Subject: [PATCH 1/1] ARM : missing corrupted reg in __do_div_asm

From: [email-masked] (陈刚(Gangchen))
Date: Wed, 30 Mar 2016 03:27:01 +0000

On 03/29/2016 06:56 PM, Dave Martin wrote:
> On Tue, Mar 29, 2016 at 11:34:18AM +0100, Russell King - ARM Linux wrote:
>> On Tue, Mar 29, 2016 at 11:26:05AM +0100, Dave Martin wrote:
>>> On Tue, Mar 29, 2016 at 12:19:49PM +0200, Arnd Bergmann wrote:
>>>> On Monday 28 March 2016 12:19:03 Chen Gang wrote:
>>>>> __xl(R0 in little endian system, or R1 in big endian system) is corrupted
>>>>> after calling __do_div64 and compiler is not informed about this in
>>>>> macro __do_div_asm. If n is used again afterwards, __xl won't be
>>>>> reloaded and n will contain incorrect value.
>>>>>
>>>>> Signed-off-by: Chen Gang <[email-masked]>
>>>>> Signed-off-by: Chen Gang <[email-masked]>
>>>>> ---
>>>> How did you find this? Did you run into this problem on a live system
>>>> or see it through inspection?
>>>>
>>>>>   arch/arm/include/asm/div64.h | 6 ++++--
>>>>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/arch/arm/include/asm/div64.h b/arch/arm/include/asm/div64.h
>>>>> index e1f0776..1a6e91a 100644
>>>>> --- a/arch/arm/include/asm/div64.h
>>>>> +++ b/arch/arm/include/asm/div64.h
>>>>> @@ -35,12 +35,14 @@ static inline uint32_t __div64_32(uint64_t *n, uint32_t base)
>>>>>          register unsigned long long __n   asm("r0") = *n;
>>>>>          register unsigned long long __res asm("r2");
>>>>>          register unsigned int __rem       asm(__xh);
>>>>> +       register unsigned int __clobber   asm(__xl);
>>>>>          asm(    __asmeq("%0", __xh)
>>>>>                  __asmeq("%1", "r2")
>>>>> +               __asmeq("%3", "r0")
>>>>> +               __asmeq("%4", "r4")
>>>>>                  __asmeq("%2", "r0")
>>>>> -               __asmeq("%3", "r4")
>>>>>                  "bl     __do_div64"
>>>>> -               : "=r" (__rem), "=r" (__res)
>>>>> +               : "=r" (__rem), "=r" (__res), "=r" (__clobber)
>>>>>                  : "r" (__n), "r" (__base)
>>>>>                  : "ip", "lr", "cc");
>>>>>          *n = __res;
>>>> Doesn't the clobber normally go in the third line along with
>>>> "ip" and "lr"?
>>> Since __xl is not used for any real argument to the asm, I think
>>> we can just add __xl to the clobber list directly, without needing
>>> to introduce an extra register variable ... no?
>> No, you can't.  The clobber list is not allowed to specify registers
>> that may be used for input or output operands, and since __xl may be
>> r0, and __n _is_ r0, you can't specify r0 in the clobber list.
> Hmm, you're right -- in which case the change looks reasonable.
>
> I wonder whether the following would be cleaner than having these
> aliased arguments:
>
> 	asm(	/* ... */
> 		"bl	__do_div64"
> 		: "+r" (__n), "=r" (__res)
> 		: "r" (__base)
> 		: "ip", "lr", "cc");
> 	*n = __res;
> 	return __n >> 32;
>
> (providing that GCC doesn't make a mess of the "easy" shift).
I tried your proposal. It didn't make any difference: this is inline 
function and gcc just ignores your trick.
> Cheers
> ---Dave