Linux Socket Filtering aka Berkeley Packet Filter (BPF) ======================================================= Introduction ------------ Linux Socket Filtering (LSF) is derived from the Berkeley Packet Filter. Though there are some distinct differences between the BSD and Linux Kernel filtering, but when we speak of BPF or LSF in Linux context, we mean the very same mechanism of filtering in the Linux kernel. BPF allows a user-space program to attach a filter onto any socket and allow or disallow certain types of data to come through the socket. LSF follows exactly the same filter code structure as BSD's BPF, so referring to 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 worry about 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 filter code passes the kernel check on it, you then immediately begin filtering data on that socket. You can also detach filters from your socket via the SO_DETACH_FILTER option. This will probably not be used much since when you close a socket that has a filter on it the filter is automagically removed. The other less common case may be adding a different filter on the same socket where you had another filter that is still running: the kernel takes care of removing the old one and placing your new one in its place, assuming your filter has passed the checks, otherwise if it fails the old filter will remain on that socket. SO_LOCK_FILTER option allows to lock the filter attached to a socket. Once set, a filter cannot be removed or changed. This allows one process to setup a socket, attach a filter, lock it then drop privileges and be assured that the filter will be kept until the socket is closed. The biggest user of this construct might be libpcap. Issuing a high-level filter command like `tcpdump -i em1 port 22` passes through the libpcap internal compiler that generates a structure that can eventually be loaded via SO_ATTACH_FILTER to the kernel. `tcpdump -i em1 port 22 -ddd` displays what is being placed into this structure. Although we were only speaking about sockets here, BPF in Linux is used in many more places. There's xt_bpf for netfilter, cls_bpf in the kernel qdisc layer, SECCOMP-BPF (SECure COMPuting [1]), and lots of other places such as team driver, PTP code, etc where BPF is being used. [1] Documentation/prctl/seccomp_filter.txt Original BPF paper: Steven McCanne and Van Jacobson. 1993. The BSD packet filter: a new architecture for user-level packet capture. In Proceedings of the USENIX Winter 1993 Conference Proceedings on USENIX Winter 1993 Conference Proceedings (USENIX'93). USENIX Association, Berkeley, CA, USA, 2-2. [http://www.tcpdump.org/papers/bpf-usenix93.pdf] Structure --------- User space applications include which contains the following 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 contains a code, jt, jf and k value. jt and jf are jump offsets and k a generic value 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 in follow-up example) is being passed to the kernel through setsockopt(2). Example ------- #include #include #include #include /* ... */ /* 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 socket in order to let all IPv4/IPv6 packets with port 22 pass. The rest will be dropped for this socket. The setsockopt(2) call to SO_DETACH_FILTER doesn't need any arguments and SO_LOCK_FILTER for preventing the filter to be detached, takes an integer 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 be covered by libpcap in high-level syntax, so as an application developer you 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 BPF filters use Linux extensions that are not supported by libpcap's compiler, iii) a filter might be more complex and not cleanly implementable with libpcap's compiler, or iv) particular filter codes should be optimized differently than libpcap's internal compiler does; then in such cases writing such a filter "by hand" can be of an alternative. For example, xt_bpf and cls_bpf users might have requirements that could result in more complex filter code, or one that cannot be expressed with libpcap (e.g. different return codes for various code paths). Moreover, BPF JIT implementors may wish to manually write test cases and thus need low-level access to BPF code as well. BPF engine and instruction set ------------------------------ Under tools/net/ there's a small helper tool called bpf_asm which can be used to write low-level filters for example scenarios mentioned in the previous section. Asm-like syntax mentioned here has been implemented in bpf_asm and will be used for further explanations (instead of dealing with less readable opcodes directly, principles are the same). The syntax is closely 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 memory store", addressable from 0 to 15 A program, that is translated by bpf_asm into "opcodes" is an array that consists 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 instruction encoded. jt and jf are two 8 bit wide jump targets, one for condition "jump if true", the other one "jump if false". Eventually, element k contains a miscellaneous argument that can be interpreted in different ways depending on the given instruction in op. The instruction set consists of load, store, branch, alu, miscellaneous and return instructions that are also represented in bpf_asm syntax. This table lists all bpf_asm instructions available resp. what their underlying opcodes as defined in linux/filter.h stand for: 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 Store A into M[] stx 3 Store 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 + sub 0, 4 A - mul 0, 4 A * div 0, 4 A / mod 0, 4 A % neg 0, 4 !A and 0, 4 A & or 0, 4 A | xor 0, 4 A ^ lsh 0, 4 A << rsh 0, 4 A >> tax Copy A into X txa Copy X into A ret 4, 9 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 #k,Lt Jump to Lt if predicate is true 9 a/%a Accumulator A 10 extension BPF extension The Linux kernel also has a couple of BPF extensions that are used along with the class of load instructions by "overloading" the k argument with a negative offset + a particular extension offset. The result of such BPF extensions 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_pr skb_vlan_tag_present(skb) rand prandom_u32() These extensions can also be prefixed with '#'. Examples for low-level BPF: ** ARP packets: ldh [12] jne #0x806, drop ret #-1 drop: ret #0 ** IPv4 TCP packets: ldh [12] jne #0x800, drop ldb [23] jneq #6, drop ret #-1 drop: ret #0 ** (Accelerated) VLAN w/ id 10: ld vlan_tci jneq #10, drop ret #-1 drop: ret #0 ** icmp random packet sampling, 1 in 4 ldh [12] jne #0x800, drop ldb [23] jneq #1, drop # get a random uint32 number ld rand mod #4 jneq #1, drop ret #-1 drop: 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 */ good: ret #0x7fff0000 /* SECCOMP_RET_ALLOW */ The above example code can be placed into a file (here called "foo"), and then be passed to the bpf_asm tool for generating opcodes, output that xt_bpf and cls_bpf understands and can directly be loaded with. Example with above ARP code: $ ./bpf_asm foo 4,40 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 BPF filters that might not be obvious at first, it's good to test filters before attaching to a live system. For that purpose, there's a small tool called bpf_dbg under tools/net/ in the kernel source directory. This debugger allows for testing BPF filters against given pcap files, single stepping through the BPF 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 an alternative stdin source as a first argument, and an alternative stdout sink as a second one, e.g. `./bpf_dbg test_in.txt test_out.txt`. Other than that, a particular libreadline configuration can be set via file "~/.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-completion support (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 0 Loads a BPF filter from standard output of bpf_asm, or transformed via e.g. `tcpdump -iem1 -ddd port 22 | tr '\n' ','`. Note that for JIT debugging (next section), this command creates a temporary socket and loads the BPF code into the kernel. Thus, this will also be useful for JIT developers. > load pcap foo.pcap Loads standard tcpdump pcap file. > run [] bpf passes:1 fails:9 Runs through all packets from a pcap to account how many passes and fails the filter will generate. A limit of packets to traverse can be given. > disassemble l0: ldh [12] l1: jeq #0x800, l2, l5 l2: ldb [23] l3: jeq #0x1, l4, l5 l4: ret #0xffff l5: 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 a `run` command will walk through the pcap file continuing from the current packet and break when a breakpoint is being hit (another `run` will continue from the currently active breakpoint executing next instructions): > run -- register dump -- pc: [0] <-- program counter code: [40] jt[0] jf[0] k[12] <-- plain BPF code of current instruction curr: l0: ldh [12] <-- disassembly of current instruction A: [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 01 16: 08 00 06 04 00 01 00 14 a4 43 78 69 0a 3b 01 26 32: 00 00 00 00 00 00 0a 3b 01 01 (breakpoint) > > breakpoint breakpoints: 0 1 Prints currently set breakpoints. > step [-, +] Performs single stepping through the BPF program from the current pc offset. Thus, on each step invocation, above register dump is issued. This can go forwards and backwards in time, a plain `step` will break on the next BPF instruction, thus +1. (No `run` needs to be issued here.) > select Selects a given packet from the pcap file to continue from. Thus, on the next `run` or `step`, the BPF program is being evaluated against the user pre-selected packet. Numbering starts just as in Wireshark with 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 and s390 and can be enabled through CONFIG_BPF_JIT. The JIT compiler is transparently invoked for each attached filter from user space or for internal kernel users if it has been 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 generated opcode 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 In the kernel source tree under tools/net/, there's bpf_jit_disasm for generating disassembly out of the kernel log's hexdump: # ./bpf_jit_disasm 70 bytes emitted from JIT compiler (pass:3, flen:6) ffffffffa0069c8f + : 0: push %rbp 1: mov %rsp,%rbp 4: sub $0x60,%rsp 8: mov %rbx,-0x8(%rbp) c: mov 0x68(%rdi),%r9d 10: sub 0x6c(%rdi),%r9d 14: mov 0xd8(%rdi),%r8 1b: mov $0xc,%esi 20: callq 0xffffffffe0ff9442 25: cmp $0x800,%eax 2a: jne 0x0000000000000042 2c: mov $0x17,%esi 31: callq 0xffffffffe0ff945e 36: cmp $0x1,%eax 39: jne 0x0000000000000042 3b: mov $0xffff,%eax 40: jmp 0x0000000000000044 42: xor %eax,%eax 44: leaveq 45: retq Issuing option `-o` will "annotate" opcodes to resulting assembler instructions, which can be very useful for JIT developers: # ./bpf_jit_disasm -o 70 bytes emitted from JIT compiler (pass:3, flen:6) ffffffffa0069c8f + : 0: push %rbp 55 1: mov %rsp,%rbp 48 89 e5 4: sub $0x60,%rsp 48 83 ec 60 8: mov %rbx,-0x8(%rbp) 48 89 5d f8 c: mov 0x68(%rdi),%r9d 44 8b 4f 68 10: sub 0x6c(%rdi),%r9d 44 2b 4f 6c 14: mov 0xd8(%rdi),%r8 4c 8b 87 d8 00 00 00 1b: mov $0xc,%esi be 0c 00 00 00 20: callq 0xffffffffe0ff9442 e8 1d 94 ff e0 25: cmp $0x800,%eax 3d 00 08 00 00 2a: jne 0x0000000000000042 75 16 2c: mov $0x17,%esi be 17 00 00 00 31: callq 0xffffffffe0ff945e e8 28 94 ff e0 36: cmp $0x1,%eax 83 f8 01 39: jne 0x0000000000000042 75 07 3b: mov $0xffff,%eax b8 ff ff 00 00 40: jmp 0x0000000000000044 eb 02 42: xor %eax,%eax 31 c0 44: leaveq c9 45: retq c3 For BPF JIT developers, bpf_jit_disasm, bpf_asm and bpf_dbg provides a useful toolchain for developing and testing the kernel's JIT compiler. BPF kernel internals -------------------- Internally, for the kernel interpreter, a different instruction set format with similar underlying principles from BPF described in previous paragraphs is being used. However, the instruction set format is modelled closer to the underlying architecture to mimic native instruction sets, so that a better performance can be achieved (more details later). This new ISA is called 'eBPF' or 'internal BPF' interchangeably. (Note: eBPF which originates from [e]xtended BPF is not the same as BPF extensions! While eBPF is an ISA, BPF extensions date back to classic BPF's 'overloading' of BPF_LD | BPF_{B,H,W} | BPF_ABS instruction.) It is designed to be JITed with one to one mapping, which can also open up the possibility for GCC/LLVM compilers to generate optimized eBPF code through an eBPF backend that performs almost as fast as natively compiled code. The new instruction set was originally designed with the possible goal in mind to write programs in "restricted C" and compile into eBPF with a optional GCC/LLVM backend, so that it can just-in-time map to modern 64-bit CPUs with minimal performance overhead over two steps, that is, C -> eBPF -> native code. Currently, the new format is being used for running user BPF programs, which includes seccomp BPF, classic socket filters, cls_bpf traffic classifier, team driver's classifier for its load-balancing mode, netfilter's xt_bpf extension, PTP dissector/classifier, and much more. They are all internally converted by the kernel into the new instruction set representation and run in the eBPF interpreter. For in-kernel handlers, this all works transparently by using bpf_prog_create() for setting up the filter, resp. bpf_prog_destroy() for destroying it. The macro BPF_PROG_RUN(filter, ctx) transparently invokes eBPF interpreter or JITed code to run the filter. 'filter' is a pointer to struct bpf_prog that we got from bpf_prog_create(), and 'ctx' the given context (e.g. skb pointer). All constraints and restrictions from bpf_check_classic() apply before a conversion to the new layout is being done behind the scenes! Currently, the classic BPF format is being used for JITing on most of the architectures. Only x86-64 performs JIT compilation from eBPF instruction set, however, future work will migrate other JIT compilers as well, so that they will profit from the very same benefits. Some core changes of the new internal format: - Number of registers increase from 2 to 10: The old format had two registers A and X, and a hidden frame pointer. The new layout extends this to be 10 internal registers and a read-only frame pointer. Since 64-bit CPUs are passing arguments to functions via registers the number of args from eBPF program to in-kernel function is restricted to 5 and one register is used to accept return value from an in-kernel function. Natively, x86_64 passes first 6 arguments in registers, aarch64/ sparcv9/mips64 have 7 - 8 registers for arguments; x86_64 has 6 callee saved registers, and aarch64/sparcv9/mips64 have 11 or more callee saved registers. Therefore, eBPF calling convention is defined as: * R0 - return value from in-kernel function, and exit value for eBPF program * R1 - R5 - arguments from eBPF program to in-kernel function * R6 - R9 - callee saved registers that in-kernel function will preserve * R10 - read-only frame pointer to access stack Thus, all eBPF registers map one to one to HW registers on x86_64, aarch64, etc, and eBPF calling convention maps directly to ABIs used by the kernel on 64-bit architectures. On 32-bit architectures JIT may map programs that use only 32-bit arithmetic and may let more complex programs to be interpreted. R0 - R5 are scratch registers and eBPF program needs spill/fill them if necessary across calls. Note that there is only one eBPF program (== one eBPF main routine) and it cannot call other eBPF functions, it can only call predefined in-kernel functions, though. - Register width increases from 32-bit to 64-bit: Still, the semantics of the original 32-bit ALU operations are preserved via 32-bit subregisters. All eBPF registers are 64-bit with 32-bit lower subregisters that zero-extend into 64-bit if they are being written to. That behavior maps directly to x86_64 and arm64 subregister definition, but makes other JITs more difficult. 32-bit architectures run 64-bit internal BPF programs via interpreter. Their JITs may convert BPF programs that only use 32-bit subregisters into native instruction set and let the rest being interpreted. Operation is 64-bit, because on 64-bit architectures, pointers are also 64-bit wide, and we want to pass 64-bit values in/out of kernel functions, so 32-bit eBPF registers would otherwise require to define register-pair ABI, thus, there won't be able to use a direct eBPF register to HW register mapping and JIT would need to do combine/split/move operations for every register in and out of the function, which is complex, bug prone and slow. Another reason is the use of atomic 64-bit counters. - Conditional jt/jf targets replaced with jt/fall-through: While the original design has constructs such as "if (cond) jump_true; else jump_false;", they are being replaced into alternative constructs like "if (cond) jump_true; /* else fall-through */". - Introduces bpf_call insn and register passing convention for zero overhead calls from/to other kernel functions: Before an in-kernel function call, the internal BPF program needs to place function arguments into R1 to R5 registers to satisfy calling convention, then the interpreter will take them from registers and pass to in-kernel function. If R1 - R5 registers are mapped to CPU registers that are used for argument passing on given architecture, the JIT compiler doesn't need to emit extra moves. Function arguments will be in the correct registers and BPF_CALL instruction will be JITed as single 'call' HW instruction. This calling convention was picked to cover common call situations without performance penalty. After an in-kernel function call, R1 - R5 are reset to unreadable and R0 has a return value of the function. Since R6 - R9 are callee saved, their state is preserved across the call. For example, consider three C functions: u64 f1() { return (*_f2)(1); } u64 f2(u64 a) { return f3(a + 1, a); } u64 f3(u64 a, u64 b) { return a - b; } GCC can compile f1, f3 into x86_64: f1: movl $1, %edi movq _f2(%rip), %rax jmp *%rax f3: movq %rdi, %rax subq %rsi, %rax ret Function f2 in eBPF may look like: f2: bpf_mov R2, R1 bpf_add R1, 1 bpf_call f3 bpf_exit If f2 is JITed and the pointer stored to '_f2'. The calls f1 -> f2 -> f3 and returns will be seamless. Without JIT, __bpf_prog_run() interpreter needs to be used to call into f2. For practical reasons all eBPF programs have only one argument 'ctx' which is already placed into R1 (e.g. on __bpf_prog_run() startup) and the programs can call kernel functions with up to 5 arguments. Calls with 6 or more arguments are currently not supported, but these restrictions can be lifted if necessary in the future. On 64-bit architectures all register map to HW registers one to one. For example, x86_64 JIT compiler can map them as ... R0 - rax R1 - rdi R2 - rsi R3 - rdx R4 - rcx R5 - r8 R6 - rbx R7 - r13 R8 - r14 R9 - r15 R10 - rbp ... since x86_64 ABI mandates rdi, rsi, rdx, rcx, r8, r9 for argument passing and rbx, r12 - r15 are callee saved. Then the following internal BPF pseudo-program: bpf_mov R6, R1 /* save ctx */ bpf_mov R2, 2 bpf_mov R3, 3 bpf_mov R4, 4 bpf_mov R5, 5 bpf_call foo bpf_mov R7, R0 /* save foo() return value */ bpf_mov R1, R6 /* restore ctx for next call */ bpf_mov R2, 6 bpf_mov R3, 7 bpf_mov R4, 8 bpf_mov R5, 9 bpf_call bar bpf_add R0, R7 bpf_exit After JIT to x86_64 may look like: push %rbp mov %rsp,%rbp sub $0x228,%rsp mov %rbx,-0x228(%rbp) mov %r13,-0x220(%rbp) mov %rdi,%rbx mov $0x2,%esi mov $0x3,%edx mov $0x4,%ecx mov $0x5,%r8d callq foo mov %rax,%r13 mov %rbx,%rdi mov $0x2,%esi mov $0x3,%edx mov $0x4,%ecx mov $0x5,%r8d callq bar add %r13,%rax mov -0x228(%rbp),%rbx mov -0x220(%rbp),%r13 leaveq retq Which is in this example equivalent in C to: u64 bpf_filter(u64 ctx) { return foo(ctx, 2, 3, 4, 5) + bar(ctx, 6, 7, 8, 9); } In-kernel functions foo() and bar() with prototype: u64 (*)(u64 arg1, u64 arg2, u64 arg3, u64 arg4, u64 arg5); will receive arguments in proper registers and place their return value into '%rax' which is R0 in eBPF. Prologue and epilogue are emitted by JIT and are implicit in the interpreter. R0-R5 are scratch registers, so eBPF program needs to preserve them across the calls as defined by calling convention. For example the following program is invalid: bpf_mov R1, 1 bpf_call foo bpf_mov R0, R1 bpf_exit After the call the registers R1-R5 contain junk values and cannot be read. In the future an eBPF verifier can be used to validate internal BPF programs. Also in the new design, eBPF is limited to 4096 insns, which means that any program will terminate quickly and will only call a fixed number of kernel functions. Original BPF and the new format are two operand instructions, which helps to do one-to-one mapping between eBPF insn and x86 insn during JIT. The input context pointer for invoking the interpreter function is generic, its content is defined by a specific use case. For seccomp register R1 points to seccomp_data, for converted BPF filters R1 points to a skb. A program, that is translated internally consists of the following elements: op:16, jt:8, jf:8, k:32 ==> op:8, dst_reg:4, src_reg:4, off:16, imm:32 So far 87 internal BPF instructions were implemented. 8-bit 'op' opcode field has room for new instructions. Some of them may use 16/24/32 byte encoding. New instructions must be multiple of 8 bytes to preserve backward compatibility. Internal BPF is a general purpose RISC instruction set. Not every register and every instruction are used during translation from original BPF to new format. For example, socket filters are not using 'exclusive add' instruction, but tracing filters may do to maintain counters of events, for example. Register R9 is not used by socket filters either, but more complex filters may be running out of registers and would have to resort to spill/fill to stack. Internal BPF can used as generic assembler for last step performance optimizations, socket filters and seccomp are using it as assembler. Tracing filters may use it as assembler to generate code from kernel. In kernel usage may not be bounded by security considerations, since generated internal BPF code may be optimizing internal code path and not being exposed to the user space. Safety of internal BPF can come from a verifier (TBD). In such use cases as described, it may be used as safe instruction set. Just like the original BPF, the new format runs within a controlled environment, is deterministic and the kernel can easily prove that. The safety of the program can be determined in two steps: first step does depth-first-search to disallow loops and other CFG validation; second step starts from the first insn and descends all possible paths. It simulates execution of every insn and observes the state change of registers and stack. eBPF opcode encoding -------------------- eBPF is reusing most of the opcode encoding from classic to simplify conversion of classic BPF to eBPF. For arithmetic and jump instructions the 8-bit 'code' field is divided into three parts: +----------------+--------+--------------------+ | 4 bits | 1 bit | 3 bits | | operation code | source | instruction class | +----------------+--------+--------------------+ (MSB) (LSB) Three LSB bits store instruction class which is one of: Classic BPF classes: eBPF classes: BPF_LD 0x00 BPF_LD 0x00 BPF_LDX 0x01 BPF_LDX 0x01 BPF_ST 0x02 BPF_ST 0x02 BPF_STX 0x03 BPF_STX 0x03 BPF_ALU 0x04 BPF_ALU 0x04 BPF_JMP 0x05 BPF_JMP 0x05 BPF_RET 0x06 [ class 6 unused, for future if needed ] BPF_MISC 0x07 BPF_ALU64 0x07 When BPF_CLASS(code) == BPF_ALU or BPF_JMP, 4th bit encodes source operand ... BPF_K 0x00 BPF_X 0x08 * in classic BPF, this means: BPF_SRC(code) == BPF_X - use register X as source operand BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand * in eBPF, this means: BPF_SRC(code) == BPF_X - use 'src_reg' register as source operand BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand ... and four MSB bits store operation code. If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 [ in eBPF ], BPF_OP(code) is one of: BPF_ADD 0x00 BPF_SUB 0x10 BPF_MUL 0x20 BPF_DIV 0x30 BPF_OR 0x40 BPF_AND 0x50 BPF_LSH 0x60 BPF_RSH 0x70 BPF_NEG 0x80 BPF_MOD 0x90 BPF_XOR 0xa0 BPF_MOV 0xb0 /* eBPF only: mov reg to reg */ BPF_ARSH 0xc0 /* eBPF only: sign extending shift right */ BPF_END 0xd0 /* eBPF only: endianness conversion */ If BPF_CLASS(code) == BPF_JMP, BPF_OP(code) is one of: BPF_JA 0x00 BPF_JEQ 0x10 BPF_JGT 0x20 BPF_JGE 0x30 BPF_JSET 0x40 BPF_JNE 0x50 /* eBPF only: jump != */ BPF_JSGT 0x60 /* eBPF only: signed '>' */ BPF_JSGE 0x70 /* eBPF only: signed '>=' */ BPF_CALL 0x80 /* eBPF only: function call */ BPF_EXIT 0x90 /* eBPF only: function return */ So BPF_ADD | BPF_X | BPF_ALU means 32-bit addition in both classic BPF and eBPF. There are only two registers in classic BPF, so it means A += X. In eBPF it means dst_reg = (u32) dst_reg + (u32) src_reg; similarly, BPF_XOR | BPF_K | BPF_ALU means A ^= imm32 in classic BPF and analogous src_reg = (u32) src_reg ^ (u32) imm32 in eBPF. Classic BPF is using BPF_MISC class to represent A = X and X = A moves. eBPF is using BPF_MOV | BPF_X | BPF_ALU code instead. Since there are no BPF_MISC operations in eBPF, the class 7 is used as BPF_ALU64 to mean exactly the same operations as BPF_ALU, but with 64-bit wide operands instead. So BPF_ADD | BPF_X | BPF_ALU64 means 64-bit addition, i.e.: dst_reg = dst_reg + src_reg Classic BPF wastes the whole BPF_RET class to represent a single 'ret' operation. Classic BPF_RET | BPF_K means copy imm32 into return register and perform function exit. eBPF is modeled to match CPU, so BPF_JMP | BPF_EXIT in eBPF means function exit only. The eBPF program needs to store return value into register R0 before doing a BPF_EXIT. Class 6 in eBPF is currently unused and reserved for future use. For load and store instructions the 8-bit 'code' field is divided as: +--------+--------+-------------------+ | 3 bits | 2 bits | 3 bits | | mode | size | instruction class | +--------+--------+-------------------+ (MSB) (LSB) Size modifier is one of ... BPF_W 0x00 /* word */ BPF_H 0x08 /* half word */ BPF_B 0x10 /* byte */ BPF_DW 0x18 /* eBPF only, double word */ ... which encodes size of load/store operation: B - 1 byte H - 2 byte W - 4 byte DW - 8 byte (eBPF only) Mode modifier is one of: BPF_IMM 0x00 /* used for 32-bit mov in classic BPF and 64-bit in eBPF */ BPF_ABS 0x20 BPF_IND 0x40 BPF_MEM 0x60 BPF_LEN 0x80 /* classic BPF only, reserved in eBPF */ BPF_MSH 0xa0 /* classic BPF only, reserved in eBPF */ BPF_XADD 0xc0 /* eBPF only, exclusive add */ eBPF has two non-generic instructions: (BPF_ABS | | BPF_LD) and (BPF_IND | | BPF_LD) which are used to access packet data. They had to be carried over from classic to have strong performance of socket filters running in eBPF interpreter. These instructions can only be used when interpreter context is a pointer to 'struct sk_buff' and have seven implicit operands. Register R6 is an implicit input that must contain pointer to sk_buff. Register R0 is an implicit output which contains the data fetched from the packet. Registers R1-R5 are scratch registers and must not be used to store the data across BPF_ABS | BPF_LD or BPF_IND | BPF_LD instructions. These instructions have implicit program exit condition as well. When eBPF program is trying to access the data beyond the packet boundary, the interpreter will abort the execution of the program. JIT compilers therefore must preserve this property. src_reg and imm32 fields are explicit inputs to these instructions. For example: BPF_IND | BPF_W | BPF_LD means: R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32)) and R1 - R5 were scratched. Unlike classic BPF instruction set, eBPF has generic load/store operations: BPF_MEM | | BPF_STX: *(size *) (dst_reg + off) = src_reg BPF_MEM | | BPF_ST: *(size *) (dst_reg + off) = imm32 BPF_MEM | | BPF_LDX: dst_reg = *(size *) (src_reg + off) BPF_XADD | BPF_W | BPF_STX: lock xadd *(u32 *)(dst_reg + off16) += src_reg BPF_XADD | BPF_DW | BPF_STX: lock xadd *(u64 *)(dst_reg + off16) += src_reg Where size is one of: BPF_B or BPF_H or BPF_W or BPF_DW. Note that 1 and 2 byte atomic increments are not supported. eBPF has one 16-byte instruction: BPF_LD | BPF_DW | BPF_IMM which consists of two consecutive 'struct bpf_insn' 8-byte blocks and interpreted as single instruction that loads 64-bit immediate value into a dst_reg. Classic BPF has similar instruction: BPF_LD | BPF_W | BPF_IMM which loads 32-bit immediate value into a register. eBPF verifier ------------- The safety of the eBPF program is determined in two steps. First step does DAG check to disallow loops and other CFG validation. In particular it will detect programs that have unreachable instructions. (though classic BPF checker allows them) Second step starts from the first insn and descends all possible paths. It simulates execution of every insn and observes the state change of registers and stack. At the start of the program the register R1 contains a pointer to context and has type PTR_TO_CTX. If verifier sees an insn that does R2=R1, then R2 has now type PTR_TO_CTX as well and can be used on the right hand side of expression. If R1=PTR_TO_CTX and insn is R2=R1+R1, then R2=UNKNOWN_VALUE, since addition of two valid pointers makes invalid pointer. (In 'secure' mode verifier will reject any type of pointer arithmetic to make sure that kernel addresses don't leak to unprivileged users) If register was never written to, it's not readable: bpf_mov R0 = R2 bpf_exit will be rejected, since R2 is unreadable at the start of the program. After kernel function call, R1-R5 are reset to unreadable and R0 has a return type of the function. Since R6-R9 are callee saved, their state is preserved across the call. bpf_mov R6 = 1 bpf_call foo bpf_mov R0 = R6 bpf_exit is a correct program. If there was R1 instead of R6, it would have been rejected. load/store instructions are allowed only with registers of valid types, which are PTR_TO_CTX, PTR_TO_MAP, FRAME_PTR. They are bounds and alignment checked. For example: bpf_mov R1 = 1 bpf_mov R2 = 2 bpf_xadd *(u32 *)(R1 + 3) += R2 bpf_exit will be rejected, since R1 doesn't have a valid pointer type at the time of execution of instruction bpf_xadd. At the start R1 type is PTR_TO_CTX (a pointer to generic 'struct bpf_context') A callback is used to customize verifier to restrict eBPF program access to only certain fields within ctx structure with specified size and alignment. For example, the following insn: bpf_ld R0 = *(u32 *)(R6 + 8) intends to load a word from address R6 + 8 and store it into R0 If R6=PTR_TO_CTX, via is_valid_access() callback the verifier will know that offset 8 of size 4 bytes can be accessed for reading, otherwise the verifier will reject the program. If R6=FRAME_PTR, then access should be aligned and be within stack bounds, which are [-MAX_BPF_STACK, 0). In this example offset is 8, so it will fail verification, since it's out of bounds. The verifier will allow eBPF program to read data from stack only after it wrote into it. Classic BPF verifier does similar check with M[0-15] memory slots. For example: bpf_ld R0 = *(u32 *)(R10 - 4) bpf_exit is invalid program. Though R10 is correct read-only register and has type FRAME_PTR and R10 - 4 is within stack bounds, there were no stores into that location. Pointer register spill/fill is tracked as well, since four (R6-R9) callee saved registers may not be enough for some programs. Allowed function calls are customized with bpf_verifier_ops->get_func_proto() The eBPF verifier will check that registers match argument constraints. After the call register R0 will be set to return type of the function. Function calls is a main mechanism to extend functionality of eBPF programs. Socket filters may let programs to call one set of functions, whereas tracing filters may allow completely different set. If a function made accessible to eBPF program, it needs to be thought through from safety point of view. The verifier will guarantee that the function is called with valid arguments. seccomp vs socket filters have different security restrictions for classic BPF. Seccomp solves this by two stage verifier: classic BPF verifier is followed by seccomp verifier. In case of eBPF one configurable verifier is shared for all use cases. See details of eBPF verifier in kernel/bpf/verifier.c eBPF maps --------- 'maps' is a generic storage of different types for sharing data between kernel and userspace. The maps are accessed from user space via BPF syscall, which has commands: - create a map with given type and attributes map_fd = bpf(BPF_MAP_CREATE, union bpf_attr *attr, u32 size) using attr->map_type, attr->key_size, attr->value_size, attr->max_entries returns process-local file descriptor or negative error - lookup key in a given map err = bpf(BPF_MAP_LOOKUP_ELEM, union bpf_attr *attr, u32 size) using attr->map_fd, attr->key, attr->value returns zero and stores found elem into value or negative error - create or update key/value pair in a given map err = bpf(BPF_MAP_UPDATE_ELEM, union bpf_attr *attr, u32 size) using attr->map_fd, attr->key, attr->value returns zero or negative error - find and delete element by key in a given map err = bpf(BPF_MAP_DELETE_ELEM, union bpf_attr *attr, u32 size) using attr->map_fd, attr->key - to delete map: close(fd) Exiting process will delete maps automatically userspace programs use this syscall to create/access maps that eBPF programs are concurrently updating. maps can have different types: hash, array, bloom filter, radix-tree, etc. The map is defined by: . type . max number of elements . key size in bytes . value size in bytes Understanding eBPF verifier messages ------------------------------------ The following are few examples of invalid eBPF programs and verifier error messages as seen in the log: Program with unreachable instructions: static struct bpf_insn prog[] = { BPF_EXIT_INSN(), BPF_EXIT_INSN(), }; Error: unreachable insn 1 Program that reads uninitialized register: BPF_MOV64_REG(BPF_REG_0, BPF_REG_2), BPF_EXIT_INSN(), Error: 0: (bf) r0 = r2 R2 !read_ok Program that doesn't initialize R0 before exiting: BPF_MOV64_REG(BPF_REG_2, BPF_REG_1), BPF_EXIT_INSN(), Error: 0: (bf) r2 = r1 1: (95) exit R0 !read_ok Program that accesses stack out of bounds: BPF_ST_MEM(BPF_DW, BPF_REG_10, 8, 0), BPF_EXIT_INSN(), Error: 0: (7a) *(u64 *)(r10 +8) = 0 invalid stack off=8 size=8 Program that doesn't initialize stack before passing its address into function: BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), BPF_LD_MAP_FD(BPF_REG_1, 0), BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), BPF_EXIT_INSN(), Error: 0: (bf) r2 = r10 1: (07) r2 += -8 2: (b7) r1 = 0x0 3: (85) call 1 invalid indirect read from stack off -8+0 size 8 Program that uses invalid map_fd=0 while calling to map_lookup_elem() function: BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), BPF_LD_MAP_FD(BPF_REG_1, 0), BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), BPF_EXIT_INSN(), Error: 0: (7a) *(u64 *)(r10 -8) = 0 1: (bf) r2 = r10 2: (07) r2 += -8 3: (b7) r1 = 0x0 4: (85) call 1 fd 0 is not pointing to valid bpf_map Program that doesn't check return value of map_lookup_elem() before accessing map element: BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), BPF_LD_MAP_FD(BPF_REG_1, 0), BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0), BPF_EXIT_INSN(), Error: 0: (7a) *(u64 *)(r10 -8) = 0 1: (bf) r2 = r10 2: (07) r2 += -8 3: (b7) r1 = 0x0 4: (85) call 1 5: (7a) *(u64 *)(r0 +0) = 0 R0 invalid mem access 'map_value_or_null' Program that correctly checks map_lookup_elem() returned value for NULL, but accesses the memory with incorrect alignment: BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), BPF_LD_MAP_FD(BPF_REG_1, 0), BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1), BPF_ST_MEM(BPF_DW, BPF_REG_0, 4, 0), BPF_EXIT_INSN(), Error: 0: (7a) *(u64 *)(r10 -8) = 0 1: (bf) r2 = r10 2: (07) r2 += -8 3: (b7) r1 = 1 4: (85) call 1 5: (15) if r0 == 0x0 goto pc+1 R0=map_ptr R10=fp 6: (7a) *(u64 *)(r0 +4) = 0 misaligned access off 4 size 8 Program that correctly checks map_lookup_elem() returned value for NULL and accesses memory with correct alignment in one side of 'if' branch, but fails to do so in the other side of 'if' branch: BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), BPF_LD_MAP_FD(BPF_REG_1, 0), BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2), BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0), BPF_EXIT_INSN(), BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 1), BPF_EXIT_INSN(), Error: 0: (7a) *(u64 *)(r10 -8) = 0 1: (bf) r2 = r10 2: (07) r2 += -8 3: (b7) r1 = 1 4: (85) call 1 5: (15) if r0 == 0x0 goto pc+2 R0=map_ptr R10=fp 6: (7a) *(u64 *)(r0 +0) = 0 7: (95) exit from 5 to 8: R0=imm0 R10=fp 8: (7a) *(u64 *)(r0 +0) = 1 R0 invalid mem access 'imm' Testing ------- Next to the BPF toolchain, the kernel also ships a test module that contains various test cases for classic and internal BPF that can be executed against the BPF interpreter and JIT compiler. It can be found in lib/test_bpf.c and enabled via Kconfig: CONFIG_TEST_BPF=m After the module has been built and installed, the test suite can be executed via insmod or modprobe against 'test_bpf' module. Results of the test cases including timings in nsec can be found in the kernel log (dmesg). Misc ---- Also trinity, the Linux syscall fuzzer, has built-in support for BPF and SECCOMP-BPF kernel fuzzing. Written by ---------- The document was written in the hope that it is found useful and in order to give potential BPF hackers or security auditors a better overview of the underlying architecture. Jay Schulist Daniel Borkmann Alexei Starovoitov