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 <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/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 + <x>
    sub              0, 4                 A - <x>
    mul              0, 4                 A * <x>
    div              0, 4                 A / <x>
    mod              0, 4                 A % <x>
    neg              0, 4                 !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, 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_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 */
    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 '
  ' ','`. 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 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 + <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 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 | <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=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 <jschlst@samba.org>
  Daniel Borkmann <dborkman@redhat.com>

  Alexei Starovoitov <ast@plumgrid.com>