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

From: [email-masked] (Ben Dooks)
Date: Fri, 26 Jul 2013 11:31:42 +0100

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<[email-masked]>
>>>> ---
>>>>   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.

-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius