From f4be8225198c4616b391d13d4ba083bf40f3a0c9 Mon Sep 17 00:00:00 2001 From: Max Reitz Date: Mon, 25 Jan 2021 13:03:05 +0100 Subject: [PATCH] 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 Message-Id: <20210125120305.19520-1-mreitz@redhat.com> Reviewed-by: Laszlo Ersek Reviewed-by: Vladimir Sementsov-Ogievskiy --- 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); /*