linux-kernel - Re: A bug in read operation for /dev/zero and a proposed fix.

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Date:	Thu, 4 Jun 2009 14:01:24 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Salman Qazi <sqazi@...gle.com>, linux-kernel@...r.kernel.org,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: A bug in read operation for /dev/zero and a proposed fix.



On Thu, 4 Jun 2009, Andrew Morton wrote:
> > 
> > To fix this, I propose that when a fatal signal is pending during
> > /dev/zero read operation, we simply return and let the user process die.
> > Here is a patch that does that.
> > 
> > Signed-off-by: Salman Qazi <sqazi@...gle.com>
> > ---
> > diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> > index 8f05c38..2ffa36e 100644
> > --- a/drivers/char/mem.c
> > +++ b/drivers/char/mem.c
> > @@ -696,6 +696,11 @@ static ssize_t read_zero(struct file * file, char __user * buf,
> >   			break;
> >   		buf += chunk;
> >   		count -= chunk;
> > +		/* The exit code here doesn't actually matter, as userland
> > +		 * will never see it.
> > +		 */
> > +		if (fatal_signal_pending(current))
> > +			return -ENOMEM;
> >   		cond_resched();
> >   	}
> >   	return written ? written : -EFAULT;
> 
> OK.  I think.
> 
> It's presumptuous to return -ENOMEM: we don't _know_ that this signal
> came from the oom-killer.  It would be better to return -EINTR here.

I don't think the error matters in this case, since we literally only care 
about fatal signals, but I agree that EINTR is probably better.

That said, it would be even better to basically act as if it was a signal, 
and do something like

	return written ? written : -EINTR;

because that might allow us to simply make it _totally_ interruptible some 
day. There is nothing that says that /dev/zero shouldn't act like an 
interruptible file descriptor (like a pipe), and just return a partial 
read.

If we want to do this for 2.6.30, though, I very much agree with the 
notion of limiting it to just fatal signals, though.

			Linus
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