Python BPF disassembler and map layout parser

Debugging eBPF programs can be tricky. The clang debug flags are not
supported in all the code-loading branches yet - e.g., only load_prog()
supports BPF_DEBUG or DEBUG_BPF_REGISTER_STATE, but compiling a kprobe
with BPF(...) doesn't.

This built-in disassembler can disassemble and print the BPF code in a
similar syntax than the kernel, whenever and the number of times the
user needs it. The BPF ISA is relatively stable so it doesn't require
much maintenance.

In addition, this parser is agnostic from the original source language
(C, B, Go, etc.), and doesn't depend on a particular compiler.

Example output for trace_pid_start() in biotop:

Disassemble of BPF program trace_pid_start:
   0: (79) r1 = *(u64*)(r1 +112)
   1: (7b) *(u64*)(r10 -8) = r1
   2: (b7) r1 = 0
   3: (63) *(u32*)(r10 -16) = r1
   4: (7b) *(u64*)(r10 -24) = r1
   5: (7b) *(u64*)(r10 -32) = r1
   6: (bf) r1 = r10
   7: (07) r1 += -28
   8: (b7) r2 = 16
   9: (85) call bpf_get_current_comm#16
  10: (67) r0 <<= 32
  11: (77) r0 >>= 32
  12: (55) if r0 != 0 goto +10 <23>
  13: (85) call bpf_get_current_pid_tgid#14
  14: (63) *(u32*)(r10 -32) = r0
  15: (18) r1 = <map at fd iovisor#3>
  17:      (64-bit upper word)
  17: (bf) r2 = r10
  18: (07) r2 += -8
  19: (bf) r3 = r10
  20: (07) r3 += -32
  21: (b7) r4 = 0
  22: (85) call bpf_map_update_elem#2
  23: (b7) r0 = 0
  24: (95) exit

The fields, types and memory layouts of maps can also be printed, which
is something that can be really helpful when dealing with unaligned
accesses or packed vs unpacked structures, and currently not supported
by clang.

For a map with key:

  struct {int a; short b; struct {int c:4; int d:8;};});

and value u64 the example output is:

Layout of BPF type HASH map test_map (ID 0):
  struct {
    [0 +4] int a;
    [4 +2] short b;
    [6 +2] char[2] __pad_2;
    [8 +4] struct {
      int c:4;
      int d:8;
    } __anon0;
  } key;
  unsigned long long value;

The [X +Y] is optional and denotes the offset and the size of each
field. Note that bit-fields and padding fields are shown.

src/python/bcc/__init__.py

@@ -30,6 +30,7 @@
 from .syscall import syscall_name
 from .utils import get_online_cpus, printb, _assert_is_bytes, ArgString
 from .version import __version__
+from .disassembler import disassemble_prog, decode_map
 
 _probe_limit = 1000
 _num_open_probes = 0
@@ -400,6 +401,15 @@ def dump_func(self, func_name):
         size, = lib.bpf_function_size(self.module, func_name),
         return ct.string_at(start, size)
 
+    def disassemble_prog(self, func_name):
+        bpfstr = self.dump_func(func_name)
+        disassemble_prog(func_name, bpfstr)
+
+    def decode_table(self, table_name, sizeinfo=False):
+        table_obj = self[table_name]
+        table_type = lib.bpf_table_type_id(self.module, table_obj.map_id)
+        decode_map(table_name, table_obj, table_type, sizeinfo=sizeinfo)
+
     str2ctype = {
         u"_Bool": ct.c_bool,
         u"char": ct.c_char,