Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Sun, 10 Feb 2019 09:57:53 -0500
From: Rich Felker <>
To: Alexey Izbyshev <>
Subject: Re: __synccall: deadlock and reliance on racy /proc/self/task

On Sun, Feb 10, 2019 at 03:15:55PM +0300, Alexey Izbyshev wrote:
> On 2019-02-10 04:20, Rich Felker wrote:
> >On Sun, Feb 10, 2019 at 02:16:23AM +0100, Szabolcs Nagy wrote:
> >>* Rich Felker <> [2019-02-09 19:52:50 -0500]:
> >>> Maybe it's salvagable though. Since __block_new_threads is true, in
> >>> order for this to happen, tid J must have been between the
> >>> __block_new_threads check in pthread_create and the clone syscall at
> >>> the time __synccall started. The number of threads in such a state
> >>> seems to be bounded by some small constant (like 2) times
> >>> libc.threads_minus_1+1, computed at any point after
> >>> __block_new_threads is set to true, so sufficiently heavy presignaling
> >>> (heavier than we have now) might suffice to guarantee that all are
> >>> captured.
> >>
> >>heavier presignaling may catch more threads, but we don't
> >>know how long should we wait until all signal handlers are
> >>invoked (to ensure that all tasks are enqueued on the call
> >>serializer chain before we start walking that list)
> >
> >That's why reading /proc/self/task is still necessary. However, it
> >seems useful to be able to prove you've queued enough signals that at
> >least as many threads as could possibly exist are already in a state
> >where they cannot return from a syscall with signals unblocked without
> >entering the signal handler. In that case you would know there's no
> >more racing going on to create new threads, so reading /proc/self/task
> >is purely to get the list of threads you're waiting to enqueue
> >themselves on the chain, not to find new threads you need to signal.
> Similar to Szabolcs, I fail to see how heavier presignaling would
> help. Even if we're sure that we'll *eventually* catch all threads
> (including their future children) that were between
> __block_new_threads check in pthread_create and the clone syscall at
> the time we set __block_new_threads to 1, we still have no means to
> know whether we reached a stable state. In other words, we don't
> know when we should stop spinning in /proc/self/task loop because we
> may miss threads that are currently being created.

This seems correct.

> Also, note that __pthread_exit() blocks all signals and decrements
> libc.threads_minus_1 before exiting, so an arbitrary number of
> threads may be exiting while we're in /proc/self/task loop, and we
> know that concurrently exiting threads are related to misses.

This too -- there could in theory be unboundedly many threads that
have already decremented threads_minus_1 but haven't yet exited, and
this approach has no way to ensure that we wait for them to exit
before returning from __synccall.

I'm thinking that the problems here are unrecoverable and that we need
the thread list.


Powered by blists - more mailing lists

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.