linux-kernel - Re: [PATCH v2 2/3] ARM: uaccess: use unified assembler language syntax

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Message-ID: <3f615e62b2d3eefab0b1c17fe19bf1d2@agner.ch>
Date:   Mon, 07 Jan 2019 22:50:41 +0100
From:   Stefan Agner <stefan@...er.ch>
To:     Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:     linux@...linux.org.uk, ndesaulniers@...gle.com,
        natechancellor@...il.com, arnd@...db.de, ard.biesheuvel@...aro.org,
        peterz@...radead.org, mingo@...hat.com, will.deacon@....com,
        julien.thierry@....com, mark.rutland@....com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 2/3] ARM: uaccess: use unified assembler language
 syntax

On 05.01.2019 17:12, Nicolas Pitre wrote:
> On Thu, 3 Jan 2019, Stefan Agner wrote:
> 
>> Convert the conditional infix to a postfix to make sure this inline
>> assembly is unified syntax. Since gcc assumes non-unified syntax
>> when emitting ARM instructions, make sure to define the syntax as
>> unified.
>>
>> This allows to use LLVM's integrated assembler.
>>
>> Signed-off-by: Stefan Agner <stefan@...er.ch>
>> ---
>> Changes since v1:
>> - Explicitly use unified syntax for inline assembly
>>
>>
>>  arch/arm/include/asm/uaccess.h | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm/include/asm/uaccess.h b/arch/arm/include/asm/uaccess.h
>> index 6390a40f16e7..a50f9b4e2574 100644
>> --- a/arch/arm/include/asm/uaccess.h
>> +++ b/arch/arm/include/asm/uaccess.h
>> @@ -86,7 +86,8 @@ static inline void set_fs(mm_segment_t fs)
>>  #define __range_ok(addr, size) ({ \
>>  	unsigned long flag, roksum; \
>>  	__chk_user_ptr(addr);	\
>> -	__asm__("adds %1, %2, %3; sbcccs %1, %1, %0; movcc %0, #0" \
>> +	__asm__(".syntax unified\n" \
>> +		"adds %1, %2, %3; sbcscc %1, %1, %0; movcc %0, #0" \
> 
> Instead of sprinkling ".syntax unified" around, you could consider 
> including <asm/unified.h> when needed.

This seems not to help in this case. When I include <asm/unified.h> at
the beginning of uaccess.h GCC still seems to emit ".syntax unified".

I guess this is since we are dealing with inline assembly. As far as I
understand its really gcc which emits a ".syntax divided" by default
when compiling ARM mode. There is an option "-masm-syntax-unified" which
according to the gcc man page should do what we need, but it seems
broken, see also: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88648

I agree this patch is not particularly pretty, but it is the only
solution I found to make this inline assembly work with gcc and Clang.

--
Stefan