1 Linux Socket Filtering aka Berkeley Packet Filter (BPF)
2 =======================================================
7 Linux Socket Filtering (LSF) is derived from the Berkeley Packet Filter.
8 Though there are some distinct differences between the BSD and Linux
9 Kernel filtering, but when we speak of BPF or LSF in Linux context, we
10 mean the very same mechanism of filtering in the Linux kernel.
12 BPF allows a user-space program to attach a filter onto any socket and
13 allow or disallow certain types of data to come through the socket. LSF
14 follows exactly the same filter code structure as BSD's BPF, so referring
15 to the BSD bpf.4 manpage is very helpful in creating filters.
17 On Linux, BPF is much simpler than on BSD. One does not have to worry
18 about devices or anything like that. You simply create your filter code,
19 send it to the kernel via the SO_ATTACH_FILTER option and if your filter
20 code passes the kernel check on it, you then immediately begin filtering
23 You can also detach filters from your socket via the SO_DETACH_FILTER
24 option. This will probably not be used much since when you close a socket
25 that has a filter on it the filter is automagically removed. The other
26 less common case may be adding a different filter on the same socket where
27 you had another filter that is still running: the kernel takes care of
28 removing the old one and placing your new one in its place, assuming your
29 filter has passed the checks, otherwise if it fails the old filter will
30 remain on that socket.
32 SO_LOCK_FILTER option allows to lock the filter attached to a socket. Once
33 set, a filter cannot be removed or changed. This allows one process to
34 setup a socket, attach a filter, lock it then drop privileges and be
35 assured that the filter will be kept until the socket is closed.
37 The biggest user of this construct might be libpcap. Issuing a high-level
38 filter command like `tcpdump -i em1 port 22` passes through the libpcap
39 internal compiler that generates a structure that can eventually be loaded
40 via SO_ATTACH_FILTER to the kernel. `tcpdump -i em1 port 22 -ddd`
41 displays what is being placed into this structure.
43 Although we were only speaking about sockets here, BPF in Linux is used
44 in many more places. There's xt_bpf for netfilter, cls_bpf in the kernel
45 qdisc layer, SECCOMP-BPF (SECure COMPuting [1]), and lots of other places
46 such as team driver, PTP code, etc where BPF is being used.
48 [1] Documentation/prctl/seccomp_filter.txt
52 Steven McCanne and Van Jacobson. 1993. The BSD packet filter: a new
53 architecture for user-level packet capture. In Proceedings of the
54 USENIX Winter 1993 Conference Proceedings on USENIX Winter 1993
55 Conference Proceedings (USENIX'93). USENIX Association, Berkeley,
56 CA, USA, 2-2. [http://www.tcpdump.org/papers/bpf-usenix93.pdf]
61 User space applications include <linux/filter.h> which contains the
62 following relevant structures:
64 struct sock_filter { /* Filter block */
65 __u16 code; /* Actual filter code */
66 __u8 jt; /* Jump true */
67 __u8 jf; /* Jump false */
68 __u32 k; /* Generic multiuse field */
71 Such a structure is assembled as an array of 4-tuples, that contains
72 a code, jt, jf and k value. jt and jf are jump offsets and k a generic
73 value to be used for a provided code.
75 struct sock_fprog { /* Required for SO_ATTACH_FILTER. */
76 unsigned short len; /* Number of filter blocks */
77 struct sock_filter __user *filter;
80 For socket filtering, a pointer to this structure (as shown in
81 follow-up example) is being passed to the kernel through setsockopt(2).
86 #include <sys/socket.h>
87 #include <sys/types.h>
88 #include <arpa/inet.h>
89 #include <linux/if_ether.h>
92 /* From the example above: tcpdump -i em1 port 22 -dd */
93 struct sock_filter code[] = {
94 { 0x28, 0, 0, 0x0000000c },
95 { 0x15, 0, 8, 0x000086dd },
96 { 0x30, 0, 0, 0x00000014 },
97 { 0x15, 2, 0, 0x00000084 },
98 { 0x15, 1, 0, 0x00000006 },
99 { 0x15, 0, 17, 0x00000011 },
100 { 0x28, 0, 0, 0x00000036 },
101 { 0x15, 14, 0, 0x00000016 },
102 { 0x28, 0, 0, 0x00000038 },
103 { 0x15, 12, 13, 0x00000016 },
104 { 0x15, 0, 12, 0x00000800 },
105 { 0x30, 0, 0, 0x00000017 },
106 { 0x15, 2, 0, 0x00000084 },
107 { 0x15, 1, 0, 0x00000006 },
108 { 0x15, 0, 8, 0x00000011 },
109 { 0x28, 0, 0, 0x00000014 },
110 { 0x45, 6, 0, 0x00001fff },
111 { 0xb1, 0, 0, 0x0000000e },
112 { 0x48, 0, 0, 0x0000000e },
113 { 0x15, 2, 0, 0x00000016 },
114 { 0x48, 0, 0, 0x00000010 },
115 { 0x15, 0, 1, 0x00000016 },
116 { 0x06, 0, 0, 0x0000ffff },
117 { 0x06, 0, 0, 0x00000000 },
120 struct sock_fprog bpf = {
121 .len = ARRAY_SIZE(code),
125 sock = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
127 /* ... bail out ... */
129 ret = setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &bpf, sizeof(bpf));
131 /* ... bail out ... */
136 The above example code attaches a socket filter for a PF_PACKET socket
137 in order to let all IPv4/IPv6 packets with port 22 pass. The rest will
138 be dropped for this socket.
140 The setsockopt(2) call to SO_DETACH_FILTER doesn't need any arguments
141 and SO_LOCK_FILTER for preventing the filter to be detached, takes an
142 integer value with 0 or 1.
144 Note that socket filters are not restricted to PF_PACKET sockets only,
145 but can also be used on other socket families.
147 Summary of system calls:
149 * setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &val, sizeof(val));
150 * setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &val, sizeof(val));
151 * setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER, &val, sizeof(val));
153 Normally, most use cases for socket filtering on packet sockets will be
154 covered by libpcap in high-level syntax, so as an application developer
155 you should stick to that. libpcap wraps its own layer around all that.
157 Unless i) using/linking to libpcap is not an option, ii) the required BPF
158 filters use Linux extensions that are not supported by libpcap's compiler,
159 iii) a filter might be more complex and not cleanly implementable with
160 libpcap's compiler, or iv) particular filter codes should be optimized
161 differently than libpcap's internal compiler does; then in such cases
162 writing such a filter "by hand" can be of an alternative. For example,
163 xt_bpf and cls_bpf users might have requirements that could result in
164 more complex filter code, or one that cannot be expressed with libpcap
165 (e.g. different return codes for various code paths). Moreover, BPF JIT
166 implementors may wish to manually write test cases and thus need low-level
167 access to BPF code as well.
169 BPF engine and instruction set
170 ------------------------------
172 Under tools/net/ there's a small helper tool called bpf_asm which can
173 be used to write low-level filters for example scenarios mentioned in the
174 previous section. Asm-like syntax mentioned here has been implemented in
175 bpf_asm and will be used for further explanations (instead of dealing with
176 less readable opcodes directly, principles are the same). The syntax is
177 closely modelled after Steven McCanne's and Van Jacobson's BPF paper.
179 The BPF architecture consists of the following basic elements:
183 A 32 bit wide accumulator
184 X 32 bit wide X register
185 M[] 16 x 32 bit wide misc registers aka "scratch memory
186 store", addressable from 0 to 15
188 A program, that is translated by bpf_asm into "opcodes" is an array that
189 consists of the following elements (as already mentioned):
191 op:16, jt:8, jf:8, k:32
193 The element op is a 16 bit wide opcode that has a particular instruction
194 encoded. jt and jf are two 8 bit wide jump targets, one for condition
195 "jump if true", the other one "jump if false". Eventually, element k
196 contains a miscellaneous argument that can be interpreted in different
197 ways depending on the given instruction in op.
199 The instruction set consists of load, store, branch, alu, miscellaneous
200 and return instructions that are also represented in bpf_asm syntax. This
201 table lists all bpf_asm instructions available resp. what their underlying
202 opcodes as defined in linux/filter.h stand for:
204 Instruction Addressing mode Description
206 ld 1, 2, 3, 4, 10 Load word into A
207 ldi 4 Load word into A
208 ldh 1, 2 Load half-word into A
209 ldb 1, 2 Load byte into A
210 ldx 3, 4, 5, 10 Load word into X
211 ldxi 4 Load word into X
212 ldxb 5 Load byte into X
214 st 3 Store A into M[]
215 stx 3 Store X into M[]
219 jeq 7, 8 Jump on k == A
220 jneq 8 Jump on k != A
224 jgt 7, 8 Jump on k > A
225 jge 7, 8 Jump on k >= A
226 jset 7, 8 Jump on k & A
245 The next table shows addressing formats from the 2nd column:
247 Addressing mode Syntax Description
250 1 [k] BHW at byte offset k in the packet
251 2 [x + k] BHW at the offset X + k in the packet
252 3 M[k] Word at offset k in M[]
253 4 #k Literal value stored in k
254 5 4*([k]&0xf) Lower nibble * 4 at byte offset k in the packet
256 7 #k,Lt,Lf Jump to Lt if true, otherwise jump to Lf
257 8 #k,Lt Jump to Lt if predicate is true
259 10 extension BPF extension
261 The Linux kernel also has a couple of BPF extensions that are used along
262 with the class of load instructions by "overloading" the k argument with
263 a negative offset + a particular extension offset. The result of such BPF
264 extensions are loaded into A.
266 Possible BPF extensions are shown in the following table:
268 Extension Description
273 poff Payload start offset
274 ifidx skb->dev->ifindex
275 nla Netlink attribute of type X with offset A
276 nlan Nested Netlink attribute of type X with offset A
278 queue skb->queue_mapping
279 hatype skb->dev->type
281 cpu raw_smp_processor_id()
282 vlan_tci vlan_tx_tag_get(skb)
283 vlan_pr vlan_tx_tag_present(skb)
286 These extensions can also be prefixed with '#'.
287 Examples for low-level BPF:
305 ** (Accelerated) VLAN w/ id 10:
312 ** icmp random packet sampling, 1 in 4
317 # get a random uint32 number
324 ** SECCOMP filter example:
326 ld [4] /* offsetof(struct seccomp_data, arch) */
327 jne #0xc000003e, bad /* AUDIT_ARCH_X86_64 */
328 ld [0] /* offsetof(struct seccomp_data, nr) */
329 jeq #15, good /* __NR_rt_sigreturn */
330 jeq #231, good /* __NR_exit_group */
331 jeq #60, good /* __NR_exit */
332 jeq #0, good /* __NR_read */
333 jeq #1, good /* __NR_write */
334 jeq #5, good /* __NR_fstat */
335 jeq #9, good /* __NR_mmap */
336 jeq #14, good /* __NR_rt_sigprocmask */
337 jeq #13, good /* __NR_rt_sigaction */
338 jeq #35, good /* __NR_nanosleep */
339 bad: ret #0 /* SECCOMP_RET_KILL */
340 good: ret #0x7fff0000 /* SECCOMP_RET_ALLOW */
342 The above example code can be placed into a file (here called "foo"), and
343 then be passed to the bpf_asm tool for generating opcodes, output that xt_bpf
344 and cls_bpf understands and can directly be loaded with. Example with above
348 4,40 0 0 12,21 0 1 2054,6 0 0 4294967295,6 0 0 0,
350 In copy and paste C-like output:
353 { 0x28, 0, 0, 0x0000000c },
354 { 0x15, 0, 1, 0x00000806 },
355 { 0x06, 0, 0, 0xffffffff },
356 { 0x06, 0, 0, 0000000000 },
358 In particular, as usage with xt_bpf or cls_bpf can result in more complex BPF
359 filters that might not be obvious at first, it's good to test filters before
360 attaching to a live system. For that purpose, there's a small tool called
361 bpf_dbg under tools/net/ in the kernel source directory. This debugger allows
362 for testing BPF filters against given pcap files, single stepping through the
363 BPF code on the pcap's packets and to do BPF machine register dumps.
365 Starting bpf_dbg is trivial and just requires issuing:
369 In case input and output do not equal stdin/stdout, bpf_dbg takes an
370 alternative stdin source as a first argument, and an alternative stdout
371 sink as a second one, e.g. `./bpf_dbg test_in.txt test_out.txt`.
373 Other than that, a particular libreadline configuration can be set via
374 file "~/.bpf_dbg_init" and the command history is stored in the file
375 "~/.bpf_dbg_history".
377 Interaction in bpf_dbg happens through a shell that also has auto-completion
378 support (follow-up example commands starting with '>' denote bpf_dbg shell).
379 The usual workflow would be to ...
381 > 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
382 Loads a BPF filter from standard output of bpf_asm, or transformed via
383 e.g. `tcpdump -iem1 -ddd port 22 | tr '\n' ','`. Note that for JIT
384 debugging (next section), this command creates a temporary socket and
385 loads the BPF code into the kernel. Thus, this will also be useful for
389 Loads standard tcpdump pcap file.
393 Runs through all packets from a pcap to account how many passes and fails
394 the filter will generate. A limit of packets to traverse can be given.
398 l1: jeq #0x800, l2, l5
403 Prints out BPF code disassembly.
406 /* { op, jt, jf, k }, */
407 { 0x28, 0, 0, 0x0000000c },
408 { 0x15, 0, 3, 0x00000800 },
409 { 0x30, 0, 0, 0x00000017 },
410 { 0x15, 0, 1, 0x00000001 },
411 { 0x06, 0, 0, 0x0000ffff },
412 { 0x06, 0, 0, 0000000000 },
413 Prints out C-style BPF code dump.
416 breakpoint at: l0: ldh [12]
418 breakpoint at: l1: jeq #0x800, l2, l5
420 Sets breakpoints at particular BPF instructions. Issuing a `run` command
421 will walk through the pcap file continuing from the current packet and
422 break when a breakpoint is being hit (another `run` will continue from
423 the currently active breakpoint executing next instructions):
427 pc: [0] <-- program counter
428 code: [40] jt[0] jf[0] k[12] <-- plain BPF code of current instruction
429 curr: l0: ldh [12] <-- disassembly of current instruction
430 A: [00000000][0] <-- content of A (hex, decimal)
431 X: [00000000][0] <-- content of X (hex, decimal)
432 M[0,15]: [00000000][0] <-- folded content of M (hex, decimal)
433 -- packet dump -- <-- Current packet from pcap (hex)
435 0: 00 19 cb 55 55 a4 00 14 a4 43 78 69 08 06 00 01
436 16: 08 00 06 04 00 01 00 14 a4 43 78 69 0a 3b 01 26
437 32: 00 00 00 00 00 00 0a 3b 01 01
443 Prints currently set breakpoints.
446 Performs single stepping through the BPF program from the current pc
447 offset. Thus, on each step invocation, above register dump is issued.
448 This can go forwards and backwards in time, a plain `step` will break
449 on the next BPF instruction, thus +1. (No `run` needs to be issued here.)
452 Selects a given packet from the pcap file to continue from. Thus, on
453 the next `run` or `step`, the BPF program is being evaluated against
454 the user pre-selected packet. Numbering starts just as in Wireshark
464 The Linux kernel has a built-in BPF JIT compiler for x86_64, SPARC, PowerPC,
465 ARM and s390 and can be enabled through CONFIG_BPF_JIT. The JIT compiler is
466 transparently invoked for each attached filter from user space or for internal
467 kernel users if it has been previously enabled by root:
469 echo 1 > /proc/sys/net/core/bpf_jit_enable
471 For JIT developers, doing audits etc, each compile run can output the generated
472 opcode image into the kernel log via:
474 echo 2 > /proc/sys/net/core/bpf_jit_enable
476 Example output from dmesg:
478 [ 3389.935842] flen=6 proglen=70 pass=3 image=ffffffffa0069c8f
479 [ 3389.935847] JIT code: 00000000: 55 48 89 e5 48 83 ec 60 48 89 5d f8 44 8b 4f 68
480 [ 3389.935849] JIT code: 00000010: 44 2b 4f 6c 4c 8b 87 d8 00 00 00 be 0c 00 00 00
481 [ 3389.935850] JIT code: 00000020: e8 1d 94 ff e0 3d 00 08 00 00 75 16 be 17 00 00
482 [ 3389.935851] JIT code: 00000030: 00 e8 28 94 ff e0 83 f8 01 75 07 b8 ff ff 00 00
483 [ 3389.935852] JIT code: 00000040: eb 02 31 c0 c9 c3
485 In the kernel source tree under tools/net/, there's bpf_jit_disasm for
486 generating disassembly out of the kernel log's hexdump:
489 70 bytes emitted from JIT compiler (pass:3, flen:6)
490 ffffffffa0069c8f + <x>:
494 8: mov %rbx,-0x8(%rbp)
495 c: mov 0x68(%rdi),%r9d
496 10: sub 0x6c(%rdi),%r9d
497 14: mov 0xd8(%rdi),%r8
499 20: callq 0xffffffffe0ff9442
501 2a: jne 0x0000000000000042
503 31: callq 0xffffffffe0ff945e
505 39: jne 0x0000000000000042
507 40: jmp 0x0000000000000044
512 Issuing option `-o` will "annotate" opcodes to resulting assembler
513 instructions, which can be very useful for JIT developers:
515 # ./bpf_jit_disasm -o
516 70 bytes emitted from JIT compiler (pass:3, flen:6)
517 ffffffffa0069c8f + <x>:
524 8: mov %rbx,-0x8(%rbp)
526 c: mov 0x68(%rdi),%r9d
528 10: sub 0x6c(%rdi),%r9d
530 14: mov 0xd8(%rdi),%r8
534 20: callq 0xffffffffe0ff9442
538 2a: jne 0x0000000000000042
542 31: callq 0xffffffffe0ff945e
546 39: jne 0x0000000000000042
550 40: jmp 0x0000000000000044
559 For BPF JIT developers, bpf_jit_disasm, bpf_asm and bpf_dbg provides a useful
560 toolchain for developing and testing the kernel's JIT compiler.
564 Internally, for the kernel interpreter, a different BPF instruction set
565 format with similar underlying principles from BPF described in previous
566 paragraphs is being used. However, the instruction set format is modelled
567 closer to the underlying architecture to mimic native instruction sets, so
568 that a better performance can be achieved (more details later).
570 It is designed to be JITed with one to one mapping, which can also open up
571 the possibility for GCC/LLVM compilers to generate optimized BPF code through
572 a BPF backend that performs almost as fast as natively compiled code.
574 The new instruction set was originally designed with the possible goal in
575 mind to write programs in "restricted C" and compile into BPF with a optional
576 GCC/LLVM backend, so that it can just-in-time map to modern 64-bit CPUs with
577 minimal performance overhead over two steps, that is, C -> BPF -> native code.
579 Currently, the new format is being used for running user BPF programs, which
580 includes seccomp BPF, classic socket filters, cls_bpf traffic classifier,
581 team driver's classifier for its load-balancing mode, netfilter's xt_bpf
582 extension, PTP dissector/classifier, and much more. They are all internally
583 converted by the kernel into the new instruction set representation and run
584 in the extended interpreter. For in-kernel handlers, this all works
585 transparently by using sk_unattached_filter_create() for setting up the
586 filter, resp. sk_unattached_filter_destroy() for destroying it. The macro
587 SK_RUN_FILTER(filter, ctx) transparently invokes the right BPF function to
588 run the filter. 'filter' is a pointer to struct sk_filter that we got from
589 sk_unattached_filter_create(), and 'ctx' the given context (e.g. skb pointer).
590 All constraints and restrictions from sk_chk_filter() apply before a
591 conversion to the new layout is being done behind the scenes!
593 Currently, for JITing, the user BPF format is being used and current BPF JIT
594 compilers reused whenever possible. In other words, we do not (yet!) perform
595 a JIT compilation in the new layout, however, future work will successively
596 migrate traditional JIT compilers into the new instruction format as well, so
597 that they will profit from the very same benefits. Thus, when speaking about
598 JIT in the following, a JIT compiler (TBD) for the new instruction format is
599 meant in this context.
601 Some core changes of the new internal format:
603 - Number of registers increase from 2 to 10:
605 The old format had two registers A and X, and a hidden frame pointer. The
606 new layout extends this to be 10 internal registers and a read-only frame
607 pointer. Since 64-bit CPUs are passing arguments to functions via registers
608 the number of args from BPF program to in-kernel function is restricted
609 to 5 and one register is used to accept return value from an in-kernel
610 function. Natively, x86_64 passes first 6 arguments in registers, aarch64/
611 sparcv9/mips64 have 7 - 8 registers for arguments; x86_64 has 6 callee saved
612 registers, and aarch64/sparcv9/mips64 have 11 or more callee saved registers.
614 Therefore, BPF calling convention is defined as:
616 * R0 - return value from in-kernel function, and exit value for BPF program
617 * R1 - R5 - arguments from BPF program to in-kernel function
618 * R6 - R9 - callee saved registers that in-kernel function will preserve
619 * R10 - read-only frame pointer to access stack
621 Thus, all BPF registers map one to one to HW registers on x86_64, aarch64,
622 etc, and BPF calling convention maps directly to ABIs used by the kernel on
623 64-bit architectures.
625 On 32-bit architectures JIT may map programs that use only 32-bit arithmetic
626 and may let more complex programs to be interpreted.
628 R0 - R5 are scratch registers and BPF program needs spill/fill them if
629 necessary across calls. Note that there is only one BPF program (== one BPF
630 main routine) and it cannot call other BPF functions, it can only call
631 predefined in-kernel functions, though.
633 - Register width increases from 32-bit to 64-bit:
635 Still, the semantics of the original 32-bit ALU operations are preserved
636 via 32-bit subregisters. All BPF registers are 64-bit with 32-bit lower
637 subregisters that zero-extend into 64-bit if they are being written to.
638 That behavior maps directly to x86_64 and arm64 subregister definition, but
639 makes other JITs more difficult.
641 32-bit architectures run 64-bit internal BPF programs via interpreter.
642 Their JITs may convert BPF programs that only use 32-bit subregisters into
643 native instruction set and let the rest being interpreted.
645 Operation is 64-bit, because on 64-bit architectures, pointers are also
646 64-bit wide, and we want to pass 64-bit values in/out of kernel functions,
647 so 32-bit BPF registers would otherwise require to define register-pair
648 ABI, thus, there won't be able to use a direct BPF register to HW register
649 mapping and JIT would need to do combine/split/move operations for every
650 register in and out of the function, which is complex, bug prone and slow.
651 Another reason is the use of atomic 64-bit counters.
653 - Conditional jt/jf targets replaced with jt/fall-through:
655 While the original design has constructs such as "if (cond) jump_true;
656 else jump_false;", they are being replaced into alternative constructs like
657 "if (cond) jump_true; /* else fall-through */".
659 - Introduces bpf_call insn and register passing convention for zero overhead
660 calls from/to other kernel functions:
662 Before an in-kernel function call, the internal BPF program needs to
663 place function arguments into R1 to R5 registers to satisfy calling
664 convention, then the interpreter will take them from registers and pass
665 to in-kernel function. If R1 - R5 registers are mapped to CPU registers
666 that are used for argument passing on given architecture, the JIT compiler
667 doesn't need to emit extra moves. Function arguments will be in the correct
668 registers and BPF_CALL instruction will be JITed as single 'call' HW
669 instruction. This calling convention was picked to cover common call
670 situations without performance penalty.
672 After an in-kernel function call, R1 - R5 are reset to unreadable and R0 has
673 a return value of the function. Since R6 - R9 are callee saved, their state
674 is preserved across the call.
676 For example, consider three C functions:
678 u64 f1() { return (*_f2)(1); }
679 u64 f2(u64 a) { return f3(a + 1, a); }
680 u64 f3(u64 a, u64 b) { return a - b; }
682 GCC can compile f1, f3 into x86_64:
693 Function f2 in BPF may look like:
701 If f2 is JITed and the pointer stored to '_f2'. The calls f1 -> f2 -> f3 and
702 returns will be seamless. Without JIT, __sk_run_filter() interpreter needs to
703 be used to call into f2.
705 For practical reasons all BPF programs have only one argument 'ctx' which is
706 already placed into R1 (e.g. on __sk_run_filter() startup) and the programs
707 can call kernel functions with up to 5 arguments. Calls with 6 or more arguments
708 are currently not supported, but these restrictions can be lifted if necessary
711 On 64-bit architectures all register map to HW registers one to one. For
712 example, x86_64 JIT compiler can map them as ...
726 ... since x86_64 ABI mandates rdi, rsi, rdx, rcx, r8, r9 for argument passing
727 and rbx, r12 - r15 are callee saved.
729 Then the following internal BPF pseudo-program:
731 bpf_mov R6, R1 /* save ctx */
737 bpf_mov R7, R0 /* save foo() return value */
738 bpf_mov R1, R6 /* restore ctx for next call */
747 After JIT to x86_64 may look like:
752 mov %rbx,-0x228(%rbp)
753 mov %r13,-0x220(%rbp)
768 mov -0x228(%rbp),%rbx
769 mov -0x220(%rbp),%r13
773 Which is in this example equivalent in C to:
775 u64 bpf_filter(u64 ctx)
777 return foo(ctx, 2, 3, 4, 5) + bar(ctx, 6, 7, 8, 9);
780 In-kernel functions foo() and bar() with prototype: u64 (*)(u64 arg1, u64
781 arg2, u64 arg3, u64 arg4, u64 arg5); will receive arguments in proper
782 registers and place their return value into '%rax' which is R0 in BPF.
783 Prologue and epilogue are emitted by JIT and are implicit in the
784 interpreter. R0-R5 are scratch registers, so BPF program needs to preserve
785 them across the calls as defined by calling convention.
787 For example the following program is invalid:
794 After the call the registers R1-R5 contain junk values and cannot be read.
795 In the future a BPF verifier can be used to validate internal BPF programs.
797 Also in the new design, BPF is limited to 4096 insns, which means that any
798 program will terminate quickly and will only call a fixed number of kernel
799 functions. Original BPF and the new format are two operand instructions,
800 which helps to do one-to-one mapping between BPF insn and x86 insn during JIT.
802 The input context pointer for invoking the interpreter function is generic,
803 its content is defined by a specific use case. For seccomp register R1 points
804 to seccomp_data, for converted BPF filters R1 points to a skb.
806 A program, that is translated internally consists of the following elements:
808 op:16, jt:8, jf:8, k:32 ==> op:8, a_reg:4, x_reg:4, off:16, imm:32
810 So far 87 internal BPF instructions were implemented. 8-bit 'op' opcode field
811 has room for new instructions. Some of them may use 16/24/32 byte encoding. New
812 instructions must be multiple of 8 bytes to preserve backward compatibility.
814 Internal BPF is a general purpose RISC instruction set. Not every register and
815 every instruction are used during translation from original BPF to new format.
816 For example, socket filters are not using 'exclusive add' instruction, but
817 tracing filters may do to maintain counters of events, for example. Register R9
818 is not used by socket filters either, but more complex filters may be running
819 out of registers and would have to resort to spill/fill to stack.
821 Internal BPF can used as generic assembler for last step performance
822 optimizations, socket filters and seccomp are using it as assembler. Tracing
823 filters may use it as assembler to generate code from kernel. In kernel usage
824 may not be bounded by security considerations, since generated internal BPF code
825 may be optimizing internal code path and not being exposed to the user space.
826 Safety of internal BPF can come from a verifier (TBD). In such use cases as
827 described, it may be used as safe instruction set.
829 Just like the original BPF, the new format runs within a controlled environment,
830 is deterministic and the kernel can easily prove that. The safety of the program
831 can be determined in two steps: first step does depth-first-search to disallow
832 loops and other CFG validation; second step starts from the first insn and
833 descends all possible paths. It simulates execution of every insn and observes
834 the state change of registers and stack.
839 Next to the BPF toolchain, the kernel also ships a test module that contains
840 various test cases for classic and internal BPF that can be executed against
841 the BPF interpreter and JIT compiler. It can be found in lib/test_bpf.c and
846 After the module has been built and installed, the test suite can be executed
847 via insmod or modprobe against 'test_bpf' module. Results of the test cases
848 including timings in nsec can be found in the kernel log (dmesg).
853 Also trinity, the Linux syscall fuzzer, has built-in support for BPF and
854 SECCOMP-BPF kernel fuzzing.
859 The document was written in the hope that it is found useful and in order
860 to give potential BPF hackers or security auditors a better overview of
861 the underlying architecture.
863 Jay Schulist <jschlst@samba.org>
864 Daniel Borkmann <dborkman@redhat.com>
865 Alexei Starovoitov <ast@plumgrid.com>