(GDB) Breakpoints and Disassemble

I’ve been recently interested in reading books and articles about hacking and I found out that Hacking:The art of exploitation is just a must read title. I am following the basic tutorials on how to work with standard Linux tools and analyze your code (Programming chapter). I am not a beginner in programming but working with Linux terminal is quite new for me. I am using the latest release of Kali Linux.

Right now my simple program below should be used to analyze how stack segment works.

int main(){
  void stack_func(int a,int b, int c, int d){
    char first;
    int second;

    first = 'c';
    second = 220;
  }

  stack_func(1,2,3,4);
  return 0;
}

The first problem is I cannot add any breakpoints for internal functions. Neither mine functions like stack_func() nor functions from libraries like strcpy etc. According to the book the pending breakpoint should resolve. Mine is just ignored and program finishes.

root@root:~/Folder# gdb -q ./stack
Reading symbols from ./stack...done.
(gdb) b stack_func
Function "stack_func" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (stack_func) pending.
(gdb) run
Starting program: /root/Folder/stack 
[Inferior 1 (process 20421) exited normally]
(gdb)  

The second problem is that disassemble also doesn’t work for my function. Again according to the book I should be able to see assembler code for my function stack_func() but the result is below.

(gdb) disass stack_func()
No symbol "stack_func" in current context.
(gdb)  

I appologize for any grammatical errors in text. 🙂

Answer

The problem is that you defined stack_func inside another function. This is called nested function and it is gcc extension in GNU C. This function has a bit other symbol name than you expect. To find out it’s exact symbol name you can use nm tool:

[ tmp]$ nm a.out |grep stack_func
00000000004004a6 t stack_func.1761

And set breakpoint and disassemble in gdb:

[ tmp]$ gdb -q ./a.out 
Reading symbols from ./a.out...done.
(gdb) b 'stack_func.1761'
Breakpoint 1 at 0x4004ba: file 111.c, line 6.
(gdb) disassemble 'stack_func.1761'
Dump of assembler code for function stack_func:
   0x00000000004004a6 <+0>: push   %rbp
   0x00000000004004a7 <+1>: mov    %rsp,%rbp
   0x00000000004004aa <+4>: mov    %edi,-0x14(%rbp)
   0x00000000004004ad <+7>: mov    %esi,-0x18(%rbp)
   0x00000000004004b0 <+10>:    mov    %edx,-0x1c(%rbp)
   0x00000000004004b3 <+13>:    mov    %ecx,-0x20(%rbp)
   0x00000000004004b6 <+16>:    mov    %r10,-0x28(%rbp)
   0x00000000004004ba <+20>:    movb   $0x63,-0x1(%rbp)
   0x00000000004004be <+24>:    movl   $0xdc,-0x8(%rbp)
   0x00000000004004c5 <+31>:    nop
   0x00000000004004c6 <+32>:    pop    %rbp
   0x00000000004004c7 <+33>:    retq   
End of assembler dump.
(gdb)