Setting different breakpoints in GDB while debugging

This post, is in continuation with “Using GDB to debug functions defined in another file in C” from where will be using add.c and helloworld.c as source code and “Debugging ARM binary on Ubuntu Host using qemu ARM and GDB” from where we will be using the debugging environment for ARM.

so, lets assume we have followed both the above posts and we have helloworld as ARM ELF binary, then start gdb

$ arm-linux-gnueabihf-gcc -g -c -o helloworld_src/add.o helloworld_src/add.c

$ arm-linux-gnueabihf-gcc -g -o helloworld helloworld_src/helloworld.c helloworld_src/add.o  -static

Run the qemu ARM in one terminal as,

$ qemu-arm -L gcc-arm-none-eabi-10-2020-q4-major/lib -g 8090 ./helloworld

On second terminal start GDB

$ arm-none-eabi-gdb

(gdb) target remote:8090

Set Breakpoint using Line Number

You can set breakpoints using line number of the source code in that file, for that you must compile your binary with debugging symbols enabled, which we did using “-g” with gcc.

Check the line numbers of the source file using “li” gdb command as,

gdb) li
warning: Source file is more recent than executable.
1	#include <stdio.h>
2	
3	extern int add_two_numbers(int, int);
4	
5	int main(int argc, char **argv) {
6		int num = 0;
7		printf("helloworld\n");
8		num = add_two_numbers(3, 2);
9		printf("addition: %d\n", 3+2);
10		return 0;
(gdb) 
11	}
(gdb) 

Now, lets say you want to set breakpoint at line number 8, the type “break 8” or simply “br 8” to set breakpoint using line number i.e. “break line_number”

(gdb) br 8
Breakpoint 1 at 0x1043c: file helloworld_src/helloworld.c, line 9.

(gdb) c
Continuing.

Breakpoint 1, main (argc=1, argv=0xfffef184) at helloworld_src/helloworld.c:9
9		printf("addition: %d\n", 3+2);

As you can see, it set the breakpoint at the next valid statement on line number 9 and our program stopped at that line number after we pressed c i.e. continue.

Set Breakpoint at main

(gdb) break main
Breakpoint 1 at 0x1042e: file helloworld_src/helloworld.c, line 6.

(gdb) c
Continuing.

Breakpoint 1, main (argc=1, argv=0xfffef184) at helloworld_src/helloworld.c:6
warning: Source file is more recent than executable.
6		int num = 0;

Setting Breakpoint using Memory Address

First, we need to check the memory address using objdump as,

$ arm-linux-gnueabihf-objdump -D helloworld > helloworld_objdump.txt

then, we can check the address of main and code inside by reading from helloworld_object.txt as

00010424 <main>:
   10424:       b580            push    {r7, lr}
   10426:       b084            sub     sp, #16
   10428:       af00            add     r7, sp, #0
   1042a:       6078            str     r0, [r7, #4]
   1042c:       6039            str     r1, [r7, #0]
   1042e:       2300            movs    r3, #0
   10430:       60fb            str     r3, [r7, #12]
   10432:       4b08            ldr     r3, [pc, #32]   ; (10454 <main+0x30>)
   10434:       447b            add     r3, pc
   10436:       4618            mov     r0, r3
   10438:       f008 fc90       bl      18d5c <_IO_puts>
   1043c:       2105            movs    r1, #5
   1043e:       4b06            ldr     r3, [pc, #24]   ; (10458 <main+0x34>)
   10440:       447b            add     r3, pc
   10442:       4618            mov     r0, r3
   10444:       f004 faac       bl      149a0 <_IO_printf>
   10448:       2300            movs    r3, #0
   1044a:       4618            mov     r0, r3
   1044c:       3710            adds    r7, #16
   1044e:       46bd            mov     sp, r7
   10450:       bd80            pop     {r7, pc}
   10452:       bf00            nop
   10454:       00039950        andeq   r9, r3, r0, asr r9
   10458:       00039950        andeq   r9, r3, r0, asr r9

As we can see above, we have a code where its printing using printf, and address is, 0x00043c , so we can set the breakpoint at this address using “star zero x” and then address as,

(gdb) br *0x0001043c
Breakpoint 1 at 0x1043c: file helloworld_src/helloworld.c, line 9.

Now, if we do continue, we can see our program stopped just before printf,

(gdb) c
Continuing.

Breakpoint 1, main (argc=1, argv=0xfffef184) at helloworld_src/helloworld.c:9
warning: Source file is more recent than executable.
9		printf("addition: %d\n", 3+2);