linux-kernel - Re: [hnaz-mm:master 272/379] lib/vsprintf.c:991:13: warning: variable 'modbuildid' set but not used

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Message-ID: <8735k0isum.fsf@jogness.linutronix.de>
Date:   Wed, 02 Mar 2022 10:58:49 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     kernel test robot <lkp@...el.com>,
        Maninder Singh <maninder1.s@...sung.com>,
        kbuild-all@...ts.01.org, linux-kernel@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Vaneet Narang <v.narang@...sung.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [hnaz-mm:master 272/379] lib/vsprintf.c:991:13: warning:
 variable 'modbuildid' set but not used

On 2022-03-01, Matthew Wilcox <willy@...radead.org> wrote:
>> >    lib/vsprintf.c: In function 'va_format':
>> >    lib/vsprintf.c:1759:9: warning: function 'va_format' might be a candidate for 'gnu_printf' format attribute [-Wsuggest-attribute=format]
>> >     1759 |         buf += vsnprintf(buf, end > buf ? end - buf : 0, va_fmt->fmt, va);
>> >          |         ^~~
>> 
>> I wonder what this means.
>
> It means the compiler thinks we might want to add:
>
> __attribute__((format(gnu_printf, x, y))) to the function declaration so it
> can type-check the arguments.
>
> 'format (ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK)'
>      The 'format' attribute specifies that a function takes 'printf',
>      'scanf', 'strftime' or 'strfmon' style arguments that should be
>      type-checked against a format string.  For example, the
>      declaration:
>
>           extern int
>           my_printf (void *my_object, const char *my_format, ...)
>                 __attribute__ ((format (printf, 2, 3)));
>
>      causes the compiler to check the arguments in calls to 'my_printf'
>      for consistency with the 'printf' style format string argument
>      'my_format'.
>
>
> I haven't looked into this at all and have no idea if we should.

AFAICT it is not possible to use the gnu_printf format attribute for
this because the va_list to check is a field within the passed in struct
pointer @va_fmt.

John Ogness