Fork and stop a GDB target

Info

GDB script that enables a user to fork of a binary gdb target in a stopped/ suspended state.

I did not find any reference online on how to do this, so I am sharing this method. This can be useful if you want to interfer with the debugged program without it affecting your original debugged session.

How

It instructs the loader to load an additional library that contains code to fork and raises a stop signal on x86-64.

This uses dlopen (3) to load the library at runtime, which depends on libc being present. Loading this library at runtime allows attaching to a running process and still fork it. Alternatively, one could use LD_PRELOAD at load- time- see comment in script to use this.

In GDB, it contains commands to save the register that will be clobbered during execution of that library to the stack and pop these after executing the library code to ensure correct program state.

Usage

I have tested this using version: GNU gdb (Debian 13.1-3) 13.1.

# in this directory
make
gdb ./binary-target

# in gdb
source fork.py

# example usage (still in gdb)
b main
r

fork

# in shell (there should be two now)
pidof binary-target