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/userspace-api/seccomp_filter.rst

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 <linux/filter.h> 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 <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 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/bpf/ 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, 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 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_avail

skb_vlan_tag_present(skb)

vlan_tpid

skb->vlan_proto

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_THREAD */
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/bpf/ 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 [<n>]

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 [-<n>, +<n>]

    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 <n>

    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, RISC-V 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

When CONFIG_BPF_JIT_ALWAYS_ON is enabled, bpf_jit_enable is permanently set to 1 and setting any other value than that will return in failure. This is even the case for setting bpf_jit_enable to 2, since dumping the final JIT image into the kernel log is discouraged and introspection through bpftool (under tools/bpf/bpftool/) is the generally recommended approach instead.

In the kernel source tree under tools/bpf/, 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 + <x>:
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 + <x>:
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 32-bit architectures, whereas x86-64, aarch64, s390x, powerpc64, sparc64, arm32, riscv64, riscv32 perform JIT compilation from eBPF instruction set.

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 $0x6,%esi
    mov $0x7,%edx
    mov $0x8,%ecx
    mov $0x9,%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. An in-kernel eBPF verifier is 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 be used as a 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

BPF_JMP32 0x06

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 or BPF_JMP32 [ in eBPF ], BPF_OP(code) is one of:

BPF_JA    0x00  /* BPF_JMP only */
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 BPF_JMP only: function call */
BPF_EXIT  0x90  /* eBPF BPF_JMP only: function return */
BPF_JLT   0xa0  /* eBPF only: unsigned '<' */
BPF_JLE   0xb0  /* eBPF only: unsigned '<=' */
BPF_JSLT  0xc0  /* eBPF only: signed '<' */
BPF_JSLE  0xd0  /* eBPF only: signed '<=' */

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 used as BPF_JMP32 to mean exactly the same operations as BPF_JMP, but with 32-bit wide operands for the comparisons instead.

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 | <size> | BPF_LD) and (BPF_IND | <size> | 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 | <size> | BPF_STX:  *(size *) (dst_reg + off) = src_reg
BPF_MEM | <size> | BPF_ST:   *(size *) (dst_reg + off) = imm32
BPF_MEM | <size> | 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=SCALAR_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, PTR_TO_STACK. 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=PTR_TO_STACK, 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 PTR_TO_STACK 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

Register value tracking

In order to determine the safety of an eBPF program, the verifier must track the range of possible values in each register and also in each stack slot. This is done with struct bpf_reg_state, defined in include/linux/ bpf_verifier.h, which unifies tracking of scalar and pointer values. Each register state has a type, which is either NOT_INIT (the register has not been written to), SCALAR_VALUE (some value which is not usable as a pointer), or a pointer type. The types of pointers describe their base, as follows:

PTR_TO_CTX

Pointer to bpf_context.

CONST_PTR_TO_MAP

Pointer to struct bpf_map. “Const” because arithmetic on these pointers is forbidden.

PTR_TO_MAP_VALUE

Pointer to the value stored in a map element.

PTR_TO_MAP_VALUE_OR_NULL

Either a pointer to a map value, or NULL; map accesses (see section ‘eBPF maps’, below) return this type, which becomes a PTR_TO_MAP_VALUE when checked != NULL. Arithmetic on these pointers is forbidden.

PTR_TO_STACK

Frame pointer.

PTR_TO_PACKET

skb->data.

PTR_TO_PACKET_END

skb->data + headlen; arithmetic forbidden.

PTR_TO_SOCKET

Pointer to struct bpf_sock_ops, implicitly refcounted.

PTR_TO_SOCKET_OR_NULL

Either a pointer to a socket, or NULL; socket lookup returns this type, which becomes a PTR_TO_SOCKET when checked != NULL. PTR_TO_SOCKET is reference-counted, so programs must release the reference through the socket release function before the end of the program. Arithmetic on these pointers is forbidden.

However, a pointer may be offset from this base (as a result of pointer arithmetic), and this is tracked in two parts: the ‘fixed offset’ and ‘variable offset’. The former is used when an exactly-known value (e.g. an immediate operand) is added to a pointer, while the latter is used for values which are not exactly known. The variable offset is also used in SCALAR_VALUEs, to track the range of possible values in the register.

The verifier’s knowledge about the variable offset consists of:

  • minimum and maximum values as unsigned

  • minimum and maximum values as signed

  • knowledge of the values of individual bits, in the form of a ‘tnum’: a u64 ‘mask’ and a u64 ‘value’. 1s in the mask represent bits whose value is unknown; 1s in the value represent bits known to be 1. Bits known to be 0 have 0 in both mask and value; no bit should ever be 1 in both. For example, if a byte is read into a register from memory, the register’s top 56 bits are known zero, while the low 8 are unknown - which is represented as the tnum (0x0; 0xff). If we then OR this with 0x40, we get (0x40; 0xbf), then if we add 1 we get (0x0; 0x1ff), because of potential carries.

Besides arithmetic, the register state can also be updated by conditional branches. For instance, if a SCALAR_VALUE is compared > 8, in the ‘true’ branch it will have a umin_value (unsigned minimum value) of 9, whereas in the ‘false’ branch it will have a umax_value of 8. A signed compare (with BPF_JSGT or BPF_JSGE) would instead update the signed minimum/maximum values. Information from the signed and unsigned bounds can be combined; for instance if a value is first tested < 8 and then tested s> 4, the verifier will conclude that the value is also > 4 and s< 8, since the bounds prevent crossing the sign boundary.

PTR_TO_PACKETs with a variable offset part have an ‘id’, which is common to all pointers sharing that same variable offset. This is important for packet range checks: after adding a variable to a packet pointer register A, if you then copy it to another register B and then add a constant 4 to A, both registers will share the same ‘id’ but the A will have a fixed offset of +4. Then if A is bounds-checked and found to be less than a PTR_TO_PACKET_END, the register B is now known to have a safe range of at least 4 bytes. See ‘Direct packet access’, below, for more on PTR_TO_PACKET ranges.

The ‘id’ field is also used on PTR_TO_MAP_VALUE_OR_NULL, common to all copies of the pointer returned from a map lookup. This means that when one copy is checked and found to be non-NULL, all copies can become PTR_TO_MAP_VALUEs. As well as range-checking, the tracked information is also used for enforcing alignment of pointer accesses. For instance, on most systems the packet pointer is 2 bytes after a 4-byte alignment. If a program adds 14 bytes to that to jump over the Ethernet header, then reads IHL and addes (IHL * 4), the resulting pointer will have a variable offset known to be 4n+2 for some n, so adding the 2 bytes (NET_IP_ALIGN) gives a 4-byte alignment and so word-sized accesses through that pointer are safe. The ‘id’ field is also used on PTR_TO_SOCKET and PTR_TO_SOCKET_OR_NULL, common to all copies of the pointer returned from a socket lookup. This has similar behaviour to the handling for PTR_TO_MAP_VALUE_OR_NULL->PTR_TO_MAP_VALUE, but it also handles reference tracking for the pointer. PTR_TO_SOCKET implicitly represents a reference to the corresponding struct sock. To ensure that the reference is not leaked, it is imperative to NULL-check the reference and in the non-NULL case, and pass the valid reference to the socket release function.

Direct packet access

In cls_bpf and act_bpf programs the verifier allows direct access to the packet data via skb->data and skb->data_end pointers. Ex:

1:  r4 = *(u32 *)(r1 +80)  /* load skb->data_end */
2:  r3 = *(u32 *)(r1 +76)  /* load skb->data */
3:  r5 = r3
4:  r5 += 14
5:  if r5 > r4 goto pc+16
R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
6:  r0 = *(u16 *)(r3 +12) /* access 12 and 13 bytes of the packet */

this 2byte load from the packet is safe to do, since the program author did check if (skb->data + 14 > skb->data_end) goto err at insn #5 which means that in the fall-through case the register R3 (which points to skb->data) has at least 14 directly accessible bytes. The verifier marks it as R3=pkt(id=0,off=0,r=14). id=0 means that no additional variables were added to the register. off=0 means that no additional constants were added. r=14 is the range of safe access which means that bytes [R3, R3 + 14) are ok. Note that R5 is marked as R5=pkt(id=0,off=14,r=14). It also points to the packet data, but constant 14 was added to the register, so it now points to skb->data + 14 and accessible range is [R5, R5 + 14 - 14) which is zero bytes.

More complex packet access may look like:

R0=inv1 R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
6:  r0 = *(u8 *)(r3 +7) /* load 7th byte from the packet */
7:  r4 = *(u8 *)(r3 +12)
8:  r4 *= 14
9:  r3 = *(u32 *)(r1 +76) /* load skb->data */
10:  r3 += r4
11:  r2 = r1
12:  r2 <<= 48
13:  r2 >>= 48
14:  r3 += r2
15:  r2 = r3
16:  r2 += 8
17:  r1 = *(u32 *)(r1 +80) /* load skb->data_end */
18:  if r2 > r1 goto pc+2
R0=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R1=pkt_end R2=pkt(id=2,off=8,r=8) R3=pkt(id=2,off=0,r=8) R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)) R5=pkt(id=0,off=14,r=14) R10=fp
19:  r1 = *(u8 *)(r3 +4)

The state of the register R3 is R3=pkt(id=2,off=0,r=8) id=2 means that two r3 += rX instructions were seen, so r3 points to some offset within a packet and since the program author did if (r3 + 8 > r1) goto err at insn #18, the safe range is [R3, R3 + 8). The verifier only allows ‘add’/’sub’ operations on packet registers. Any other operation will set the register state to ‘SCALAR_VALUE’ and it won’t be available for direct packet access.

Operation r3 += rX may overflow and become less than original skb->data, therefore the verifier has to prevent that. So when it sees r3 += rX instruction and rX is more than 16-bit value, any subsequent bounds-check of r3 against skb->data_end will not give us ‘range’ information, so attempts to read through the pointer will give “invalid access to packet” error.

Ex. after insn r4 = *(u8 *)(r3 +12) (insn #7 above) the state of r4 is R4=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) which means that upper 56 bits of the register are guaranteed to be zero, and nothing is known about the lower 8 bits. After insn r4 *= 14 the state becomes R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)), since multiplying an 8-bit value by constant 14 will keep upper 52 bits as zero, also the least significant bit will be zero as 14 is even. Similarly r2 >>= 48 will make R2=inv(id=0,umax_value=65535,var_off=(0x0; 0xffff)), since the shift is not sign extending. This logic is implemented in adjust_reg_min_max_vals() function, which calls adjust_ptr_min_max_vals() for adding pointer to scalar (or vice versa) and adjust_scalar_min_max_vals() for operations on two scalars.

The end result is that bpf program author can access packet directly using normal C code as:

void *data = (void *)(long)skb->data;
void *data_end = (void *)(long)skb->data_end;
struct eth_hdr *eth = data;
struct iphdr *iph = data + sizeof(*eth);
struct udphdr *udp = data + sizeof(*eth) + sizeof(*iph);

if (data + sizeof(*eth) + sizeof(*iph) + sizeof(*udp) > data_end)
        return 0;
if (eth->h_proto != htons(ETH_P_IP))
        return 0;
if (iph->protocol != IPPROTO_UDP || iph->ihl != 5)
        return 0;
if (udp->dest == 53 || udp->source == 9)
        ...;

which makes such programs easier to write comparing to LD_ABS insn and significantly faster.

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

Pruning

The verifier does not actually walk all possible paths through the program. For each new branch to analyse, the verifier looks at all the states it’s previously been in when at this instruction. If any of them contain the current state as a subset, the branch is ‘pruned’ - that is, the fact that the previous state was accepted implies the current state would be as well. For instance, if in the previous state, r1 held a packet-pointer, and in the current state, r1 holds a packet-pointer with a range as long or longer and at least as strict an alignment, then r1 is safe. Similarly, if r2 was NOT_INIT before then it can’t have been used by any path from that point, so any value in r2 (including another NOT_INIT) is safe. The implementation is in the function regsafe(). Pruning considers not only the registers but also the stack (and any spilled registers it may hold). They must all be safe for the branch to be pruned. This is implemented in states_equal().

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'

Program that performs a socket lookup then sets the pointer to NULL without checking it:

BPF_MOV64_IMM(BPF_REG_2, 0),
BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_2, -8),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
BPF_MOV64_IMM(BPF_REG_3, 4),
BPF_MOV64_IMM(BPF_REG_4, 0),
BPF_MOV64_IMM(BPF_REG_5, 0),
BPF_EMIT_CALL(BPF_FUNC_sk_lookup_tcp),
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN(),

Error:

0: (b7) r2 = 0
1: (63) *(u32 *)(r10 -8) = r2
2: (bf) r2 = r10
3: (07) r2 += -8
4: (b7) r3 = 4
5: (b7) r4 = 0
6: (b7) r5 = 0
7: (85) call bpf_sk_lookup_tcp#65
8: (b7) r0 = 0
9: (95) exit
Unreleased reference id=1, alloc_insn=7

Program that performs a socket lookup but does not NULL-check the returned value:

BPF_MOV64_IMM(BPF_REG_2, 0),
BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_2, -8),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
BPF_MOV64_IMM(BPF_REG_3, 4),
BPF_MOV64_IMM(BPF_REG_4, 0),
BPF_MOV64_IMM(BPF_REG_5, 0),
BPF_EMIT_CALL(BPF_FUNC_sk_lookup_tcp),
BPF_EXIT_INSN(),

Error:

0: (b7) r2 = 0
1: (63) *(u32 *)(r10 -8) = r2
2: (bf) r2 = r10
3: (07) r2 += -8
4: (b7) r3 = 4
5: (b7) r4 = 0
6: (b7) r5 = 0
7: (85) call bpf_sk_lookup_tcp#65
8: (95) exit
Unreleased reference id=1, alloc_insn=7

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.