linux-kernel - Re: [PATCH v2] MIPS: Fix incorrect NULL check in local_flush_tlb

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Date:	Sun, 6 Jul 2014 00:50:34 +0200
From:	Emil Goode <emilgoode@...il.com>
To:	Jonas Gorski <jogo@...nwrt.org>
Cc:	Ralf Baechle <ralf@...ux-mips.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	John Crispin <blogic@...nwrt.org>,
	MIPS Mailing List <linux-mips@...ux-mips.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kernel-janitors@...r.kernel.org
Subject: Re: [PATCH v2] MIPS: Fix incorrect NULL check in
 local_flush_tlb_page()

Hello,

On Sat, Jul 05, 2014 at 09:10:44PM +0200, Jonas Gorski wrote:
> On Sat, Jul 5, 2014 at 8:26 PM, Emil Goode <emilgoode@...il.com> wrote:
> > We check that the struct vm_area_struct pointer vma is NULL and then
> > dereference it a few lines below. The intent must have been to make sure
> > that vma is not NULL and then to check the value from cpu_context() for
> > the condition to be true.
> >
> > Signed-off-by: Emil Goode <emilgoode@...il.com>
> > ---
> >
> > v2: Updated the commit message with a better explanation.
> >
> >  arch/mips/mm/tlb-r3k.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/mips/mm/tlb-r3k.c b/arch/mips/mm/tlb-r3k.c
> > index d657493..6546758 100644
> > --- a/arch/mips/mm/tlb-r3k.c
> > +++ b/arch/mips/mm/tlb-r3k.c
> > @@ -158,7 +158,7 @@ void local_flush_tlb_page(struct vm_area_struct *vma, unsigned long page)
> >  {
> >         int cpu = smp_processor_id();
> >
> > -       if (!vma || cpu_context(cpu, vma->vm_mm) != 0) {
> > +       if (vma && cpu_context(cpu, vma->vm_mm) != 0) {
> 
> Sorry for replying "too late", but grepping through the kernel code I
> fail to find any caller that does not dereference vma before calling
> (local)flush_tlb_page(). Also both tlb-4k and tlb-8k assume vma cannot
> be NULL, so I would say it is safe to assume vma is never NULL, and
> the NULL check can be removed completely.
> 
> Also it looks like this "bug" was there since at least 2.6.12, and
> never seem to have bitten anyone.

Yes, the bug pre-dates GIT history and I agree that it is most unlikely
that it ever caused a problem. I will send a new patch that removes the
NULL check of vma.

Best regards,

Emil Goode
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