[PATCH v1 1/1] bug: Mark __warn_printf() with __printf() attribute

[PATCH v1 1/1] bug: Mark __warn_printf() with __printf() attribute

[Andy Shevchenko]

__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@linux.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;
-- 
2.50.1

Re: [PATCH v1 1/1] bug: Mark __warn_printf() with __printf() attribute

[Brendan Jackman]

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@linux.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@google.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

Re: [PATCH v1 1/1] bug: Mark __warn_printf() with __printf() attribute

[Andy Shevchenko]

+Cc: Arnd (just FYI)

On Mon, Dec 08, 2025 at 03:16:18PM +0100, 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@linux.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;

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH v1 1/1] bug: Mark __warn_printf() with __printf() attribute

[Arnd Bergmann]

On Wed, Feb 4, 2026, at 16:59, Andy Shevchenko wrote:
> +Cc: Arnd (just FYI)
>
> On Mon, Dec 08, 2025 at 03:16:18PM +0100, 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@linux.intel.com>

Yes, I think this is the correct fix. However, Brendan's patch is
now in the tip tree as d36067d6ea00 ("bug: Hush
suggest-attribute=format for __warn_printf()").

I hadn't seen your patch but did notice as well that the one
that was merged is not ideal. I decided against sending a fixup
because it looks harmless, but if you want to rebase your patch,
feel free to add

Acked-by: Arnd Bergmann <arnd@arndb.de>

      Arnd