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Date: Thu, 18 Feb 2016 13:24:58 -0500 (EST) From: Nicolas Pitre <nicolas.pitre@...aro.org> To: Arnd Bergmann <arnd@...db.de> cc: Russell King <linux@....linux.org.uk>, linux-arm-kernel@...ts.infradead.org, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Jon Medhurst <tixy@...aro.org>, Marc Zyngier <marc.zyngier@....com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH 3/3] [RESEND] ARM: kprobes: use "I" constraint for inline assembly offsets On Thu, 18 Feb 2016, 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. I don't follow you. >From the gcc manual: '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 >From the ARM ARM: LDR<c> <Rt>, [<Rn>{, #+/-<imm12>}] where imm12 is a constant between 0 and 4095. So J is really the appropriate constraint here. Sure, in this case it is very likely that I would just works given that offset_of() is unlikely to exceed shifted 8 bits and that's what people use in most cases. But strictly speaking it's J that perfectly matches the LDR/STR instructions. > > 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"); > } > > -- > 2.7.0 > >
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