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

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Date:	Thu, 14 Jun 2007 16:58:43 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Roland McGrath <roland@...hat.com>,
	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>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [BUG] ptraced process waiting on syscall may return kernel internal errnos

On 06/14, Rafael J. Wysocki wrote:
>
> Sorry for being late, I've just realized that you are discussing the freezer
> here. ;-)

my fault, I was going to cc you but forgot, sorry!

> On Wednesday, 13 June 2007 17:15, Oleg Nesterov wrote:
>
> > > @@ -105,7 +105,11 @@ static int recalc_sigpending_tsk(struct 
> > >  		set_tsk_thread_flag(t, TIF_SIGPENDING);
> > >  		return 1;
> > >  	}
> > > -	clear_tsk_thread_flag(t, TIF_SIGPENDING);
> > > +	/*
> > > +	 * We must never clear the flag in another thread, or in current
> > > +	 * when it's possible the current syscall is returning -ERESTART*.
> > > +	 * So we don't clear it here, and only callers who know they should do.
> > > +	 */
> > >  	return 0;
> > >  }
> > 
> > This breaks cancel_freezing(). Somehow we should clear TIF_SIGPENDING for
> > kernel threads. Otherwise we may have subtle failures if try_to_freeze_tasks()
> > fails.
> 
> Well, the only code path in which we'd want to call cancel_freezing() for kernel
> threads is when the freezing of kernel threads.  However, this only happens if
> one of the kernel threads declares itself as freezable and the fails to call
> try_to_freeze(), which is a bug.

But this happens? We know a lot of reasons why try_to_freeze() can fail just
because some kthread waits for already frozen task.

>                                   Thus I don't think that we need to worry
> about that case too much.

Well, we can have very subtle problems because a kernel thread may run with
TIF_SIGPENDING forever. This means in particualar that any wait_event_interruptible()
can't succeed. I think this is worse than explicit failure (like -ERESSTART... leak),
because it is hard to reproduce/debug.

> Moreover, I'm not sure that it's a good idea at all to send signals to kernel
> threads from the freezer, since in fact we only need to wake them up to make
> them call try_to_freeze() (after we've set TIF_FREEZE for them).

Yes! I completely agree.

Oleg.

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