Using Debuggers

Debuggers

Debugger is a tool for dynamic analysis program execution path and memory usage

• High-level: step-by-step C source execution and variable inspection
• Low-level: assembler instructions execution and memory/register dump

Compiling for debug: cc -O0 -g program.c -o binaryprog

• -O0 — turn off optimization. The higher level of optimization is, the more “reorganization” of assembler code is done. Debugging optimized binary is tricky, because there is no strict correspondence of C source code and result assembler code. More than that, some unneeded constructions usually “optimized out”
• -g — include C source references in binary file (you still need the source file itself)
• -o binaryprog — set result binary file name

Debugging with GDB`

1. Example program:

   #include <stdio.h>
   #define SIZE 10
   
   int A[SIZE] = {7,6,5,4,3,2,1,0,1,2};
   int B[SIZE];
   
   void fun(int *a, int *b, int len) {
       int i;
   
       for(i=0; i<len; i++)
           b[i] = a[i];
   }
   
   int main(int argc, char *argv[]) {
       fun(A, B, 100500);
       return 0;
   }
   

2. Start with gdb binaryprog:

3. First set a breakpoint to main() function (it always exists in C program):

   (gdb) b main
   Breakpoint 1 at 0x400758: file arrr.c, line 15.
   

4. Run program until breakpoint is reached with r:

   (gdb) r
   Starting program: /home/andrewt/src/arrr
   Breakpoint 1, main (argc=1, argv=0x7fbf7854) at arrr.c:15
   15              fun(A, B, 100500);
   

5. Step into function call with s:

   (gdb) s
   fun (a=0x4108c0 <A>, b=0x410950 <B>, len=100500) at arrr.c:10
   10              for(i=0; i<len; i++)
   

6. Run next c code (executing function call as “one” instruction) with n:

   (gdb) n
   11                      b[i] = a[i];
   (gdb) n
   10              for(i=0; i<len; i++)
   (gdb)
   11                      b[i] = a[i];
   (gdb)
   10              for(i=0; i<len; i++)
   (gdb)
   11                      b[i] = a[i];
   

   Also you can just press enter for repeating last command.

7. Print a variable with p:

   (gdb) p a
   $1 = (int *) 0x4108c0 <A>
   (gdb) p B
   $2 = {7, 6, 0, 0, 0, 0, 0, 0, 0, 0}
   (gdb) p i
   $3 = 2
   

   ($1, $2 etc. is the number of expressions printed).

8. Add an expression to display list to display it on all command with display:

   (gdb) display i
   1: i = 2
   (gdb) n
   10              for(i=0; i<len; i++)
   1: i = 2
   (gdb)
   11                      b[i] = a[i];
   1: i = 3
   (gdb)
   10              for(i=0; i<len; i++)
   1: i = 3
   

9. Quit with q:

   (gdb) q
   A debugging session is active.
   
        Inferior 1 [process 10578] will be killed.
   
   Quit anyway? (y or n) y
   

Using Breakpoints

[tatarnikov@akos]$ gdb arrr
...
(gdb) b fun
Breakpoint 1 at 0x4006c8: file arrr.c, line 10.
(gdb) r
Starting program: /home/andrewt/src/arrr
Breakpoint 1, fun (a=0x4108c0 <A>, b=0x410950 <B>, len=100500) at arrr.c:10
10              for(i=0; i<len; i++)
(gdb) bt
#0  fun (a=0x4108c0 <A>, b=0x410950 <B>, len=100500) at arrr.c:10
#1  0x00400774 in main (argc=1, argv=0x7fbf7854) at arrr.c:15
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x004006f8 in fun (a=0x4108c0 <A>, b=0x410950 <B>, len=100500) at arrr.c:11
11                      b[i] = a[i];
(gdb) p a[i]
$1 = 1
(gdb) p b[i]
Cannot access memory at address 0x414000
(gdb) p i
$2 = 3500
(gdb) info breakpoints
Num     Type           Disp Enb Address    What
1       breakpoint     keep y   0x004006c8 in fun at arrr.c:10
        breakpoint already hit 1 time
(gdb) delete 1
(gdb) info breakpoints
No breakpoints or watchpoints.

• bt — show “backtrace”, stack of functions call
• c — continue
• delete № — delete breakpoint №
• also u — execute code until block or function ends
• also there are “watchpoints” — program stops after a variable or expression changed

Other Debuggers

In Linux Ubuntu (SSH):

• gdb -tui binaryprog — with window of code, console
• cgdb binaryprog — almost like gdb -tui, but better, console

GUI:

• nemiver binaryprog — simple graphical debugger

Homework

Pick up your favorite debugger and learn how to use it.

References

• Debugger (Wikipedia)
• GNU Debugger (Wikipedia)
• GDB documentation