[Bug 52213] Thread crashes when pthread_exit is called in a SIGQUIT handler

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=52213

--- Comment #11 from Giovanni Mascellani <gmascellani at codeweavers.com> ---
I think I'm getting an idea of what is happening here: when a thread is
started, its stack pointer is stored in amd64_thread_data()->exit_frame by
signal_start_thread() and syscall_frame is used whenever the execution flow is
in a UNIX library. When exiting a thread, signal_exit_thread() resets the stack
to the exit_frame and then calls pthread_exit().

The problem is that pthread_exit() doesn't like this arrangement: basically the
only thing pthread_exit() does is unwinding the thread's stack, so that all
cleanup procedures, registered for example with pthread_cleanup_push(), are
executed. But since we already unwound the stack under its feet (and possibly
rewrote a part of it inside the SIGQUIT handler), this doesn't work anymore.

In the particular case on my test program, inside pthread_cond_wait() a cleanup
procedure is established (to properly abort the wait if the stack is unwound),
and since its address is overwritten inside the SIGQUIT handler, a segmentation
fault happens.

I am not sure of what is the correct way to fix this problem. I'd be tempted to
say that we can do away with exit_frame and just let pthread unwind the stack,
but I am not sure what exit_frame is supposed to help with. Why aren't we using
the stack allocated by pthread, instead of transitioning to our own syscall
stack?

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