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>
---
util/coroutine-sigaltstack.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/util/coroutine-sigaltstack.c b/util/coroutine-sigaltstack.c
index aade82afb8..e99b8a4f9c 100644
--- a/util/coroutine-sigaltstack.c
+++ b/util/coroutine-sigaltstack.c
@@ -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);
/*