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

From: [email-masked] (陈刚(Gangchen))
Date: Thu, 31 Mar 2016 07:56:05 +0000

On 03/30/2016 10:07 PM, Dave Martin wrote:
> On Wed, Mar 30, 2016 at 03:27:01AM +0000, ??(Gangchen) wrote:
>> 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.
> What doesn't work for you when using this method?
>
> Why does the fact that this is an inline function make a difference?
With the help of other colleagues, I understand your proposal now.
I create a patch and I can verify that it works!

Should I submit it, as it seems better than this one I sent?

Cheers ---Dave