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

[Jon Medhurst (Tixy)]

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 â4095 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 â4095 to 4095 

So the current use of 'J' seems correct to me.

[1] https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html#Machine-Constraints


> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  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");
>  }
>