Subject: [PATCH v2] arm: fix memset-related crashes caused by recent GCC (4.7.2) optimizations

From: [email-masked] (Alexander Holler)
Date: Tue, 12 Mar 2013 11:06:23 +0100

Am 11.03.2013 05:10, schrieb Nicolas Pitre:
> On Sun, 10 Mar 2013, Russell King - ARM Linux wrote:
>
>> On Sun, Mar 10, 2013 at 06:06:11PM +0100, Alexander Holler wrote:
>>> Am 07.03.2013 16:17, schrieb Russell King - ARM Linux:
>>>> On Wed, Mar 06, 2013 at 08:15:17PM +0100, Dirk Behme wrote:
>>>>> Am 11.02.2013 13:57, schrieb Ivan Djelic:
>>>>>> Recent GCC versions (e.g. GCC-4.7.2) perform optimizations based on
>>>>>> assumptions about the implementation of memset and similar functions.
>>>>>> The current ARM optimized memset code does not return the value of
>>>>>> its first argument, as is usually expected from standard implementations.
>>>
>>> I've just tried this patch with kernel 4.8.2 on an armv5-system where I
>>> use gcc 4.7.2 since several months and where most parts of the system
>>> are compiled with gcc 4.7.2 too.
>>>
>>> And I had at least one problem which manifested itself with
>>
>> Yes, the patch _is_ wrong.  Reverted.  I was trusting Nicolas' review
>> of it, but the patch is definitely wrong.
>
> Worse: it is in v3.9-rc2 already.
>
> Here's a fix.  Patch system?
>
> ---------- >8
> Subject: fix the memset fix
>
> Commit 455bd4c430b0 ("ARM: 7668/1: fix memset-related crashes caused by
> recent GCC (4.7.2) optimizations") attempted to fix a compliance issue
> with the memset return value.  However the memset itself was broken by
> that patch in the misaligned pointer case.
>
> This fixes the above by branching over the entry code from the
> misaligned fixup code to avoid reloading the original pointer.
>
> Also, because the function entry alignment is wrong in the Thumb mode
> compilation, that fixup code is moved to the end.
>
> While at it, the entry instructions are slightly reworked to help dual
> issue pipelines.
>
> Signed-off-by: Nicolas Pitre <[email-masked]>
> ---
>
> diff --git a/arch/arm/lib/memset.S b/arch/arm/lib/memset.S
> index d912e7397e..94b0650ea9 100644
> --- a/arch/arm/lib/memset.S
> +++ b/arch/arm/lib/memset.S
> @@ -14,31 +14,15 @@
>
>   	.text
>   	.align	5
> -	.word	0
> -
> -1:	subs	r2, r2, #4		@ 1 do we have enough
> -	blt	5f			@ 1 bytes to align with?
> -	cmp	r3, #2			@ 1
> -	strltb	r1, [ip], #1		@ 1
> -	strleb	r1, [ip], #1		@ 1
> -	strb	r1, [ip], #1		@ 1
> -	add	r2, r2, r3		@ 1 (r2 = r2 - (4 - r3))
> -/*
> - * The pointer is now aligned and the length is adjusted.  Try doing the
> - * memset again.
> - */
>
>   ENTRY(memset)
> -/*
> - * Preserve the contents of r0 for the return value.
> - */
> -	mov	ip, r0
> -	ands	r3, ip, #3		@ 1 unaligned?
> -	bne	1b			@ 1
> +	ands	r3, r0, #3		@ 1 unaligned?
> +	mov	ip, r0			@ preserve r0 as return value
> +	bne	6f			@ 1
>   /*
>    * we know that the pointer in ip is aligned to a word boundary.
>    */
> -	orr	r1, r1, r1, lsl #8
> +1:	orr	r1, r1, r1, lsl #8
>   	orr	r1, r1, r1, lsl #16
>   	mov	r3, r1
>   	cmp	r2, #16
> @@ -127,4 +111,13 @@ ENTRY(memset)
>   	tst	r2, #1
>   	strneb	r1, [ip], #1
>   	mov	pc, lr
> +
> +6:	subs	r2, r2, #4		@ 1 do we have enough
> +	blt	5b			@ 1 bytes to align with?
> +	cmp	r3, #2			@ 1
> +	strltb	r1, [ip], #1		@ 1
> +	strleb	r1, [ip], #1		@ 1
> +	strb	r1, [ip], #1		@ 1
> +	add	r2, r2, r3		@ 1 (r2 = r2 - (4 - r3))
> +	b	1b
>   ENDPROC(memset)
>

Tested-by: Alexander Holler

It works since 2 days on my busy 24/7 armv5-thingy without me having 
seen an error. A quick test on an armv7-system hasn't revealed any 
problems too.

Thanks,

Alexander