08. Using a debugger

Debugger

an instrument of 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:

0. Example program:

      1 #include <stdio.h>
      2 #define SIZE 10
      3 
      4 int A[SIZE] = {7,6,5,4,3,2,1,0,1,2};
      5 int B[SIZE];
      6 
      7 void fun(int *a, int *b, int len) {
      8         int i;
      9 
     10         for(i=0; i<len; i++)
     11                 b[i] = a[i];
     12 }
     13 
     14 int main(int argc, char *argv[]) {
     15         fun(A, B, 100500);
     16         return 0;
     17 }
   

1. start with gdb binaryprog

2. 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.

3. run program until breakpoint is reached with r

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

4. 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++)

5. 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

6. 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)

7. 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

8. 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

• [user@sugon]$ gdb arrr
  ...
  (gdb) b fun
  Breakpoint 1 at 0x4006c8: file arrr.c, line 10.
  (gdb) r
  Starting program: /home/george/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 №

GDB documentation

• also u — execute code until block or function ends

• also there are watchpoints — program stops after a variable or expression changed

Other debuggers

On sugon:

• gdb -tui binaryprog — with window of code, console

On local machine (linux):

• cgdb binaryprog — almost like gdb -tui, but better, console

• nemiver binaryprog — simple graphical debugger

  • try it

  • find analogs of n, s, u, breakpoint etc.

• [[gdbgui|https://www.gdbgui.com] — python logic, JS UI through a browser

  • installing:

      $ pip3 install gdbgui --user
      (installation protocol)

  • using:

      $ firefox & # (or snother browser)
      $ ~/.local/bin/gdbgui binaryprog

  • binary is located at /home/<username>/.local/bin, you need to either add this path ti $PATH with PATH=$PATH:$HOME/local/bin or use full path to start as displayed above

H/W

Pick up your favorite debugger and learn how to use it