduction¶
Linux Socket Filtering (LSF) is derived from the Berkeley Packet Filter.Though there are some distinct differences between the BSD and LinuxKernel filtering, but when we speak of BPF or LSF in Linux context, wemean the very same mechanism of filtering in the Linux kernel.
BPF allows a user-space program to attach a filter onto any socket andallow or disallow certain types of data to come through the socket. LSFfollows exactly the same filter code structure as BSD’s BPF, so referringto 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 worryabout 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 filtercode passes the kernel check on it, you then immediately begin filteringdata on that socket.
You can also detach filters from your socket via the SO_DETACH_FILTERoption. This will probably not be used much since when you close a socketthat has a filter on it the filter is automagically removed. The otherless common case may be adding a different filter on the same socket whereyou had another filter that is still running: the kernel takes care ofremoving the old one and placing your new one in its place, assuming yourfilter has passed the checks, otherwise if it fails the old filter willremain on that socket.
SO_LOCK_FILTER option allows to lock the filter attached to a socket. Onceset, a filter cannot be removed or changed. This allows one process tosetup a socket, attach a filter, lock it then drop privileges and beassured that the filter will be kept until the socket is closed.
The biggest user of this construct might be libpcap. Issuing a high-levelfilter command liketcpdump -i em1 port 22 passes through the libpcapinternal compiler that generates a structure that can eventually be loadedvia SO_ATTACH_FILTER to the kernel.tcpdump -i em1 port 22 -ddddisplays what is being placed into this structure.
Although we were only speaking about sockets here, BPF in Linux is usedin many more places. There’s xt_bpf for netfilter, cls_bpf in the kernelqdisc layer, SECCOMP-BPF (SECure COMPuting[1]), and lots of other placessuch as team driver, PTP code, etc where BPF is being used.
[1]Seccomp BPF (SECure COMPuting with filters)
Original BPF paper:
Steven McCanne and Van Jacobson. 1993. The BSD packet filter: a newarchitecture for user-level packet capture. In Proceedings of theUSENIX Winter 1993 Conference Proceedings on USENIX Winter 1993Conference 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 thefollowing 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 containsa code, jt, jf and k value. jt and jf are jump offsets and k a genericvalue 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 infollow-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 socketin order to let all IPv4/IPv6 packets with port 22 pass. The rest willbe dropped for this socket.
The setsockopt(2) call to SO_DETACH_FILTER doesn’t need any argumentsand SO_LOCK_FILTER for preventing the filter to be detached, takes aninteger 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 becovered by libpcap in high-level syntax, so as an application developeryou 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 BPFfilters use Linux extensions that are not supported by libpcap’s compiler,iii) a filter might be more complex and not cleanly implementable withlibpcap’s compiler, or iv) particular filter codes should be optimizeddifferently than libpcap’s internal compiler does; then in such caseswriting such a filter “by hand” can be of an alternative. For example,xt_bpf and cls_bpf users might have requirements that could result inmore complex filter code, or one that cannot be expressed with libpcap(e.g. different return codes for various code paths). Moreover, BPF JITimplementors may wish to manually write test cases and thus need low-levelaccess to BPF code as well.
BPF engine and instruction set¶
Under tools/bpf/ there’s a small helper tool called bpf_asm which canbe used to write low-level filters for example scenarios mentioned in theprevious section. Asm-like syntax mentioned here has been implemented inbpf_asm and will be used for further explanations (instead of dealing withless readable opcodes directly, principles are the same). The syntax isclosely 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 memorystore”, addressable from 0 to 15
A program, that is translated by bpf_asm into “opcodes” is an array thatconsists 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 instructionencoded. jt and jf are two 8 bit wide jump targets, one for condition“jump if true”, the other one “jump if false”. Eventually, element kcontains a miscellaneous argument that can be interpreted in differentways depending on the given instruction in op.
The instruction set consists of load, store, branch, alu, miscellaneousand return instructions that are also represented in bpf_asm syntax. Thistable lists all bpf_asm instructions available resp. what their underlyingopcodes as defined in linux/filter.h stand for:
Instruction
Addressing mode
Description
ld
1, 2, 3, 4, 12
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, 12
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, 9, 10
Jump on A == <x>
jneq
9, 10
Jump on A != <x>
jne
9, 10
Jump on A != <x>
jlt
9, 10
Jump on A < <x>
jle
9, 10
Jump on A <= <x>
jgt
7, 8, 9, 10
Jump on A > <x>
jge
7, 8, 9, 10
Jump on A >= <x>
jset
7, 8, 9, 10
Jump on A & <x>
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
!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, 11
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
x/%x,Lt,Lf
Jump to Lt if true, otherwise jump to Lf
9
#k,Lt
Jump to Lt if predicate is true
10
x/%x,Lt
Jump to Lt if predicate is true
11
a/%a
Accumulator A
12
extension
BPF extension
The Linux kernel also has a couple of BPF extensions that are used alongwith the class of load instructions by “overloading” the k argument witha negative offset + a particular extension offset. The result of such BPFextensions 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->hash
cpu
raw_smp_processor_id()vlan_tci
skb_vlan_tag_get(skb)
vlan_avail
skb_vlan_tag_present(skb)
vlan_tpid
skb->vlan_proto
rand
get_random_u32()
These extensions can also be prefixed with ‘#’.Examples for low-level BPF:
ARP packets:
ldh [12]jne #0x806, dropret #-1drop: ret #0
IPv4 TCP packets:
ldh [12]jne #0x800, dropldb [23]jneq #6, dropret #-1drop: ret #0
icmp random packet sampling, 1 in 4:
ldh [12]jne #0x800, dropldb [23]jneq #1, drop# get a random uint32 numberld randmod #4jneq #1, dropret #-1drop: 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_THREAD */good: ret #0x7fff0000 /* SECCOMP_RET_ALLOW */
Examples for low-level BPF extension:
Packet for interface index 13:
ld ifidxjneq #13, dropret #-1drop: ret #0
(Accelerated) VLAN w/ id 10:
ld vlan_tcijneq #10, dropret #-1drop: ret #0
The above example code can be placed into a file (here called “foo”), andthen be passed to the bpf_asm tool for generating opcodes, output that xt_bpfand cls_bpf understands and can directly be loaded with. Example with aboveARP code:
$ ./bpf_asm foo4,40 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 BPFfilters that might not be obvious at first, it’s good to test filters beforeattaching to a live system. For that purpose, there’s a small tool calledbpf_dbg under tools/bpf/ in the kernel source directory. This debugger allowsfor testing BPF filters against given pcap files, single stepping through theBPF 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 analternative stdin source as a first argument, and an alternative stdoutsink as a second one, e.g../bpf_dbg test_in.txt test_out.txt.
Other than that, a particular libreadline configuration can be set viafile “~/.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-completionsupport (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 0Loads a BPF filter from standard output of bpf_asm, or transformed viae.g.
tcpdump-iem1-dddport22|tr'\n'','. Note that for JITdebugging (next section), this command creates a temporary socket andloads the BPF code into the kernel. Thus, this will also be useful forJIT 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 failsthe filter will generate. A limit of packets to traverse can be given.
disassemble:
l0: ldh [12]l1: jeq #0x800, l2, l5l2: ldb [23]l3: jeq #0x1, l4, l5l4: ret #0xffffl5: 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 arun commandwill walk through the pcap file continuing from the current packet andbreak when a breakpoint is being hit (anotherrun will continue fromthe currently active breakpoint executing next instructions):
run:
-- register dump --pc: [0] <-- program countercode: [40] jt[0] jf[0] k[12] <-- plain BPF code of current instructioncurr: l0: ldh [12] <-- disassembly of current instructionA: [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 0116: 08 00 06 04 00 01 00 14 a4 43 78 69 0a 3b 01 2632: 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 pcoffset. Thus, on each step invocation, above register dump is issued.This can go forwards and backwards in time, a plainstep will breakon the next BPF instruction, thus +1. (Norun needs to be issued here.)
select <n>
Selects a given packet from the pcap file to continue from. Thus, onthe nextrun orstep, the BPF program is being evaluated againstthe user pre-selected packet. Numbering starts just as in Wiresharkwith index 1.
quit
Exits bpf_dbg.
JIT compiler¶
The Linux kernel has a built-in BPF JIT compiler for x86_64, SPARC,PowerPC, ARM, ARM64, MIPS, RISC-V, s390, and ARC and can be enabled throughCONFIG_BPF_JIT. The JIT compiler is transparently invoked for eachattached filter from user space or for internal kernel users if it hasbeen 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 generatedopcode 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
When CONFIG_BPF_JIT_ALWAYS_ON is enabled, bpf_jit_enable is permanently set to 1 andsetting any other value than that will return in failure. This is even the case forsetting bpf_jit_enable to 2, since dumping the final JIT image into the kernel logis discouraged and introspection through bpftool (under tools/bpf/bpftool/) is thegenerally recommended approach instead.
In the kernel source tree under tools/bpf/, there’s bpf_jit_disasm forgenerating disassembly out of the kernel log’s hexdump:
# ./bpf_jit_disasm70 bytes emitted from JIT compiler (pass:3, flen:6)ffffffffa0069c8f + <x>:0: push %rbp1: mov %rsp,%rbp4: sub $0x60,%rsp8: mov %rbx,-0x8(%rbp)c: mov 0x68(%rdi),%r9d10: sub 0x6c(%rdi),%r9d14: mov 0xd8(%rdi),%r81b: mov $0xc,%esi20: callq 0xffffffffe0ff944225: cmp $0x800,%eax2a: jne 0x00000000000000422c: mov $0x17,%esi31: callq 0xffffffffe0ff945e36: cmp $0x1,%eax39: jne 0x00000000000000423b: mov $0xffff,%eax40: jmp 0x000000000000004442: xor %eax,%eax44: leaveq45: retqIssuing option `-o` will "annotate" opcodes to resulting assemblerinstructions, which can be very useful for JIT developers:# ./bpf_jit_disasm -o70 bytes emitted from JIT compiler (pass:3, flen:6)ffffffffa0069c8f + <x>:0: push %rbp 551: mov %rsp,%rbp 48 89 e54: sub $0x60,%rsp 48 83 ec 608: mov %rbx,-0x8(%rbp) 48 89 5d f8c: mov 0x68(%rdi),%r9d 44 8b 4f 6810: sub 0x6c(%rdi),%r9d 44 2b 4f 6c14: mov 0xd8(%rdi),%r8 4c 8b 87 d8 00 00 001b: mov $0xc,%esi be 0c 00 00 0020: callq 0xffffffffe0ff9442 e8 1d 94 ff e025: cmp $0x800,%eax 3d 00 08 00 002a: jne 0x0000000000000042 75 162c: mov $0x17,%esi be 17 00 00 0031: callq 0xffffffffe0ff945e e8 28 94 ff e036: cmp $0x1,%eax 83 f8 0139: jne 0x0000000000000042 75 073b: mov $0xffff,%eax b8 ff ff 00 0040: jmp 0x0000000000000044 eb 0242: xor %eax,%eax 31 c044: leaveq c945: retq c3
For BPF JIT developers, bpf_jit_disasm, bpf_asm and bpf_dbg provides a usefultoolchain for developing and testing the kernel’s JIT compiler.
BPF kernel internals¶
Internally, for the kernel interpreter, a different instruction setformat with similar underlying principles from BPF described in previousparagraphs is being used. However, the instruction set format is modelledcloser to the underlying architecture to mimic native instruction sets, sothat a better performance can be achieved (more details later). This newISA is called eBPF. See theBPF Documentation for details. (Note: eBPF whichoriginates from [e]xtended BPF is not the same as BPF extensions! WhileeBPF is an ISA, BPF extensions date back to classic BPF’s ‘overloading’of BPF_LD | BPF_{B,H,W} | BPF_ABS instruction.)
The new instruction set was originally designed with the possible goal inmind to write programs in “restricted C” and compile into eBPF with a optionalGCC/LLVM backend, so that it can just-in-time map to modern 64-bit CPUs withminimal performance overhead over two steps, that is, C -> eBPF -> native code.
Currently, the new format is being used for running user BPF programs, whichincludes seccomp BPF, classic socket filters, cls_bpf traffic classifier,team driver’s classifier for its load-balancing mode, netfilter’s xt_bpfextension, PTP dissector/classifier, and much more. They are all internallyconverted by the kernel into the new instruction set representation and runin the eBPF interpreter. For in-kernel handlers, this all works transparentlyby usingbpf_prog_create() for setting up the filter, resp.bpf_prog_destroy() for destroying it. The functionbpf_prog_run(filter, ctx) transparently invokes eBPF interpreter or JITedcode to run the filter. ‘filter’ is a pointer tostructbpf_prog that wegot frombpf_prog_create(), and ‘ctx’ the given context (e.g.skb pointer). All constraints and restrictions frombpf_check_classic() applybefore a conversion to the new layout is being done behind the scenes!
Currently, the classic BPF format is being used for JITing on most32-bit architectures, whereas x86-64, aarch64, s390x, powerpc64,sparc64, arm32, riscv64, riscv32, loongarch64, arc perform JIT compilationfrom eBPF instruction set.
Testing¶
Next to the BPF toolchain, the kernel also ships a test module that containsvarious test cases for classic and eBPF that can be executed againstthe BPF interpreter and JIT compiler. It can be found in lib/test_bpf.c andenabled via Kconfig:
CONFIG_TEST_BPF=m
After the module has been built and installed, the test suite can be executedvia insmod or modprobe against ‘test_bpf’ module. Results of the test casesincluding timings in nsec can be found in the kernel log (dmesg).
Misc¶
Also trinity, the Linux syscall fuzzer, has built-in support for BPF andSECCOMP-BPF kernel fuzzing.
Written by¶
The document was written in the hope that it is found useful and in orderto give potential BPF hackers or security auditors a better overview ofthe underlying architecture.
Jay Schulist <jschlst@samba.org>
Daniel Borkmann <daniel@iogearbox.net>
Alexei Starovoitov <ast@kernel.org>