Re: ptrace attach in multi-threaded processes

On Mon, Jul 11, 2016 at 06:19:38PM -0700, Mark Johnston wrote:
> Hi,
> 
> It seems to be possible for ptrace(PT_ATTACH) to race with the delivery
> of a signal to the same process. ptrace(PT_ATTACH) sets P_TRACED and
> sends SIGSTOP to a thread in the target process. Consider the case where
> a signal is delivered to a second thread, and both threads are executing
> ast() concurrently. The two threads will both call issignal() and from
> there call ptracestop() because P_TRACED is set, though they will be
> serialized by the proc lock. If the thread receiving SIGSTOP wins the
> race, it will suspend first and set p->p_xthread. The second thread will
> also suspend in ptracestop(), overwriting the p_xthread field set by the
> first thread. Later, ptrace(PT_DETACH) will unsuspend the threads, but
> it will set td->td_xsig only in the second thread. This means that the
> first thread will return SIGSTOP from ptracestop() and subsequently
> suspend the process, which seems rather incorrect.
Why ?  In particular, why delivering STOP after attach, in the described
situation, is perceived as incorrect ?  Parallel STOPs, one from attach,
and other from kill(2), must result in two stops.

The bit about overwriting p_xsig/p_xthread indeed initially sound worrysome,
but probably not too much.  The only consequence of reassigning p_xthread
is the selection of the 'lead' thread in sys_process.c, it seems.

> 
> The above is just a theory to explain an unexpectedly-stopped
> multi-threaded process that I've observed. Is there some mechanism I'm
> missing that prevents multiple threads from suspending in ptracestop()
> at the same time? If not, then I think that's the root of the problem,
> since p_xthread is pretty clearly not meant to be overwritten this way.
Again, why ?

Note the comment 
		 * Just make wait() to work, the last stopped thread
                 * will win.
which seems to point to the situation.

> Moreover, in my scenario I see a thread with TDB_XSIG set even after
> ptrace(PT_DETACH) was called (P_TRACED is cleared).
This is interesting, we indeed do not clear the flag consistently.
But again, the only consequence seems to be a possible invalid reporting
of events.