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

[Robin Murphy]

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 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.

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@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");
> >   }
>
>
>
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