Change on rax register during debug session with gdb does not affect code execution

Question

I'm planning to participate to some of the Capture the flags (CTF) challenges, in the near future. For that reason, I've decided to study assembly. As of now I'm focusing on the usage of the CPU registers. Following some examples that I have found on internet, I tried to debug a very simple "Hello World" program written in C, to see how the CPU registers are used. My environment is Linux and GCC version 11. I compiled my code with the -g flag, in order to include debug symbols.

Following is my very simple C source code:

#include <iostream>

int main (int argc, char** argv)
{
  char message_c_str[] = "Hello World from C!";
  printf("%s\n", message_c_str);

  return 0;
}

Studying the disassembly of the main function, I understand that the string containing the message gets stored inside the RAX (and RDX registers?), before calling the printf function:

└─$ objdump -M intel -D main| grep -A20 main.:
0000000000001159 <main>:
  1159:       55                      push   rbp
  115a:       48 89 e5                mov    rbp,rsp
  115d:       48 83 ec 30             sub    rsp,0x30
  1161:       89 7d dc                mov    DWORD PTR [rbp-0x24],edi
  1164:       48 89 75 d0             mov    QWORD PTR [rbp-0x30],rsi
  1168:       48 b8 48 65 6c 6c 6f    movabs rax,0x6f57206f6c6c6548
  116f:       20 57 6f 
  1172:       48 ba 72 6c 64 20 66    movabs rdx,0x6d6f726620646c72
  1179:       72 6f 6d 
  117c:       48 89 45 e0             mov    QWORD PTR [rbp-0x20],rax
  1180:       48 89 55 e8             mov    QWORD PTR [rbp-0x18],rdx
  1184:       c7 45 f0 20 43 21 00    mov    DWORD PTR [rbp-0x10],0x214320
  118b:       48 8d 45 e0             lea    rax,[rbp-0x20]
  118f:       48 89 c7                mov    rdi,rax
  1192:       e8 b9 fe ff ff          call   1050 <puts@plt>
  1197:       b8 00 00 00 00          mov    eax,0x0
  119c:       c9                      leave  
  119d:       c3                      ret

I thought to start a debug session and try to change the RAX on the fly, just for the sake of seeing if I was able to change the string content before printing it on the command line. Unfortunately, even though it seems that I can change the RAX value, the program still prints the hard coded message. So, I'm not sure why I cannot change it. Am I missing to run any gdb command after updating the value of RAX?

Following is my debug session with the issue:

┌──(alexis㉿kali)-[~/Desktop/Hacking/hello_world]
└─$ gdb -q main
Reading symbols from main...
(gdb) break main
Breakpoint 1 at 0x1168: file /home/alexis/Desktop/Hacking/hello_world/main.cpp, line 5.
(gdb) run
Starting program: /home/alexis/Desktop/Hacking/hello_world/main 

Breakpoint 1, main (argc=1, argv=0x7fffffffdf58) at 
/home/alexis/Desktop/Hacking/hello_world/main.cpp:5
5           char message_c_str[] = "Hello World from C!";
(gdb) info register rax
rax            0x555555555159      93824992235865
(gdb) next
6           printf("%s\n", message_c_str);
(gdb) info register rax
rax            0x6f57206f6c6c6548  8022916924116329800
(gdb) set $rax=0x6361636361
(gdb) info register rax
rax            0x6361636361        426835665761
(gdb) next
Hello World from C!
8           return 0;
(gdb) 

You can see that the code still prints "Hello World from C!", even if the RAX register changed. Why?

Answer 1

The string is only temporarily in rax+rdx. In the following lines it is placed on the stack and the address goes to rdi, that is used by puts.

What's important here is to understand that one line of source code is translated to multiple lines of assembly. When you change the rax on line printf("%s\n", message_c_str); the string is already pushed on the stack and rax only keeps an old value as it wasn't overwritten by anything. It is no longer the string that's being printed.

To accomplish your goal you would have to change the string on the stack or change it in rax before it's being pushed onto it (so before your next command).

Also be aware that next advances one source code line. If you want to move one assembly instruction use nexti - with that you have more control about what gets executed.

Answer 2

You're using next (whole block of asm corresponding to a C source line), not nexti or stepi (aka ni or si) to step by asm instruction.

And you made a debug build so GCC doesn't keep anything in registers across C statements. The points where execution stops with next are the ones where the compiler-generated instructions are about to load or LEA a new RAX, so its current value is dead and doesn't matter.

(And it's only using RAX at all because it's a debug build with GCC; otherwise things like lea rax,[rbp-0x20] / mov rdi,rax would LEA straight into RDI, instead of uselessly using RAX as a temporary. Return value from writing an unused parameter when falling off the end of a non-void function Or for mov-immediate to memory, there's no mov r/m64, imm64, only to register, so those moves to RAX and RDX do make sense.)

---

If you wanted to have it print something different, you could si until after movabs rax,0x6f57206f6c6c6548 but before mov QWORD PTR [rbp-0x20],rax, and at that point change the initializer for part of the string data. (Which is in RAX at that point.) e.g. introducing a 0x00 byte will terminate the C string.

Or right before the call puts, you could set $rdi = $rdi+5 to be like puts(message_c_str + 5).

layout reg or layout asm (use layout next / prev to fix the display if its broken) are helpful for seeing where execution is. See other GDB asm tips at the bottom of https://stackoverflow.com/tags/x86/info