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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"); >> } >> > > > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@...ts.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel >
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