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coroutine-sigaltstack: Add SIGUSR2 mutex 

Disposition (action) for any given signal is global for the process.
When two threads run coroutine-sigaltstack's qemu_coroutine_new()
concurrently, they may interfere with each other: One of them may revert
the SIGUSR2 handler to SIG_DFL, between the other thread (a) setting up
coroutine_trampoline() as the handler and (b) raising SIGUSR2.  That
SIGUSR2 will then terminate the QEMU process abnormally.

We have to ensure that only one thread at a time can modify the
process-global SIGUSR2 handler.  To do so, wrap the whole section where
that is done in a mutex.

Alternatively, we could for example have the SIGUSR2 handler always be
coroutine_trampoline(), so there would be no need to invoke sigaction()
in qemu_coroutine_new().  Laszlo has posted a patch to do so here:

  https://lists.nongnu.org/archive/html/qemu-devel/2021-01/msg05962.html

However, given that coroutine-sigaltstack is more of a fallback
implementation for platforms that do not support ucontext, that change
may be a bit too invasive to be comfortable with it.  The mutex proposed
here may negatively impact performance, but the change is much simpler.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210125120305.19520-1-mreitz@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
This commit is contained in:
Max Reitz 2021-01-25 13:03:05 +01:00
parent 59c9466d58
commit f4be822519
1 changed files with 9 additions and 0 deletions
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9
util/coroutine-sigaltstack.c
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@ -157,6 +157,7 @@ Coroutine *qemu_coroutine_new(void)
sigset_t sigs;
sigset_t osigs;
sigjmp_buf old_env;
static pthread_mutex_t sigusr2_mutex = PTHREAD_MUTEX_INITIALIZER;
/* The way to manipulate stack is with the sigaltstack function. We
* prepare a stack, with it delivering a signal to ourselves and then
@ -186,6 +187,12 @@ Coroutine *qemu_coroutine_new(void)
sa.sa_handler = coroutine_trampoline;
sigfillset(&sa.sa_mask);
sa.sa_flags = SA_ONSTACK;
/*
* sigaction() is a process-global operation. We must not run
* this code in multiple threads at once.
*/
pthread_mutex_lock(&sigusr2_mutex);
if (sigaction(SIGUSR2, &sa, &osa) != 0) {
abort();
}
@ -234,6 +241,8 @@ Coroutine *qemu_coroutine_new(void)
* Restore the old SIGUSR2 signal handler and mask
*/
sigaction(SIGUSR2, &osa, NULL);
pthread_mutex_unlock(&sigusr2_mutex);
pthread_sigmask(SIG_SETMASK, &osigs, NULL);
/*