Re: [Qemu-devel] [PATCH RFC] main loop: fix some accesses made in sighandler context

[Anthony Liguori <anthony@codemonkey.ws>]

On 09/15/2011 12:22 PM, Laszlo Ersek wrote:
> Make variables volatile ("sig_atomic_t" should cover "int" and "pid_t").
>
> Also replace calls to functions that are not required to be async-signal-safe
> [1]. (I haven't checked if any signal masks and/or previous suspension of the
> interrupted thread keep the current calls safe.)
>
> termsig_handler()
>    ->  qemu_system_killed(): shutdown_signal, shutdown_pid, no_shutdown [2]
>      ->  qemu_system_shutdown_request(): shutdown_requested
>        ->  qemu_notify_event()
>          ->  qemu_event_increment(): fprintf(), strerror(), exit()
>
> [1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04_03_03
> [2] http://lists.nongnu.org/archive/html/qemu-devel/2011-09/msg01757.html
>
> "checkpatch.pl" warned four times about "volatile", and considered the
> zero-initialization of "no_shutdown" (which has static storage duration) an
> error.
>
> Build tested only. Please CC me on any followup, I'm not subscribed. Thank you.
>
> Signed-off-by: Laszlo Ersek<lersek@redhat.com>
> ---
>   cpus.c   |   13 ++++++++++---
>   sysemu.h |    2 +-
>   vl.c     |    6 +++---
>   3 files changed, 14 insertions(+), 7 deletions(-)
>
> diff --git a/cpus.c b/cpus.c
> index 54c188c..ed51247 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -289,9 +289,16 @@ static void qemu_event_increment(void)
>
>       /* EAGAIN is fine, a read must be pending.  */
>       if (ret<  0&&  errno != EAGAIN) {
> -        fprintf(stderr, "qemu_event_increment: write() failed: %s\n",
> -                strerror(errno));
> -        exit (1);
> +        int len;
> +        char buf[128];
> +
> +        /* Don't bother with strerror_[rl]. Make a single attempt to write. */
> +        len = snprintf(buf, sizeof buf,
> +                       "qemu_event_increment: write() failed: %d\n", errno);

I don't think you can rely on snprintf being signal safe.  I think you should 
just exit on failure.

OpenBSD lists snprintf as signal safe, but "probably not on other systems."

Regards,

Anthony Liguori