How to solve “ptrace operation not permitted” when trying to attach GDB to a process?

Question

I\'m trying to attach a program with gdb but it returns:

Attaching to process 29139
Could not attach to process. If your uid matches the ui

Answer 1

I was going to answer this old question as it is unaccepted and any other answers are not got the point. The real answer may be already written in /etc/sysctl.d/10-ptrace.conf as it is my case under Ubuntu. This file says:

For applications launching crash handlers that need PTRACE, exceptions can be registered by the debugee by declaring in the segfault handler specifically which process will be using PTRACE on the debugee: prctl(PR_SET_PTRACER, debugger_pid, 0, 0, 0);

So just do the same thing as above: keep /proc/sys/kernel/yama/ptrace_scope as 1 and add prctl(PR_SET_PTRACER, debugger_pid, 0, 0, 0); in the debugee. Then the debugee will allow debugger to debug it. This works without sudo and without reboot.

Usually, debugee also need to call waitpid to avoid exit after crash so debugger can find the pid of debugee.