linux-kernel - Re: [BUG] ptraced process waiting on syscall may return kernel internal errnos

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Date:	Wed, 13 Jun 2007 16:01:53 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Oleg Nesterov <oleg@...sign.ru>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>
Subject: Re: [BUG] ptraced process waiting on syscall may return kernel
	internal errnos

> The freezer is crap... news at 11. Maybe a quick hack would be to let it
> clear sigpending if tsk->mm == NULL but that's ugly. Note that there's
> anything pretty about the freezer anyway...

I think it might be made not too unreasonable by adding a TASK_FROZEN state.
But I am still persuaded by my "hide in the corner" plan.

> Well.. why was it _and_wake() in the first place anyway ? Or do I miss
> something ? Why would we need to wake a thread for which we are removing
> signals ?

The bug was about a case where recalc_sigpending_tsk would set
TIF_SIGPENDING when it hadn't been set before (wants_signal).  It has
nothing to do with the rm_from_queue_full being done there.  It's just a
violation of the necessary rule that when you set TIF_SIGPENDING on another
thread you better call signal_wake_up on it.

> What about something like:
> 
> 			do {
> 				rm_from_queue_full(&mask, &t->pending);
> -				recalc_sigpending_and_wake(t);
> 				t = next_thread(t);
> 			} while (t != current);
> +			recalc_sigpending();

There is no need for the +, just the -.  The calling thread is the one
where know there is certainly no perturbation of behavior due to leaving
TIF_SIGPENDING set rather than clearing it.  It's just going to exit the
syscall and deal with signal state properly on the way out either way.
Doing recalc_sigpending is an unnecessary optimization of the corner case.

> So at the end of the day, easier to test it inside dequeue_signal().

Before completely revamping the whole set of entrypoints to be saner all
around, yes.

Thanks,
Roland
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