Re: [Qemu-devel] [PATCH] qemu-ga: guest-shutdown: use only async-signal-safe functions

[Luiz Capitulino]

On Mon, 14 May 2012 11:51:13 -0600
Eric Blake <address@hidden> wrote:

> On 05/14/2012 11:40 AM, Luiz Capitulino wrote:
> > POSIX mandates[1] that a child process of a multi-thread program uses
> > only async-signal-safe functions before exec(). We consider qemu-ga
> > to be multi-thread, because it uses glib.
> > 
> > However, qmp_guest_shutdown() uses functions that are not
> > async-signal-safe. Fix it the following way:
> > 
> > - fclose() -> reopen_fd_to_null()
> > - execl() -> execle()
> > - exit() -> _exit()
> > - drop slog() usage (which is not safe)
> > 
> >   [1] http://pubs.opengroup.org/onlinepubs/009695399/functions/fork.html
> > 
> 
> >  # @guest-shutdown:
> >  #
> >  # Initiate guest-activated shutdown. Note: this is an asynchronous
> > -# shutdown request, with no guaruntee of successful shutdown. Errors
> > -# will be logged to guest's syslog.
> > +# shutdown request, with no guaruntee of successful shutdown.
> 
> As long as you are changing docs, fix the typo:
> 
> s/guaruntee/guarantee/
> 
> > +++ b/qga/commands-posix.c
> > @@ -57,16 +57,13 @@ void qmp_guest_shutdown(bool has_mode, const char 
> > *mode, Error **err)
> >      if (pid == 0) {
> >          /* child, start the shutdown */
> >          setsid();
> > -        fclose(stdin);
> > -        fclose(stdout);
> > -        fclose(stderr);
> > -
> > -        ret = execl("/sbin/shutdown", "shutdown", shutdown_flag, "+0",
> > -                    "hypervisor initiated shutdown", (char*)NULL);
> > -        if (ret) {
> > -            slog("guest-shutdown failed: %s", strerror(errno));
> > -        }
> > -        exit(!!ret);
> > +        reopen_fd_to_null(0);
> > +        reopen_fd_to_null(1);
> > +        reopen_fd_to_null(2);
> 
> I prefer the POSIX-mandated macros STDIN_FILENO, STDOUT_FILENO, and
> STDERR_FILENO, but don't know if qemu intends to rely on them (according
> to gnulib, at least older mingw lacked those macro names from
> <unistd.h>).  So I won't make you change this.
> 
> > +
> > +        ret = execle("/sbin/shutdown", "shutdown", shutdown_flag, "+0",
> > +                    "hypervisor initiated shutdown", (char*)NULL, environ);
> 
> Where was 'environ' declared?  POSIX says that environ must exist, but
> that it is the one variable where you must declare it yourself rather
> than getting it from a public header.  (For convenience, glibc declares
> environ in <unistd.h> when using _GNU_SOURCE, but when you are asking
> for strict standards namespace compliance, it disappears.)

I'll declare it then.

> 
> > +        _exit(!!ret);
> 
> Why are we even bothering with ret?  If execle() returns, we _know_ we
> had a failure, and !!ret will always be 1.

True, will drop it.