Question

I’m trying to inspect a buffer which contains a binary formatted message, but also contains string data. As an example, I’m using this C code:

int main (void) {
    char buf[100] = "\x01\x02\x03\x04String Data\xAA\xBB\xCC";

    return 0;
}

I’d like to get a hex dump of what’s in buf, of a format similar to xxd (I don’t care if it’s an exact match, what I’m really looking for is a hex dump side by side with printable chars).

Inside GDB I can use something like:

(gdb) x /100bx buf
0x7fffffffdf00: 0x01    0x02    0x03    0x04    0x53    0x74    0x72    0x69
0x7fffffffdf08: 0x6e    0x67    0x20    0x44    0x61    0x74    0x61    0xaa
0x7fffffffdf10: 0xbb    0xcc    0x00    0x00    0x00    0x00    0x00    0x00
0x7fffffffdf18: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x7fffffffdf20: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x7fffffffdf28: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x7fffffffdf30: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x7fffffffdf38: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x7fffffffdf40: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x7fffffffdf48: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x7fffffffdf50: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x7fffffffdf58: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00

which is fine, but it’s hard to pick out strings that way… or I can use

(gdb) x /100bs buf
0x7fffffffdf00:  "\001\002\003\004String Data\252\273\314"
0x7fffffffdf13:  ""
0x7fffffffdf14:  ""
0x7fffffffdf15:  ""
0x7fffffffdf16:  ""
0x7fffffffdf17:  ""
...

which makes it hard to read the binary part… the actual messages I’m dealing with have plenty of ascii nul’s in them, too, so really it just looks like a mess.

The best I can come up with is to do this:

(gdb) dump binary memory dump.bin buf buf+100

and then

$ xxd dump.bin
0000000: 0102 0304 5374 7269 6e67 2044 6174 61aa  ....String Data.
0000010: bbcc 0000 0000 0000 0000 0000 0000 0000  ................
0000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
0000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
0000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
0000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
0000060: 0000 0000                                ....

but that’s a pain to do that every time. I figured somebody out there has wanted this before, so wondering if anybody has found a way to do it inside gdb. Plus you lose the addresses from the original memory this way.

I’m using GDB 7.4 with python support built in, so I’m open to the idea of using a pretty printer or similar, but I don’t know how to set that up.

Answer 1

(gdb) define xxd
>dump binary memory dump.bin $arg0 $arg0+$arg1
>shell xxd dump.bin
>end
(gdb) xxd &j 10 
0000000: 0000 0000 0000 0000 0000 0000 4d8c a7f7  ............M...
0000010: ff7f 0000 0000 0000 0000 0000 c8d7 ffff  ................
0000020: ff7f 0000 0000 0000

Seems easy enough 😉

You could likely write a Python script (modern GDB versions have embedded Python interpreter) to do the same, and get rid of the need to "shell out".

Update:

Here is a possible Python implementation (save this into xxd.py):

class XXD(gdb.Command):
  def __init__(self):
    super(XXD, self).__init__("xxd", gdb.COMMAND_USER)

  def _PrintLine(self, offset, bytes, size):
      print('{:08x}: '.format(offset), end='')
      todo = size
      while todo >= 4:
        print(''.join('{:02x}'.format(b) for b in bytes[0:4]), end='')
        todo -= 4
        bytes = bytes[3:]
        if todo:
          print(' ', end='')

      # Print any remaining bytes
      print(''.join('{:02x}'.format(b) for b in bytes[0:todo]), end='')

      print()
      return size

  def invoke(self, arg, from_tty):
    args = arg.split()
    if len(args) != 2:
      print("xxd: <addr> <count>")
      return
    size = int(args[1])
    addr = gdb.parse_and_eval(args[0])
    inferior = gdb.inferiors()[0]
    bytes = inferior.read_memory(addr, size).tobytes()

    offset = int(addr)
    while size > 0:
      n = self._PrintLine(offset, bytes, min(len(bytes), 16))
      size -= n
      offset += n
      bytes = bytes[n:]

XXD()

Use it like so:

// Sample program x.c
char foo[] = "abcdefghijklmopqrstuvwxyz";
int main() { return 0; }

gcc -g x.c
gdb -q ./a.out

(gdb) source xxd.py
Temporary breakpoint 1, main () at x.c:3
3       int main() { return 0; }

(gdb) xxd &foo[0] 18
00404030: 61626364 64656667 6768696a 6a6b6c6d
00404040: 7273