Why can't gdb read memory if pwntools is used to send input?

Question

Here is the program which gdb is attached to:

prog.c

#include <stdio.h>

void dummy(char* s)
{

}

int main()
{
    char buf[512];
    scanf("%s", buf);
    printf("%s\n", buf);
    dummy(buf);
    return 0;
}

It is compiled with: gcc prog.c o prog

This is the script which drives the program:

from pwn import *

p = process("./prog")
raw_input('>>')
p.sendline('A')

Here's the sequence of operation I perform:

1. Run the script in one bash tab. It launches prog
2. In another bash tab: sudo gdb -p `pgrep prog`. gdb attaches itself to the running process
3. Set a breakpoint on dummy call in gdb: b dummy
4. Press c in gdb to continue
5. Hit Enter in the script to continue
6. gdb gives up by saying: 0x000056446a5af764 <dummy+4>: Cannot access memory at address 0x56446a5af764

If instead of feeding the input programmatically, I launch the program manually, attach gdb and feed the input myself, the breakpoint is correctly hit.

What is the problem in the script?

Answer 1

The process dies before/while gdb connects to it, as your python script finishes. Use

p.interactive()

at the end of your script to keep it running.