Date: Fri, 29 May 2015 11:35:25 +0300 From: Solar Designer <solar@...nwall.com> To: john-dev@...ts.openwall.com Subject: Re: Signal handling within john formats. Hi Sayantan, On Thu, May 28, 2015 at 03:23:27PM +0530, Sayantan Datta wrote: > On Wed, May 27, 2015 at 1:01 PM, Sayantan Datta <std2048@...il.com> wrote: > > I'd like to use signal handling functions - alarm() and sigaction() within > > a format. As you're probably aware, you should not. > > Howver using them disrupts status checking, even if I restore to > > the previous signal handler after I'm done using signals. > > > > The sequence of function calls within the format is as follows: > > > > sigaction(SIGALARM, &set_signal, &old_signal); > > alarm(x); > > . > > . > > . > > alarm(0); > > sigaction(SIGALARM, &old_signal, NULL); > > > > > > But the above sequence fails to restore normal JtR signal handling. So, > > what's missing? > Turns out restoring previous signal handler is not enough; alarm() function > interferes with setitimer() function. So, set setitimer() must also be > saved before using alarm() and then restored after calling alarm(). How exactly setitimer() interacts with alarm() may vary across systems. You should not mix them. You may do what you need with setitimer() alone, but even then you still have a race condition between replacing the signal handler and replacing the timer settings (and ditto when restoring them). To avoid it, you may block/unblock the signal. Once finally implemented correctly, this should actually work... but it gets complicated. As an alternative, you could add support for registering of your additional handlers (or just one additional handler) to the global alarm handler in JtR. This would be JtR-specific, which is good in that it won't introduce dependency on extra features of the underlying system working correctly. Better yet, though, you'd avoid the need for this. Why exactly do you need it? Let's discuss the actual problem you're trying to solve first. Thanks, Alexander
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