Using gdb as a monitor?

Question

Can GDB be used as if it was a traditional assembly monitor?

As soon as you step into eg. library code it returns:

No function contains program counter for selected frame

GDB the debugger is able to step into unknown code but GDB the UI stops working.

In this related question you can find a pair of proposed solutions but neither quite satisfies me.

What if the binary library doesn't come with a debugging symbols package? What if the program jumps into run-time generated code?

Disassembling the code isn't really a solution either as the UI ignores it, most importantly the values of registers aren't updated until you return to the original known code. info registers works, but that is hardly interactive.

Any suggestion?

Matthew Slattery · Accepted Answer

You can do this sort of thing with the display command.

display/i $pc will disassemble the current instruction just before the prompt is printed each time:

(gdb) b main
Breakpoint 1 at 0x80483b5: file hw.c, line 5.
(gdb) display/i $pc
(gdb) r
Starting program: /tmp/hw

Breakpoint 1, main () at hw.c:5
5         puts("Hello world");
1: x/i $pc
0x80483b5 :    movl   $0x8048490,(%esp)

Now step an instruction (then just keep hitting Enter to repeat):

(gdb) si
0x080483bc      5         puts("Hello world");
1: x/i $pc
0x80483bc :    call   0x80482d4 
(gdb) 
0x080482d4 in puts@plt ()
1: x/i $pc
0x80482d4 :   jmp    *0x804959c
Current language:  auto; currently asm
(gdb) 
0x080482da in puts@plt ()
1: x/i $pc
0x80482da : push   $0x10
(gdb) 
0x080482df in puts@plt ()
1: x/i $pc
0x80482df :        jmp    0x80482a4 <_init+48>

It still works when we get to this point:

(gdb) 
0x080482a4 in ?? ()
1: x/i $pc
0x80482a4 <_init+48>:   pushl  0x804958c
(gdb) 
0x080482aa in ?? ()
1: x/i $pc
0x80482aa <_init+54>:   jmp    *0x8049590
(gdb) 
0xb7f052d0 in _dl_runtime_resolve () from /lib/ld-linux.so.2
1: x/i $pc
0xb7f052d0 <_dl_runtime_resolve>:       push   %eax

More than one display expression can be active at once (use undisplay to remove them). For example, to watch what happens to %eax:

(gdb) display/x $eax
2: /x $eax = 0xbf90ab34
(gdb) si
0xb7f052d1 in _dl_runtime_resolve () from /lib/ld-linux.so.2
2: /x $eax = 0xbf90ab34
1: x/i $pc
0xb7f052d1 <_dl_runtime_resolve+1>:     push   %ecx
(gdb) 
0xb7f052d2 in _dl_runtime_resolve () from /lib/ld-linux.so.2
2: /x $eax = 0xbf90ab34
1: x/i $pc
0xb7f052d2 <_dl_runtime_resolve+2>:     push   %edx
(gdb) 
0xb7f052d3 in _dl_runtime_resolve () from /lib/ld-linux.so.2
2: /x $eax = 0xbf90ab34
1: x/i $pc
0xb7f052d3 <_dl_runtime_resolve+3>:     mov    0x10(%esp),%edx
(gdb) 
0xb7f052d7 in _dl_runtime_resolve () from /lib/ld-linux.so.2
2: /x $eax = 0xbf90ab34
1: x/i $pc
0xb7f052d7 <_dl_runtime_resolve+7>:     mov    0xc(%esp),%eax

...and here the change in %eax can be seen:

(gdb) 
0xb7f052db in _dl_runtime_resolve () from /lib/ld-linux.so.2
2: /x $eax = 0xb7f0d668
1: x/i $pc
0xb7f052db <_dl_runtime_resolve+11>:    call   0xb7eff780 <_dl_fixup>
(gdb)

Using gdb as a monitor?

Answers (2)

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