linux-kernel - Re: A bug about system call on ARM

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Date:	Thu, 30 May 2013 23:54:56 -0400 (EDT)
From:	Nicolas Pitre <nico@...xnic.net>
To:	Will Deacon <will.deacon@....com>
Cc:	"Wang, Yalin" <Yalin.Wang@...ymobile.com>,
	'richard -rw- weinberger' <richard.weinberger@...il.com>,
	"'linux-arch@...r.kernel.org'" <linux-arch@...r.kernel.org>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	"'linux-arm-kernel@...ts.infradead.org'" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: A bug about system call on ARM

On Thu, 30 May 2013, Will Deacon wrote:

> On Thu, May 30, 2013 at 10:09:49AM +0100, Will Deacon wrote:
> > On Thu, May 30, 2013 at 02:41:42AM +0100, Wang, Yalin wrote:
> > > If you have some patch for this issue,
> > > I can do the test for it .
> > 
> > I'll have a look at cooking something which uses an exception table entry
> > to rewind the PC and retry the system call. That's simpler than directly
> > injecting a user page fault from the system call path.
> 
> Ok, please can you try the following?
> 
> Will
> 
> --->8
> 
> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> index bc5bc0a..855926e 100644
> --- a/arch/arm/kernel/entry-common.S
> +++ b/arch/arm/kernel/entry-common.S
> @@ -361,6 +361,15 @@ ENTRY(vector_swi)
>  	str	r8, [sp, #S_PSR]		@ Save CPSR
>  	str	r0, [sp, #S_OLD_R0]		@ Save OLD_R0
>  	zero_fp
> +	enable_irq
> +	ct_user_exit
> +
> +#ifdef CONFIG_ALIGNMENT_TRAP
> +	ldr	ip, __cr_alignment
> +	ldr	ip, [ip]
> +	mcr	p15, 0, ip, c1, c0		@ update control register
> +#endif

This is wrong.  you must set up the align bit in the control register 
_before_ enabling IRQs or an IRQ handler might run without alignment 
fixup.

Otherwise the patch looks good to me.


Nicolas
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