filter: doc: improve BPF documentation

This patch significantly updates the BPF documentation and describes
its internal architecture, Linux extensions, and handling of the
kernel's BPF and JIT engine, plus documents how development can be
facilitated with the help of bpf_dbg, bpf_asm, bpf_jit_disasm.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 561 insertions, 47 deletions

diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt
index cdb3e40b9d14..a06b48d2f5cc 100644
--- a/Documentation/networking/filter.txt
+++ b/Documentation/networking/filter.txt
@@ -1,49 +1,563 @@
-filter.txt: Linux Socket Filtering
-Written by: Jay Schulist <jschlst@samba.org>
+Linux Socket Filtering aka Berkeley Packet Filter (BPF)
+=======================================================
 
 Introduction
-============
-
-	Linux Socket Filtering is derived from the Berkeley
-Packet Filter. There are some distinct differences between
-the BSD and Linux Kernel Filtering.
-
-Linux Socket Filtering (LSF) 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 the BSD Berkeley Packet Filter
-(BPF), so referring to the BSD bpf.4 manpage is very helpful in
-creating filters.
-
-LSF is much simpler than BPF. 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.
-
-Examples
-========
-
-Ioctls-
-setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &Filter, sizeof(Filter));
-setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &value, sizeof(value));
-setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER, &value, sizeof(value));
-
-See the BSD bpf.4 manpage and the BSD Packet Filter paper written by
-Steven McCanne and Van Jacobson of Lawrence Berkeley Laboratory.
+------------
+
+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->rxhash
+  cpu                                   raw_smp_processor_id()
+  vlan_tci                              vlan_tx_tag_get(skb)
+  vlan_pr                               vlan_tx_tag_present(skb)
+
+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
+
+** SECCOMP filter example:
+
+  ld [4]                  /* offsetof(struct seccomp_data, arch) */
+  jne #0xc000003e, bad    /* AUDIT_ARCH_X86_64 */
+  ld [0]                  /* offsetof(struct seccomp_data, nr) */
+  jeq #15, good           /* __NR_rt_sigreturn */
+  jeq #231, good          /* __NR_exit_group */
+  jeq #60, good           /* __NR_exit */
+  jeq #0, good            /* __NR_read */
+  jeq #1, good            /* __NR_write */
+  jeq #5, good            /* __NR_fstat */
+  jeq #9, good            /* __NR_mmap */
+  jeq #14, good           /* __NR_rt_sigprocmask */
+  jeq #13, good           /* __NR_rt_sigaction */
+  jeq #35, good           /* __NR_nanosleep */
+  bad: ret #0             /* SECCOMP_RET_KILL */
+  good: ret #0x7fff0000   /* SECCOMP_RET_ALLOW */
+
+The above example code can be placed into a file (here called "foo"), and
+then be passed to the bpf_asm tool for generating opcodes, output that xt_bpf
+and cls_bpf understands and can directly be loaded with. Example with above
+ARP code:
+
+$ ./bpf_asm foo
+4,40 0 0 12,21 0 1 2054,6 0 0 4294967295,6 0 0 0,
+
+In copy and paste C-like output:
+
+$ ./bpf_asm -c foo
+{ 0x28,  0,  0, 0x0000000c },
+{ 0x15,  0,  1, 0x00000806 },
+{ 0x06,  0,  0, 0xffffffff },
+{ 0x06,  0,  0, 0000000000 },
+
+In particular, as usage with xt_bpf or cls_bpf can result in more complex BPF
+filters that might not be obvious at first, it's good to test filters before
+attaching to a live system. For that purpose, there's a small tool called
+bpf_dbg under tools/net/ in the kernel source directory. This debugger allows
+for testing BPF filters against given pcap files, single stepping through the
+BPF code on the pcap's packets and to do BPF machine register dumps.
+
+Starting bpf_dbg is trivial and just requires issuing:
+
+# ./bpf_dbg
+
+In case input and output do not equal stdin/stdout, bpf_dbg takes an
+alternative stdin source as a first argument, and an alternative stdout
+sink as a second one, e.g. `./bpf_dbg test_in.txt test_out.txt`.
+
+Other than that, a particular libreadline configuration can be set via
+file "~/.bpf_dbg_init" and the command history is stored in the file
+"~/.bpf_dbg_history".
+
+Interaction in bpf_dbg happens through a shell that also has auto-completion
+support (follow-up example commands starting with '>' denote bpf_dbg shell).
+The usual workflow would be to ...
+
+> load bpf 6,40 0 0 12,21 0 3 2048,48 0 0 23,21 0 1 1,6 0 0 65535,6 0 0 0
+  Loads a BPF filter from standard output of bpf_asm, or transformed via
+  e.g. `tcpdump -iem1 -ddd port 22 | tr '\n' ','`. Note that for JIT
+  debugging (next section), this command creates a temporary socket and
+  loads the BPF code into the kernel. Thus, this will also be useful for
+  JIT developers.
+
+> load pcap foo.pcap
+  Loads standard tcpdump pcap file.
+
+> run [<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 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.
+
+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>