From: Sebastian Gottschall <s.gottschall@newmedia-net.de>
To: musl@lists.openwall.com
Subject: [musl] race condition in sem_wait
Date: Wed, 19 Feb 2020 00:29:33 +0100 [thread overview]
Message-ID: <c06792ad-859d-5514-96fe-c97564596fe5@newmedia-net.de> (raw)
In-Reply-To: <20200218222325.la2lz2yiny6rm47u@gentoo-zen2700x>
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
next prev parent reply other threads:[~2020-02-19 12:00 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-18 19:38 [musl] Locale support considered harmful noise Jacob Welsh
2020-02-18 21:42 ` A. Wilcox
2020-02-18 22:23 ` Hadrien Lacour
2020-02-18 23:29 ` Sebastian Gottschall [this message]
2020-02-19 0:46 ` [musl] race condition in sem_wait Sebastian Gottschall
2020-02-19 3:39 ` Rich Felker
2020-02-19 8:26 ` Sebastian Gottschall
2020-02-19 14:13 ` Rich Felker
2020-02-19 4:07 ` Bobby Bingham
2020-02-19 21:28 ` [musl] Locale support considered harmful noise Jacob Welsh
2020-02-19 22:06 ` Rich Felker
2020-02-19 3:36 ` Rich Felker
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