CODEWITHSUNDEEP
cdebugginggdb
user10598786
user10598786

Reputation:

GDB skips shared library breakpoint

Im using a debugger with a simple C program, im trying to set a breakpoint with a shared library, but GDB skips this breakpoint entirely.

Im trying to use GDB with a simple C program to learn about GDB. I set 3 breakpoints, 1 at line 7, one at the strcpy function, and one at line 8. I try to set a breakpoint in my program involving a shared library (specifically "break strcpy"), but every time I run the program and press "c", the program skips breakpoint 2 entirely

#include <stdio.h>
#include <string.h>

int main() {
    char str_a[20];

    strcpy(str_a, "Hello, world!\n");
    printf(str_a);
}

Whenever I run the program in the debugger, it stops normally at breakpoint 1, which is expected, but then whenever I press "c" to continue to breakpoint 2, it skips breakpoint 2 entirely and just shows the output breakpoint 3 is supposed to have. Is this something to do with GDB's handling of shared libraries?

EDIT: Here is the disassembly

    0x0000555555555145 <+0>:    push   rbp
    0x0000555555555146 <+1>:    mov    rbp,rsp
    0x0000555555555149 <+4>:    sub    rsp,0x20
    0x000055555555514d <+8>:    lea    rax,[rbp-0x20]
    0x0000555555555151 <+12>:   lea    rsi,[rip+0xeac]        # 0x555555556004
    0x0000555555555158 <+19>:   mov    rdi,rax
    0x000055555555515b <+22>:   call   0x555555555030 <strcpy@plt>
    0x0000555555555160 <+27>:   lea    rax,[rbp-0x20]
    0x0000555555555164 <+31>:   mov    rdi,rax
    0x0000555555555167 <+34>:   mov    eax,0x0
    0x000055555555516c <+39>:   call   0x555555555040 <printf@plt>
    0x0000555555555171 <+44>:   mov    eax,0x0
    0x0000555555555176 <+49>:   leave  
    0x0000555555555177 <+50>:   ret

Upvotes: 0

Views: 853

Answers (1)

Employed Russian
Employed Russian

Reputation: 213375

You didn't specify your platform. I suspect it's Linux with GLIBC.

The reason GDB behaves this way is that strcpy is not a normal function, but a GNU IFUNC.

Try setting breakpoint on __strcpy_sse2_unaligned and see this answer.

Update:

the debugger spits out this error whenever it reaches breakpoint 2, "../sysdeps/x86_64/multiarch/strcpy-sse2-unaligned.S: No such file or directory.

  1. That isn't an error.
  2. The fact that it reaches that breakpoint confirms that this answer is correct.
  3. You can simply treat __strcpy_sse2_unaligned as an alias to strcpy. Setting a breakpoint there is (on your system) equivalent to setting it on strcpy.

Upvotes: 1

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