Re: [PATCH] Fix Warnining in arch/x86/kernel/signal.c

[Ingo Molnar <mingo@elte.hu>]

* Subrata Modak <subrata@linux.vnet.ibm.com> wrote:

> On Wed, 2009-05-13 at 11:20 +0900, Hiroshi Shimamoto wrote:
> > Subrata Modak wrote:
> > > Hi,
> > > 
> > >> On Tue, 2009-05-12 at 17:16 +0200, Andi Kleen wrote:
> > >> On Tue, May 12, 2009 at 05:16:14PM +0530, Subrata Modak wrote:
> > >>
> > >> Hi Subrata,
> > >>
> > >>> With gcc (GCC) 4.4.1 20090429 (prerelease), i get the following build warning:
> > >> Patch looks good (you can add a 
> > >> Reviewed-by: Andi Kleen <ak@linux.intel.com>)
> > >> But I don't maintain this code anymore. Please resend to x86@kernel.org
> > >> cc linux-kernel@vger.kernel.org for merge.
> > >>
> > >> Thanks,
> > >>
> > >> -Andi
> > > 
> > > 
> > > With gcc (GCC) 4.4.1 20090429 (prerelease), i get the following build warning:
> > > 
> > > CC      arch/x86/kernel/signal.o
> > > arch/x86/kernel/signal.c: In function ‘sys_sigreturn’:
> > > arch/x86/kernel/signal.c:573: warning: ‘set.sig[1]’ may be used uninitialized in this function
> > > 
> > > On investigation i found that this is because of the evaluation
> > > precedence of the expression below:
> > > 
> > > 569 unsigned long sys_sigreturn(struct pt_regs *regs)
> > > 570 {
> > > 571         struct sigframe __user *frame;
> > > 572         unsigned long ax;
> > > 573         sigset_t set;
> > > 574 
> > > 575         frame = (struct sigframe __user *)(regs->sp - 8);
> > > 576 
> > > 577         if (!access_ok(VERIFY_READ, frame, sizeof(*frame)))
> > > 578                 goto badframe;
> > > 579         if (__get_user(set.sig[0], &frame->sc.oldmask) || (_NSIG_WORDS > 1
> > > 580                 && __copy_from_user(&set.sig[1], &frame->extramask,
> > > 581                                     sizeof(frame->extramask))))
> > > 
> > > The initialization for set.sig[1] may not occur if
> > > 	__get_user(set.sig[0], &frame->sc.oldmask)
> > > evalutes to true. So, the compiler is complaining.
> > > 
> > > I have devised a small patch for this which wanes away this warning
> > > without changing the conditional evaluation criteria. Let me know if
> > > you like this patch.
> > > 
> > > 582                 goto badframe;
> > > 583 
> > > 584         sigdelsetmask(&set, ~_BLOCKABLE);
> > > 585         spin_lock_irq(&current->sighand->siglock);
> > > 586         current->blocked = set;
> > > 587         recalc_sigpending();
> > > 588         spin_unlock_irq(&current->sighand->siglock);
> > > 589 
> > > 590         if (restore_sigcontext(regs, &frame->sc, &ax))
> > > 591                 goto badframe;
> > > 592         return ax;
> > > 593 
> > > 594 badframe:
> > > 595         signal_fault(regs, frame, "sigreturn");
> > > 596 
> > > 597         return 0;
> > > 598 }
> > > 
> > > Signed-Off-By: Subrata Modak <subrata@linux.vnet.ibm.com>
> > > Reviewed-by: Andi Kleen <ak@linux.intel.com>
> > > To: <x86@kernel.org>
> > > Cc: Linux Kernel <linux-kernel@vger.kernel.org>
> > > Cc: Andi Kleen <andi@firstfloor.org>
> > > Cc: Andi Kleen <ak@linux.intel.com>
> > > Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
> > > Cc: Sachin P Sant <sachinp@linux.vnet.ibm.com>
> > > Subject: [PATCH] Fix Warnining in arch/x86/kernel/signal.c
> > > ---
> > > 
> > > --- a/arch/x86/kernel/signal.c	2009-05-12 10:59:24.000000000 +0530
> > > +++ b/arch/x86/kernel/signal.c	2009-05-12 16:57:32.000000000 +0530
> > > @@ -576,9 +576,10 @@ unsigned long sys_sigreturn(struct pt_re
> > >  
> > >  	if (!access_ok(VERIFY_READ, frame, sizeof(*frame)))
> > >  		goto badframe;
> > > -	if (__get_user(set.sig[0], &frame->sc.oldmask) || (_NSIG_WORDS > 1
> > > -		&& __copy_from_user(&set.sig[1], &frame->extramask,
> > > -				    sizeof(frame->extramask))))
> > > +
> > > +        if ( (__copy_from_user(&set.sig[1], &frame->extramask,
> > > +                sizeof(frame->extramask)) && _NSIG_WORDS > 1) || 
> > > +                __get_user(set.sig[0], &frame->sc.oldmask))
> > >  		goto badframe;
> > 
> > I'm not sure why this eliminates that warning.
> > set.sig[0] may not be initialized too, if __copy_from_user() failed.
> 
> True, but only when either or both of __copy_from_user() and
> (_NSIG_WORDS > 1) fails. But in all instances set.sig[1] gets
> initialized.
> 
> > I don't have enough time to look at this right now, sorry.
> > 
> > Another question, __copy_from_user() will be called even if
> > _NSIG_WORDS is less than 2, perhaps it never occurs.
> > I think, to check _NSIG_WORDS > 1 before calling __copy_from_user()
> > is better.
> 
> Fine. Let Ingo/Thomas/Peter decide whether they would like this fix or
> drop it.

If you get the Acked-by from Hiroshi-san it looks good to me. He 
modified this code last.

	Ingo