Re: [PATCH v4 07/14] arm64: Add support for suppressing warning backtraces

Guenter Roeck
2025-03-19T13:13:27Z

On 3/19/25 01:05, Christophe Leroy wrote:
>
>
> Le 18/03/2025 à 16:59, Will Deacon a écrit :
>> On Thu, Mar 13, 2025 at 05:40:59PM +0100, Alessandro Carminati wrote:
>>> On Thu, Mar 13, 2025 at 1:25 PM Will Deacon <will@kernel.org> wrote:
>>>>
>>>> On Thu, Mar 13, 2025 at 11:43:22AM +0000, Alessandro Carminati wrote:
>>>>> diff --git a/arch/arm64/include/asm/bug.h b/arch/arm64/include/asm/bug.h
>>>>> index 28be048db3f6..044c5e24a17d 100644
>>>>> --- a/arch/arm64/include/asm/bug.h
>>>>> +++ b/arch/arm64/include/asm/bug.h
>>>>> @@ -11,8 +11,14 @@
>>>>>
>>>>>   #include <asm/asm-bug.h>
>>>>>
>>>>> +#ifdef HAVE_BUG_FUNCTION
>>>>> +# define __BUG_FUNC  __func__
>>>>> +#else
>>>>> +# define __BUG_FUNC  NULL
>>>>> +#endif
>>>>> +
>>>>>   #define __BUG_FLAGS(flags)                           \
>>>>> -     asm volatile (__stringify(ASM_BUG_FLAGS(flags)));
>>>>> +     asm volatile (__stringify(ASM_BUG_FLAGS(flags, %c0)) : : "i" (__BUG_FUNC));
>>>>
>>>> Why is 'i' the right asm constraint to use here? It seems a bit odd to
>>>> use that for a pointer.
>>>
>>> I received this code as legacy from a previous version.
>>> In my review, I considered the case when HAVE_BUG_FUNCTION is defined:
>>> Here, __BUG_FUNC is defined as __func__, which is the name of the
>>> current function as a string literal.
>>> Using the constraint "i" seems appropriate to me in this case.
>>>
>>> However, when HAVE_BUG_FUNCTION is not defined:
>>> __BUG_FUNC is defined as NULL. Initially, I considered it literal 0,
>>> but after investigating your concern, I found:
>>>
>>> ```
>>> $ echo -E "#include <stdio.h>\n#include <stddef.h>\nint main()
>>> {\nreturn 0;\n}" | aarch64-linux-gnu-gcc -E -dM - | grep NULL
>>> #define NULL ((void *)0)
>>> ```
>>>
>>> I realized that NULL is actually a pointer that is not a link time
>>> symbol, and using the "i" constraint with NULL may result in undefined
>>> behavior.
>>>
>>> Would the following alternative definition for __BUG_FUNC be more convincing?
>>>
>>> ```
>>> #ifdef HAVE_BUG_FUNCTION
>>>      #define __BUG_FUNC __func__
>>> #else
>>>      #define __BUG_FUNC (uintptr_t)0
>>> #endif
>>> ```
>>> Let me know your thoughts.
>>
>> Thanks for the analysis; I hadn't noticed this specific issue, it just
>> smelled a bit fishy. Anyway, the diff above looks better, thanks.
>
> That propably deserves a comment.
>
> Doesn't sparse and/or checkpatch complain about 0 being used in lieu of NULL ?
>

__BUG_FUNC is only used as parameter to asm code, not as pointer.

From the diff:

- : : "i" (__FILE__), "i" (__LINE__), \
+ : : "i" (__FILE__), "i" (__BUG_FUNC), "i" (__LINE__),\

The use is quite similar to __FILE__ and __LINE__. It might even be possible
and appropriate to just define __BUG_FUNC as 0 if HAVE_BUG_FUNCTION is not defined.

Guenter