Re: [PATCH 1/2] tracing: work around -Wmissing-format-attribute warning

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On Wed, Jun 03 2026, Rasmus Villemoes <ravi@prevas.dk> wrote:

> On Wed, Jun 03 2026, "Arnd Bergmann" <arnd@arndb.de> wrote:
>
>> On Wed, Jun 3, 2026, at 09:15, Rasmus Villemoes wrote:
>>> On Tue, Jun 02 2026, "Arnd Bergmann" <arnd@arndb.de> wrote:
>>>> On Tue, Jun 2, 2026, at 20:59, Andy Shevchenko wrote:
>>>>> On Tue, Jun 02, 2026 at 05:07:05PM +0200, Arnd Bergmann wrote:
>>>
>>> May I suggest a different approach, that avoids having that extra
>>> function emitted (which presumably compiles to a single jump
>>> instruction, but still, with retpoline and CFI and all that it all adds
>>> up): Keep the declaration of __vsnprintf() in the header without the
>>> __print() attribute, but then do
>>>
>>> int __vsnprintf(char *buf, size_t size, const char *fmt_str, va_list args)
>>>    __alias(vsnprintf);
>>>
>>> in vsprintf.c. Aside from reusing the same entry point, I could well
>>> imagine a compiler some day complaining about seeing the printf
>>> attribute applied in a local extra declaration but not having it in the
>>> header file.
>>>
>>> Presumably it will need its own EXPORT_SYMBOL if any of the intended
>>> users are modular, and it certainly still needs a comment.
>>
>> I had tried that earlier but given up because the attributes have to
>> match exactly.
>>
>> This definition works with all currently supported versions of gcc,
>> but may have to change when the there is a new version that adds
>> even more attributes:
>>
>> int
>> __printf(3, 0)
>> __attribute__((nothrow))
>> __attribute__((nonnull(1)))
>> __vsnprintf(char *__restrict buf, size_t size,
>>             const char * __restrict fmt_str, va_list args)
>>                __alias(vsnprintf);
>>
>
> Ah, I see. The documentation for the alias attribute does say that the
> types have to match, but I didn't know that the nothrow and nonnull
> attributes were considered part of the type identity. Oddly enough, if
> one does
>
>   typeof(vsnprintf) __vsnprintf __alias(vsnprintf);
>
> that still fails, but only complains about nothrow, not nonnull.
>
> I don't remember what minimum gcc we currently require, but gcc 9
> introduced another attribute that is apperently meant for cases like
> this: 'copy'. This seems to build:
>
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 9f359b31c8d1..c1402d375429 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2988,6 +2988,9 @@ int vsnprintf(char *buf, size_t size, const char *fmt_str, va_list args)
>  }
>  EXPORT_SYMBOL(vsnprintf);
>  
> +int __vsnprintf(char *buf, size_t size, const char *fmt_str, va_list args)
> +       __alias(vsnprintf) __attribute__((__copy__(vsnprintf)));
> +
>  /**
>   * vscnprintf - Format a string and place it in a buffer
>   * @buf: The buffer to place the result into
>
> That at least should handle any future "gcc knows this-or-that about the
> vsnprintf function". But I don't know if clang supports that copy
> mechanism or if the minimum supported gcc is too old.

Ah, so we already have __copy in compiler-attributes.h, stating that
it's not supported by clang. Our current minimum gcc version is 8. But
judging from the commit message for c0d9782f5, perhaps it's not actually
a problem that it just expands to nothing for gcc 8, as the "less
restrictive attribute" warning was also introduced with gcc 9, and the
__copy macro is already used for module init/exit functions. Which also
suggests that it might not be a problem that clang doesn't support it.

Rasmus