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Message-ID: <c06792ad-859d-5514-96fe-c97564596fe5@newmedia-net.de>
Date: Wed, 19 Feb 2020 00:29:33 +0100
From: Sebastian Gottschall <s.gottschall@...media-net.de>
To: musl@...ts.openwall.com
Subject: race condition in sem_wait
Hello
i discovered recently a race condition while playing with threads and
sem_wait/sem_post
sem_wait may fail with errno set EAGAIN which is not valid since only
sem_trywait is able to set that errno code.
this was causing a bug with a later select() and accept() which failed
since accept does not work if errno is set to EAGAIN.
from my point of view the bug is in sem_timedwait.c
if (!sem_trywait(sem)) return 0;
int spins = 100;
while (spins-- && sem->__val[0] <= 0 && !sem->__val[1]) a_spin();
while (sem_trywait(sem)) {
the fist sem_trywait will fail with -1 and sets EAGAIN. but the second
sem_trywait will not fail and does return 0. the problem now is that
errno is still present and not reset.
this may cause if sem_post is called from a second thread on the same
semaphore.
of course the same bug affects sem_timedwait itself.
so i assume sem_wait is not thread safe which is bad and is not follow
the posix specification
or am i wrong here?
Sebastian
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