linux-kernel - Re: [PATCH v1 1/1] bug: Mark __warn_printf() with _

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Message-ID: <DEZJF6CZNIOW.2GO6OSM2IKT6S@google.com>
Date: Tue, 16 Dec 2025 09:35:56 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
	Peter Zijlstra <peterz@...radead.org>, <linux-kernel@...r.kernel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v1 1/1] bug: Mark __warn_printf() with __printf() attribute

On Mon Dec 8, 2025 at 2:16 PM UTC, Andy Shevchenko wrote:
> __warn_printf() is using printf() type of format, and compiler
> is not happy about them as is:
>
> lib/bug.c:187:25: error: function ‘__warn_printf’ might be a candidate for ‘gnu_printf’ format attribute [-Werror=suggest-attribute=format]
>   187 |                         vprintk(fmt, *args);
>       |                         ^~~~~~~
>
> Fix the compilation errors by adding __printf() attribute.
>
> Fixes: 5c47b7f3d1a9 ("bug: Add BUG_FORMAT_ARGS infrastructure")
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> ---
>  lib/bug.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/bug.c b/lib/bug.c
> index edd9041f89f3..d7db57280ab7 100644
> --- a/lib/bug.c
> +++ b/lib/bug.c
> @@ -173,7 +173,7 @@ struct bug_entry *find_bug(unsigned long bugaddr)
>  	return module_find_bug(bugaddr);
>  }
>  
> -static void __warn_printf(const char *fmt, struct pt_regs *regs)
> +static __printf(1, 0) void __warn_printf(const char *fmt, struct pt_regs *regs)
>  {
>  	if (!fmt)
>  		return;

In [0] I fixed this error by disabling the warning locally. My rationale
(which I didn't write in the patch, sorry) was that the pt_regs argument
doesn't correspond to any specifier in the format string so this was a
false positive. 

However, I didn't realise you can specify 0 for the final arg - the GCC
docs[1] say:

> For functions where the arguments are not available to be checked
> (such as vprintf), specify the third parameter as zero. In this case
> the compiler only checks the format string for consistency. 

So I think what what we get here is a check that the format string isn't
garbage, without any expectation that theres a %p or whatever for the
pt_regs.

Maybe it's worth noting this down in the commit message?

Other than that, thanks, this seems superior to my patch:

Reviewed-by: Brendan Jackman <jackmanb@...gle.com>

[0] https://lore.kernel.org/all/20251207-warn-printf-gcc-v1-1-b597d612b94b@google.com/

[1] https://gcc.gnu.org/onlinedocs/gcc-3.2/gcc/Function-Attributes.html