linux-kernel - Re: [PATCH 3/3] [RESEND] ARM: kprobes: use "I" constraint for inline assembly offsets

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Message-ID: <56C61486.9070803@arm.com>
Date:	Thu, 18 Feb 2016 18:59:18 +0000
From:	Robin Murphy <robin.murphy@....com>
To:	"Jon Medhurst (Tixy)" <tixy@...aro.org>,
	Arnd Bergmann <arnd@...db.de>
Cc:	Russell King <linux@....linux.org.uk>,
	Nicolas Pitre <nico@...aro.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/3] [RESEND] ARM: kprobes: use "I" constraint for inline
 assembly offsets

On 18/02/16 18:12, Jon Medhurst (Tixy) wrote:
> On Thu, 2016-02-18 at 18:05 +0100, Arnd Bergmann wrote:
>> build-testing with clang showed that the "J" constraint does not take
>> positive arguments on clang when building in for Thumb-2:
>>
>> core.c:540:3: error: invalid operand for inline asm constraint 'J'
>>
>> This has been reported as llvm bug https://llvm.org/bugs/show_bug.cgi?id=26061
>>
>> However, looking at the source code in depth, I found that the
>> kernel is also wrong, and it should not use "J" at all, but should
>> use "I" to pass an immediate argument to the inline assembly when that
>> is used as an offset to an 'ldr' instruction rather than the 'sub'
>> argument.
>
> This patch doesn't seem correct to me.
>
> The ARM ARM says the immediate offset to an ARM ldr instructions is "any
> value in the range 0-4095" and offsets may be added or subtracted,
> leading to values from −4095 to 4095".
>
> And GCC machine constraints [1] says
>
> I
>      Integer that is valid as an immediate operand in a data processing
>      instruction. That is, an integer in the range 0 to 255 rotated by a
>      multiple of 2
> J
>      Integer in the range −4095 to 4095
>
> So the current use of 'J' seems correct to me.

Hmm, Arnd reports the failure when building for Thumb-2, and the code 
under #ifdef CONFIG_THUMB2_KERNEL contains an ldrd, which takes a 
different immediate of the form imm8 * 4. Maybe it's just operand %5 
which needs fixing, although I don't see that a suitable constraint for 
that actually exists...

Robin.

> [1] https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html#Machine-Constraints
>
>
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>> ---
>>   arch/arm/probes/kprobes/core.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/arm/probes/kprobes/core.c b/arch/arm/probes/kprobes/core.c
>> index a4ec240ee7ba..4b34b40ca917 100644
>> --- a/arch/arm/probes/kprobes/core.c
>> +++ b/arch/arm/probes/kprobes/core.c
>> @@ -570,10 +570,10 @@ void __kprobes jprobe_return(void)
>>   		:
>>   		: "r" (kcb->jprobe_saved_regs.ARM_sp),
>>   		  "I" (sizeof(struct pt_regs) * 2),
>> -		  "J" (offsetof(struct pt_regs, ARM_sp)),
>> -		  "J" (offsetof(struct pt_regs, ARM_pc)),
>> -		  "J" (offsetof(struct pt_regs, ARM_cpsr)),
>> -		  "J" (offsetof(struct pt_regs, ARM_lr))
>> +		  "I" (offsetof(struct pt_regs, ARM_sp)),
>> +		  "I" (offsetof(struct pt_regs, ARM_pc)),
>> +		  "I" (offsetof(struct pt_regs, ARM_cpsr)),
>> +		  "I" (offsetof(struct pt_regs, ARM_lr))
>>   		: "memory", "cc");
>>   }
>>
>
>
>
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>

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