linux-kernel - Re: [PATCH 2/2] ARM: fix framepointer check in unwind

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Date:	Thu, 05 Dec 2013 17:10:08 +0400
From:	Konstantin Khlebnikov <k.khlebnikov@...sung.com>
To:	Jean Pihet <jean.pihet@...aro.org>
Cc:	Russell King <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Will Deacon <will.deacon@....com>,
	Vyacheslav Tyrtov <v.tyrtov@...sung.com>
Subject: Re: [PATCH 2/2] ARM: fix framepointer check in unwind_frame

On Thu, 2013-12-05 at 13:45 +0100, Jean Pihet wrote:
> On 5 December 2013 09:34, Konstantin Khlebnikov
> <k.khlebnikov@...sung.com> wrote:
> > This patch fixes corner case when (fp + 4) overflows unsigned long,
> > for example: fp = 0xFFFFFFFF -> fp + 4 == 3.
> >
> > Signed-off-by: Konstantin Khlebnikov <k.khlebnikov@...sung.com>
> > ---
> >  arch/arm/kernel/stacktrace.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm/kernel/stacktrace.c b/arch/arm/kernel/stacktrace.c
> > index 00f79e5..af4e8c8 100644
> > --- a/arch/arm/kernel/stacktrace.c
> > +++ b/arch/arm/kernel/stacktrace.c
> > @@ -31,7 +31,7 @@ int notrace unwind_frame(struct stackframe *frame)
> >         high = ALIGN(low, THREAD_SIZE);
> >
> >         /* check current frame pointer is within bounds */
> > -       if (fp < (low + 12) || fp + 4 >= high)
> > +       if (fp < low + 12 || fp > high - 4)
> Shouldn't that use the '>=' comparison?

nope, fp == high-4 is fine:

-------------------+ 
... -5 -4 -3 -2 -1 | -0
-------------------+
        ^             ^
        +---- fp      +----- high
 
stack frame contains four 'unsigned long' values: {fp, sp, lr, pc}.
'fp' points to 'pc', so totally there are 16 bytes: fp-12..fp+3

> 
> Jean
> >                 return -EINVAL;
> >
> >         /* restore the registers from the stack frame */
> >
> > --
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