Re: [Qemu-devel] [PATCH] catch signals

[Daniel P. Berrange]

On Wed, Aug 06, 2008 at 11:20:39AM +0200, Gerd Hoffmann wrote:
> Daniel P. Berrange wrote:
> > On Tue, Aug 05, 2008 at 06:09:39PM +0200, Gerd Hoffmann wrote:
> >>  
> >> +#ifndef _WIN32
> >> +static void termsig_handler(int signal)
> >> +{
> >> +    switch (signal) {
> >> +    case SIGSEGV:
> >> +    case SIGBUS:
> >> +        /* returning from signal handler most likely isn't going to work 
> >> */
> >> +        fprintf(stderr, "qemu: got signal %d (%s), taking emergency 
> >> exit\n",
> >> +                signal, strsignal(signal));
> >> +        exit(1);
> > 
> > Neither of these functions are on the POSIX  async-signal-safe list,
> > so their use from signal handlers is not a good idea.
> 
> We are in dead water already and also don't plan to ever return from the
>  signal handler.  Is it really a problem then?

Yes, because if you further corrupt state in the signal handler it makes
debugging what went wrong even harder than it already is. In SEGV/BUS
case you really want to be able to get a decent core dump to analyse, so
the utmost care should be taken to avoid further messing up state. 

The STDIO libs are not re-entrant safe so if the original code was in a
STDIO function, and a SEGV comes in on another thread, the signal handler
will likely deadlock in a mutex. 'exit' will call into STDIO to flush
buffers, so will suffer the same problem.

> I want to be able to do cleanups (well, at least attempt) even in case
> of a segfault.  If exit() + atexit handlers isn't going to fill the bill
> we'll have to create some signal-save emergency cleanup handlers.

You could use 'sigaltstack' to setup an alternate pre-allocated stack
and carefully code the handler so it only uses pre-allocated memory,
or mem from the alt-stack & signal-safe functions. If you need something
really complex, you could 'fork' a cleanup program and communicate any
info to it using a pipe.

Regards,
Daniel
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