linux-kernel - Re: [PATCH] printf: add _

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Message-ID: <aTRss_EHjqH21NEV@smile.fi.intel.com>
Date: Sat, 6 Dec 2025 19:49:39 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Tamir Duberstein <tamird@...nel.org>
Cc: Petr Mladek <pmladek@...e.com>, Steven Rostedt <rostedt@...dmis.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Kees Cook <kees@...nel.org>, linux-kernel@...r.kernel.org,
	oe-kbuild-all@...ts.linux.dev, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] printf: add __printf attribute

On Sat, Dec 06, 2025 at 12:13:34PM -0500, Tamir Duberstein wrote:
> On Sat, Dec 6, 2025 at 11:11 AM Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com> wrote:
> > On Sat, Dec 06, 2025 at 08:19:09AM -0500, Tamir Duberstein wrote:

...

> > > -static void
> > > +static void __printf(2, 3)
> >
> > 3?!
> >
> > I think it should be (2, 0). Yes, the both users call it with "%p..." in format
> > string, but the second parameter tells compiler to check the variadic
> > arguments, which are absent here. Changing 'const void *p' to '...' will align
> > it with the given __printf() attribute, but I don't know if this what we want.
> 
> The second parameter is the first-to-check, it is not specific to
> variadic arguments. Using 0 means that no arguments are checkable, so
> the compiler only validates the format string itself and won’t
> diagnose mismatches with `p`. This works whether or not we later
> change `const void *p` to `...`.

Yes, but this is fragile. As I explained it works only because we supply
the format string stuck to "%p", anything else will require reconsidering
the function prototypes. So, strictly speaking this should be (2, 0) if
we leave const void *p as is.

-- 
With Best Regards,
Andy Shevchenko