[PATCH] ARM: kdgb: use .inst for data to be assembled as intruction

[Dave Martin]

On Fri, Jul 26, 2013 at 11:31:42AM +0100, Ben Dooks wrote:
> On 25/07/13 19:09, Dave Martin wrote:
> >On Thu, Jul 25, 2013 at 06:13:18PM +0100, Ben Dooks wrote:
> >>On 25/07/13 18:11, Dave Martin wrote:
> >>>On Thu, Jul 25, 2013 at 03:49:38PM +0100, Ben Dooks wrote:
> >>>>The arch_kgdb_breakpoint() function uses an inline assembly directive
> >>>>to assemble a specific instruction using .word. This means the linker
> >>>>will not treat is as an instruction, and therefore incorrectly swap
> >>>>the endian-ness if running BE8.
> >>>>
> >>>>Note, not tested, please comment if this is wrong.
> >>>>
> >>>>Signed-off-by: Ben Dooks<ben.dooks at codethink.co.uk>
> >>>>---
> >>>>  arch/arm/include/asm/kgdb.h |    2 +-
> >>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>>diff --git a/arch/arm/include/asm/kgdb.h b/arch/arm/include/asm/kgdb.h
> >>>>index 48066ce..76227c8 100644
> >>>>--- a/arch/arm/include/asm/kgdb.h
> >>>>+++ b/arch/arm/include/asm/kgdb.h
> >>>>@@ -41,7 +41,7 @@
> >>>>
> >>>>  static inline void arch_kgdb_breakpoint(void)
> >>>>  {
> >>>>-	asm(".word 0xe7ffdeff");
> >>>>+	asm(".inst 0xe7ffdeff");
> >>>
> >>>Yikes, this isn't going to work in a Thumb kernel.
> >>>
> >>>We should make HAVE_ARCH_KGDB depend on !THUMB2_KERNEL until/unless that
> >>>gets fixed...  It looks like the incompatibilities may be more extensive
> >>>than just this one instruction.
> >>>
> >>>
> >>>For the ARM case, similarly to the other patches, please use the __inst
> >>>macros from<asm/opcodes.h>   instead of emitting the opcode explicitly.
> >>
> >>See previous objections to that, plus they're marked for internal use
> >>only!
> >
> >Ditto my counterarguments.  I'm not emotionally attached to __inst*(), but
> >we should use one or the other: either .inst or __inst(), not a mixture.
> >However, the __inst macros work for inline asm and .S, and do more than
> >just emitting a single opcode; see opcodes-virt.h for example, so while
> >removing them isn't rocket science, it would involve churn in a few places.
> >
> >
> >Note, the "Don't use these directly" comment only applies to the triple-
> >underscored helpers.
> >
> >The broader "using these macros directly is poor practice" comment was
> >an attempt to engourage people to write the likes of
> >
> >#define __KGDB_BKPT_INSTR __inst_arm(0xxe7ffdeff)
> >
> >static inline void arch_kgdb_breakpoint(void)
> >{
> >	asm(__KGDB_BKPT_INSTR);
> >}
> >
> >...on the basis that this ought to be more readable.
> >
> >But this is a bit moot in a situation like this where the opcode is
> >only used in one place, by itself, in a wrapper whose name makes the
> >intent clear anyway.
> 
> Ok, I will look into changing the patches in the next day or so
> and do the same fixes for kprobes.

OK, thanks.

Cheers
---Dave