Re: How to display instructions around the current instuction?

[Stefan Puiu via Gdb <gdb@sourceware.org>]

On Thu, Jan 28, 2021 at 7:06 PM Peng Yu <pengyu.ut@gmail.com> wrote:
>
> Hi Stefan,
>
> > display/10i $rip-10
> >
> > I don't think x86-64 instructions have fixed width, so that won't
> > always print the same number of instructions before the current one,
> > but it's probably a good approximation.
>
> Have you tried it? If $rip-5 is not right at the start of an

Well, I said "Have you tried...?", not "here, that's the answer". But
since you ask so nicely, yes, I've tried it. It's something I use
often on Linux, and it works. In fact, I've just tried it with a
simple program and it works. On my Linux (Ubuntu 20.04 x86-64).

But wait, you have MacOS; you didn't mention that in your initial email.

> instruction, it does not even disassmble correctly. Please test your
> recommendation before sending it out. It is not appropriate to waste
> other people's time.

Please don't be rude to people who are trying to help you. And what do
you know, there's plenty of wrong answers on stackoverflow, and plenty
that haven't been deleted. You still have to figure them out.

>
> (gdb) 1: x/10i $rip-5
> <error: No registers.>
> (gdb) Temporary breakpoint 1 at 0x401134
> Starting program: /mnt/macosx/pengy/mrrdir/test/gnu/gdb/cmd/display/main.exe
>
> Temporary breakpoint 1, 0x0000000000401134 in main ()
> 1: x/10i $rip-5
>    0x40112f <frame_dummy+15>:   add    BYTE PTR [rbp+0x48],dl
>    0x401132 <main+2>:   mov    ebp,esp
> => 0x401134 <main+4>:   sub    rsp,0x10
>    0x401138 <main+8>:   movabs rdi,0x402004
>    0x401142 <main+18>:  mov    al,0x0
>    0x401144 <main+20>:  call   0x401030 <printf@plt>
>    0x401149 <main+25>:  xor    ecx,ecx
>    0x40114b <main+27>:  mov    DWORD PTR [rbp-0x4],eax
>    0x40114e <main+30>:  mov    eax,ecx
>    0x401150 <main+32>:  add    rsp,0x10
> (gdb) 0x0000000000401138 in main ()
> 1: x/10i $rip-5
>    0x401133 <main+3>:   in     eax,0x48
>    0x401135 <main+5>:   sub    esp,0x10
> => 0x401138 <main+8>:   movabs rdi,0x402004
>    0x401142 <main+18>:  mov    al,0x0
>    0x401144 <main+20>:  call   0x401030 <printf@plt>
>    0x401149 <main+25>:  xor    ecx,ecx
>    0x40114b <main+27>:  mov    DWORD PTR [rbp-0x4],eax
>    0x40114e <main+30>:  mov    eax,ecx
>    0x401150 <main+32>:  add    rsp,0x10
>    0x401154 <main+36>:  pop    rbp
> (gdb) 0x0000000000401142 in main ()
> 1: x/10i $rip-5
>    0x40113d <main+13>:  add    BYTE PTR [rax],al
>    0x40113f <main+15>:  add    BYTE PTR [rax],al
>    0x401141 <main+17>:  add    BYTE PTR [rax-0x1181800],dh
>    0x401147 <main+23>:  (bad)
>    0x401148 <main+24>:  push   QWORD PTR [rcx]
>    0x40114a <main+26>:  leave
>    0x40114b <main+27>:  mov    DWORD PTR [rbp-0x4],eax
>    0x40114e <main+30>:  mov    eax,ecx
>    0x401150 <main+32>:  add    rsp,0x10
>    0x401154 <main+36>:  pop    rbp
>
> --
> Regards,
> Peng