Re: [PATCH 0/2] fix the traced mt-exec deadlock

ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman) writes:

ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman) writes:

quoted

The big lesson for me, and what was not obvious from your change
description is that we are changing the user space visible semantics
of exec+ptrace and that cred_guard_mutex is not at all the problem (as
we always take cred_guard_mutex in a killable or interruptible way).

Just to follow up.

Because the cred_guard_mutex is fine as is we don't need to move
de_thread out from under cred_guard_mutex.  We just need to change
de_thread to wait until all of the other threads are zombies.
Which should remove about half your proposed patch.

The other key thing is that knowning it isn't cred_guard_mutex let's us
know that this kind of deadlock goes all of the way back to when
CLONE_THREAD was merged into the kernel.

Insteresingly enough looking at zap_other_threads and notify_count I
have found a second bug.  When a multi-threaded processes becomes a
zombie we don't send the notification to the parent process until the
non-leader threads have been reaped.  Which means ptrace can mess up
sending SIGCHLD to the parent.

Bah. I was misreading the code.  Nothing but exec uses notify_count
and group_exit_task.

Eric