Re: [PATCH] crypto: arm64/sha: use %c constraint code in ASM_EXPORT

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 5 April 2017 at 18:21, Robin Murphy <robin.murphy@arm.com> wrote:
> On 05/04/17 18:08, Ard Biesheuvel wrote:
>> Hoi Matthias!
>>
>> On 5 April 2017 at 17:56, Matthias Kaehlcke <mka@chromium.org> wrote:
>>> From: Greg Hackmann <ghackmann@google.com>
>>>
>>> The current definition of ASM_EXPORT doesn't work properly with clang,
>>> according to https://bugs.llvm.org//show_bug.cgi?id=27250#c3 it relies on
>>> gcc specific behavior. Change the constraint from an intermediate to an
>>> output expression which works with both gcc and clang.
>>>
>>> From: Greg Hackmann <ghackmann@google.com>
>>> Commit-message-by: Matthias Kaehlcke <mka@chromium.org>
>>> Signed-off-by: Greg Hackmann <ghackmann@google.com>
>>> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
>>> ---
>>>  arch/arm64/crypto/sha1-ce-glue.c | 2 +-
>>>  arch/arm64/crypto/sha2-ce-glue.c | 2 +-
>>>  2 files changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/arm64/crypto/sha1-ce-glue.c b/arch/arm64/crypto/sha1-ce-glue.c
>>> index aefda9868627..c71e94ba0e43 100644
>>> --- a/arch/arm64/crypto/sha1-ce-glue.c
>>> +++ b/arch/arm64/crypto/sha1-ce-glue.c
>>> @@ -18,7 +18,7 @@
>>>  #include <linux/module.h>
>>>
>>>  #define ASM_EXPORT(sym, val) \
>>> -       asm(".globl " #sym "; .set " #sym ", %0" :: "I"(val));
>>> +       asm(".globl " #sym "; .set " #sym ", %c0" :: "I"(val));
>>>
>>>  MODULE_DESCRIPTION("SHA1 secure hash using ARMv8 Crypto Extensions");
>>>  MODULE_AUTHOR("Ard Biesheuvel <ard.biesheuvel@linaro.org>");
>>> diff --git a/arch/arm64/crypto/sha2-ce-glue.c b/arch/arm64/crypto/sha2-ce-glue.c
>>> index 7cd587564a41..381b5fb2dcb2 100644
>>> --- a/arch/arm64/crypto/sha2-ce-glue.c
>>> +++ b/arch/arm64/crypto/sha2-ce-glue.c
>>> @@ -18,7 +18,7 @@
>>>  #include <linux/module.h>
>>>
>>>  #define ASM_EXPORT(sym, val) \
>>> -       asm(".globl " #sym "; .set " #sym ", %0" :: "I"(val));
>>> +       asm(".globl " #sym "; .set " #sym ", %c0" :: "I"(val));
>>>
>>>  MODULE_DESCRIPTION("SHA-224/SHA-256 secure hash using ARMv8 Crypto Extensions");
>>>  MODULE_AUTHOR("Ard Biesheuvel <ard.biesheuvel@linaro.org>");
>>
>> I am fine with this change, although I would really like to add a
>> better reference to the commit log. It is *very* difficult to find any
>> documentation regarding non-trivial uses of inline asm constraints,
>> and if %c0 is the correct syntax, surely we can quote something better
>> than a LLVM bugzilla entry? Also, where does the distinction between
>> 'intermediate' vs 'output' expression come from?
>
> FWIW, GCC docs do say (under the helpfully-obvious "x86 operand
> modifiers" section[1]):
>
>         c       Require a constant operand and print the constant
>                 expression with no punctuation.
>
> Which more or less makes sense in this this context too. As an aside,
> though, since this is emitting a general integer argument to an
> assembler directive, and not an operand to an ADD instruction, how come
> we're using "I" and not "i" as the constraint in the first place?
>

No reason. "I" came to mind when writing the code, and worked as
expected. Perhaps we should just fix that at the same time.