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

Just want to emphasize a related answer. Let's say that you're root and you've done:

strace -p 700

and get:

strace: attach: ptrace(PTRACE_SEIZE, 700): Operation not permitted

Check:

cat /proc/700/status | grep TracerPid

If you see something like TracerPid: 12, i.e. not zero, that's the PID of the program that is already using the ptrace system call. Both gdb and strace use it, and there can only be one active at a time.