Date: Sat, 18 Mar 2006 13:09:25 -0800
From: Andrew Morton <akpm@osdl.org>
To: tglx@linutronix.de
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, trini@kernel.crashing.org
Subject: Re: [patch 1/2] Validate itimer timeval from userspace
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Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Sat, 2006-03-18 at 12:31 -0800, Andrew Morton wrote:
> > Thomas Gleixner <tglx@linutronix.de> wrote:
> > >
> > > On Sat, 2006-03-18 at 12:07 -0800, Andrew Morton wrote:
> > > 
> > > > From my reading, 2.4's sys_setitimer() will normalise the incoming timeval
> > > > rather than rejecting it.  And I think 2.6.13 did that too.
> > > > 
> > > > It would be bad of us to change this behaviour, even if that's what the
> > > > spec says we should do - because we can break existing applications.
> > > > 
> > > > So I think we're stuck with it - we should normalise and then accept such
> > > > timevals.  And we should have a big comment explaining how we differ from
> > > > the spec, and why.
> > > 
> > > Hmm. How do you treat a negative value ?
> > > 
> > 
> > In the same way as earlier kernels did!
> > 
> > Unless, of course, those kernels did something utterly insane.  In that
> > case we'd need to have a little think.
> 
> It was caught by:
> 
> timeval_to_jiffies(const struct timeval *value)
> {
>         unsigned long sec = value->tv_sec;
>         long usec = value->tv_usec;
> 
>         if (sec >= MAX_SEC_IN_JIFFIES)
> sec = MAX_SEC_IN_JIFFIES;
> ....
> }
> 
> The conversion of long to unsigned long converted a negative value to
> the maximum timeout.
> 
> It's not utterly insane, but it does not make much sense either.
> 
> Of course I can convert it that way, if we want to keep this "help
> sloppy programmers aid" alive.
> 

It would be strange to set an alarm for 0xffffffff seconds in the future
but yeah, unless we can point at a reason why nobody could have ever been
doing that, we should turn this into permanent, documented behaviour of
Linux 2.6 and earlier, I'm afraid.

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