GBD Breakpoints

Software Breakpoint.

Hardware Breakpoint.

break

Linespec Location
break 10 Line 10 current file
break hello.c::10 Line 10, file hello.c
break -3 Current line -3
break +3 Current line +3
break sum Function sum
break hello.c::sum Function sum in hello.c
break A::sum Function A::sum
Explicit Location
break -qualified sum Match sum, not A::sum
Address Location
break *sum+16 Instruction 16 in sum
break 0x0x400670 Instruction at 0x400670
Regex Location
rbreak sum.* Function sum.*
rbreak demo.c:sum.* Function sum.* in hello.c
rbreak . All function in current program
rbreak demo.c:. All function in file
Condition
break sum if x > 3 Conditional
break sum inferior 1 Inferior specific
break sum inferior 1 if x > 3 Inferior + condition
break sum thread 1 Thread specific
break sum thread 1 if x > 3 Thread + condition
condition 2 if x > 5 Change condition of break 2
Command
command Command for current break
command 2 Command for break 2 
command 2
    info args
    backtrace
    continue
end
Temporary Break
tbreak sum Auto delete after hit
Management
info break List of breakpoints
enable break [ids] Enable breakpoints
disable break [ids] Disable breakpoints
delete break [ids] Delete breakpoints
clear <locspec> Delete all breakpoints at locspec
save break <file> Save breakpoints to file
source <file> Load breakpoints from file

hbreak

Set hbreak at a function in a mmap region.

File math.c

int sum(int x, int y)
{
    return x + y;
}

int sub(int x, int y)
{
    return x - y;
}

File main.c

#include <sys/mman.h>
#include <stddef.h>
#include <fcntl.h>
#include <stdio.h>

int main()
{
    // load math.o to a new memory region
    int fd = open("math.o", O_RDONLY);
    void *addr = mmap(
        NULL, // kernel choose start addr for region
        4096, // length of the region
        PROT_READ|PROT_EXEC,    // protection mode
        MAP_PRIVATE,            // visible mode
        fd,                     // file to load
        0                       // offset in file
    );

    // find address of .text segment
    void *text = addr + 0x0040;

    // find address of sum and sub functions
    void *sum = text + 0x0000;
    void *sub = text + 0x0018;

    // call sum(5, 4)
    int isum = ((int (*)(int, int))sum)(5, 4);

    // call sub(5, 4)
    int isum = ((int (*)(int, int))sub)(5, 4);

    printf("isum=%d, isub=%d\n", isum, isum);
    // output: isum=9, isum=1

    return 0;
}

Run.

$ gcc -c math.c -o math.o
$ gcc -g main.c -o main
$ ./main

Inspect math.o

$ readelf --section-headers --wide math.o
  [Nr] Name     Type        Address             Off     Size    ES  Flg Lk Inf Al
  [ 1] .text    PROGBITS    0000000000000000    000040  00002e  00  AX  0   0  1

$ readelf --symbols math.o
   Num:    Value          Size Type    Bind   Vis      Ndx Name     
     3: 0000000000000000    24 FUNC    GLOBAL DEFAULT    1 sum
     4: 0000000000000018    22 FUNC    GLOBAL DEFAULT    1 sub

Inspect process memory.

(gdb) break main.c:27
(gdb) run
(gdb) info proc mappings
      Start Addr           End Addr       Size     Offset  Perms  objfile
  0x7ffff7ffa000     0x7ffff7ffb000     0x1000        0x0  r-xp   math.o

Illustrate math.o and process memory.

+---------------------+
|       ........      |
+---------------------+
| .text               | 0x0040
|  + sum()            |         + 0x0000
|  + sub()            |         + 0x0018
+---------------------+
+---------------------+
|       ........      |
+---------------------+
| math.o              | 0xfffff7ff5000
|   .text             | 0xfffff7ff5040
|   ├── sum()         |         + 0x0000
|   └── sub()         |         + 0x0018
+---------------------+

Add symbols and hbreak.

(gdb) add-symbol-file math.o 0x7ffff7ffa040
(gdb) info function sub
0x00007ffff7ffa058  sub

(gdb) hbreak sub
Hardware assisted breakpoint 5 at 0x7ffff7ffa060

(gdb) continue
Breakpoint 5, 0x00007ffff7ffa060 in sub ()

(gdb) backtrace
#0  0x00007ffff7ffa060 in sub ()
#1  0x000055555555521d in main () at main.c:30

Reference