[PATCH] ARM: teach __asmeq that r12 and ip are the same register

[PATCH] ARM: teach __asmeq that r12 and ip are the same register

[Ard Biesheuvel]

The __asmeq macro is used inside inline asm statements to ensure that
register asm variables that explicitly specify a register are mapped
correctly onto those registers when used in inline asm input and output
constraints. However, the string based matching fails to take into account
that 'ip' may also be referred to as 'r12', (e.g., by clang), causing false
negatives.

Fix this by making __asmeq consider the ("ip","r12") and ("r12","ip")
cases specifically.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
Even though clang shouldn't be susceptible to the same bug, there is still
value in retaining these assertions there, so instead of just turning
__asmeq into a nop if __clang__ is defined, let's make it work correctly.

 arch/arm/include/asm/compiler.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/arm/include/asm/compiler.h b/arch/arm/include/asm/compiler.h
index 8155db2f7fa1..c3a316a59710 100644
--- a/arch/arm/include/asm/compiler.h
+++ b/arch/arm/include/asm/compiler.h
@@ -8,8 +8,11 @@
  * This string is meant to be concatenated with the inline asm string and
  * will cause compilation to stop on mismatch.
  * (for details, see gcc PR 15089)
+ * For compatibility with clang, we have to explicitly take the equivalence
+ * of 'r12' and 'ip into account as well.
  */
-#define __asmeq(x, y)  ".ifnc " x "," y " ; .err ; .endif\n\t"
+#define __asmeq(x, y)  ".ifnc " x "," y " ; .ifnc " x y ",ipr12 ; " \
+	".ifnc " x y ",r12ip ; .err ; .endif ; .endif ; .endif\n\t"
 
 
 #endif /* __ASM_ARM_COMPILER_H */
-- 
1.8.3.2

[PATCH] ARM: teach __asmeq that r12 and ip are the same register

[Nicolas Pitre]

On Wed, 28 Jan 2015, Ard Biesheuvel wrote:

> The __asmeq macro is used inside inline asm statements to ensure that
> register asm variables that explicitly specify a register are mapped
> correctly onto those registers when used in inline asm input and output
> constraints. However, the string based matching fails to take into account
> that 'ip' may also be referred to as 'r12', (e.g., by clang), causing false
> negatives.
> 
> Fix this by making __asmeq consider the ("ip","r12") and ("r12","ip")
> cases specifically.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

gcc is likely to do the same substitution with r10 ("sl") and r11 
("fp").  Might as well add them right away for completeness.

Then, does clang call sp, lr, pc as r13, r14 and r15? I guess not as 
this is probably against the latest ARM assembly conventions.


> ---
> Even though clang shouldn't be susceptible to the same bug, there is still
> value in retaining these assertions there, so instead of just turning
> __asmeq into a nop if __clang__ is defined, let's make it work correctly.
> 
>  arch/arm/include/asm/compiler.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/include/asm/compiler.h b/arch/arm/include/asm/compiler.h
> index 8155db2f7fa1..c3a316a59710 100644
> --- a/arch/arm/include/asm/compiler.h
> +++ b/arch/arm/include/asm/compiler.h
> @@ -8,8 +8,11 @@
>   * This string is meant to be concatenated with the inline asm string and
>   * will cause compilation to stop on mismatch.
>   * (for details, see gcc PR 15089)
> + * For compatibility with clang, we have to explicitly take the equivalence
> + * of 'r12' and 'ip into account as well.
>   */
> -#define __asmeq(x, y)  ".ifnc " x "," y " ; .err ; .endif\n\t"
> +#define __asmeq(x, y)  ".ifnc " x "," y " ; .ifnc " x y ",ipr12 ; " \
> +	".ifnc " x y ",r12ip ; .err ; .endif ; .endif ; .endif\n\t"
>  
>  
>  #endif /* __ASM_ARM_COMPILER_H */
> -- 
> 1.8.3.2
> 
> 

[PATCH] ARM: teach __asmeq that r12 and ip are the same register

[Ard Biesheuvel]

On 28 January 2015 at 21:46, Nicolas Pitre <nicolas.pitre@linaro.org> wrote:
> On Wed, 28 Jan 2015, Ard Biesheuvel wrote:
>
>> The __asmeq macro is used inside inline asm statements to ensure that
>> register asm variables that explicitly specify a register are mapped
>> correctly onto those registers when used in inline asm input and output
>> constraints. However, the string based matching fails to take into account
>> that 'ip' may also be referred to as 'r12', (e.g., by clang), causing false
>> negatives.
>>
>> Fix this by making __asmeq consider the ("ip","r12") and ("r12","ip")
>> cases specifically.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>
> gcc is likely to do the same substitution with r10 ("sl") and r11
> ("fp").  Might as well add them right away for completeness.
>

I just checked with my 4.8 gcc toolchain: r11 indeed gets emitted as
fp when used in inline asm, but r10 is just r10.
So I should update the patch to include fp as well.

> Then, does clang call sp, lr, pc as r13, r14 and r15? I guess not as
> this is probably against the latest ARM assembly conventions.
>

@Behan, could you answer this one?

>> ---
>> Even though clang shouldn't be susceptible to the same bug, there is still
>> value in retaining these assertions there, so instead of just turning
>> __asmeq into a nop if __clang__ is defined, let's make it work correctly.
>>
>>  arch/arm/include/asm/compiler.h | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm/include/asm/compiler.h b/arch/arm/include/asm/compiler.h
>> index 8155db2f7fa1..c3a316a59710 100644
>> --- a/arch/arm/include/asm/compiler.h
>> +++ b/arch/arm/include/asm/compiler.h
>> @@ -8,8 +8,11 @@
>>   * This string is meant to be concatenated with the inline asm string and
>>   * will cause compilation to stop on mismatch.
>>   * (for details, see gcc PR 15089)
>> + * For compatibility with clang, we have to explicitly take the equivalence
>> + * of 'r12' and 'ip into account as well.
>>   */
>> -#define __asmeq(x, y)  ".ifnc " x "," y " ; .err ; .endif\n\t"
>> +#define __asmeq(x, y)  ".ifnc " x "," y " ; .ifnc " x y ",ipr12 ; " \
>> +     ".ifnc " x y ",r12ip ; .err ; .endif ; .endif ; .endif\n\t"
>>
>>
>>  #endif /* __ASM_ARM_COMPILER_H */
>> --
>> 1.8.3.2
>>
>>

[PATCH] ARM: teach __asmeq that r12 and ip are the same register

[Behan Webster]

On 01/28/15 14:00, Ard Biesheuvel wrote:
> On 28 January 2015 at 21:46, Nicolas Pitre <nicolas.pitre@linaro.org> wrote:
>> On Wed, 28 Jan 2015, Ard Biesheuvel wrote:
>>
>>> The __asmeq macro is used inside inline asm statements to ensure that
>>> register asm variables that explicitly specify a register are mapped
>>> correctly onto those registers when used in inline asm input and output
>>> constraints. However, the string based matching fails to take into account
>>> that 'ip' may also be referred to as 'r12', (e.g., by clang), causing false
>>> negatives.
>>>
>>> Fix this by making __asmeq consider the ("ip","r12") and ("r12","ip")
>>> cases specifically.
>>>
>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> gcc is likely to do the same substitution with r10 ("sl") and r11
>> ("fp").  Might as well add them right away for completeness.
>>
> I just checked with my 4.8 gcc toolchain: r11 indeed gets emitted as
> fp when used in inline asm, but r10 is just r10.
> So I should update the patch to include fp as well.
>
>> Then, does clang call sp, lr, pc as r13, r14 and r15? I guess not as
>> this is probably against the latest ARM assembly conventions.
>>
> @Behan, could you answer this one?
These are aliased in clang as one would expect.

Behan

-- 
Behan Webster
behanw at converseincode.com