Re: [PATCH] coroutine-sigaltstack: Add SIGUSR2 mutex

[Laszlo Ersek <lersek@redhat.com>]

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 <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);
>  
>      /*
> 

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thanks!
Laszlo