On 01/25/21 13:03, Max Reitz wrote: > 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 <mre...@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); > > /* >
Reviewed-by: Laszlo Ersek <ler...@redhat.com> Thanks! Laszlo
