objdump and gdb spits different assembly results

Question

you'll need to save the image to read the instructions in the image

well, here's the problem.

I'm working on a bomblab just for killtime and I'm stuck with the issue

of different assembly code generation of objdump and gdb.

my laptop is running 64-bit Ubuntu 14.04 LTS and

I've installed 32bit glibc to work on bomblab.(since it's compiled in 32-bit env)

well, first the symbols aren't showing and I don't really get what gdb is spitting out.

Can someone explain me what's going on?

Answer 1

When you placed a breakpoint at phase_2, gdb patched the running image to place an int3 instruction at 0x8048b50, which is just after phase_2's function prologue has finished setting up the stack frame. So instead of

0x8048b50: 8b  mov edx,DWORD PTR [ebp+0x8]
0x8048b51: 55
0x8048b52: 08

you now have

0x8048b50: cc  int3
0x8048b51: 55  push ebp
0x8048b52: 08  first byte of an 'or' instruction

and that is what gdb is displaying. Eventually things get synced back up and the disassemble command starts to display the correct series of instructions.

The difference in what is displayed won't affect correct execution of the program; before proceeding from the breakpoint, gdb will either place that 8b byte back into the image or it will synthetically execute the mov instruction and then continue execution at 0x8048b53.