Re: [PATCH] arm: Add unwinding annotations for 64bit division functions

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On Mon, September 19, 2011 4:22 pm, Nicolas Pitre wrote:
> On Mon, 19 Sep 2011, Laura Abbott wrote:
>
>> The 64bit division functions never had unwinding annotations
>> added. This prevents a backtrace from being printed within
>> the function and if a division by 0 occurs. Add the annotations.
>>
>> Signed-off-by: Laura Abbott <lauraa@xxxxxxxxxxxxxx>
>> ---
>>  arch/arm/lib/div64.S |    8 ++++++++
>>  1 files changed, 8 insertions(+), 0 deletions(-)
>>
>> diff --git a/arch/arm/lib/div64.S b/arch/arm/lib/div64.S
>> index faa7748..e55c484 100644
>> --- a/arch/arm/lib/div64.S
>> +++ b/arch/arm/lib/div64.S
>> @@ -13,6 +13,7 @@
>>   */
>>
>>  #include <linux/linkage.h>
>> +#include <asm/unwind.h>
>>
>>  #ifdef __ARMEB__
>>  #define xh r0
>> @@ -44,6 +45,7 @@
>>   */
>>
>>  ENTRY(__do_div64)
>> +UNWIND(.fnstart)
>>
>>  	@ Test for easy paths first.
>>  	subs	ip, r4, #1
>> @@ -189,7 +191,12 @@ ENTRY(__do_div64)
>>  	moveq	yh, xh
>>  	moveq	xh, #0
>>  	moveq	pc, lr
>> +UNWIND(.fnend)
>>
>> +UNWIND(.fnstart)
>> +UNWIND(.pad #4)
>> +UNWIND(.save {lr})
>> +Ldiv0_64:
>
> Why this phony fnend+fnstart here?
>
If a division by 0 occurs, we need to be able to access the saved LR on
the stack which is setup right before calling the __div0 function. This
can't go at the top of __do_div64 because if we try to do a backtrace from
within __do_div64 the annotation won't be correct as the LR was never
saved on the stack. (yes, __do_div64 is all register math but it's still
possible to take a prefetch abort in the middle of that function. Taking a
prefetch abort in the middle of __do_div64 is what found this issue in the
first place.) This setup handles both cases correctly.

>
> Nicolas
>

Laura

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>


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