linux-kernel - Re: [PATCH 5/9] surface3_power: avoid format string truncation warning

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Message-ID: <CAHp75VeTq=eyumbRWiQeLnpbTOZJR=R_7=+RkKtY2YcL-z3isQ@mail.gmail.com>
Date: Wed, 27 Mar 2024 12:58:12 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Arnd Bergmann <arnd@...nel.org>, llvm@...ts.linux.dev, 
	Hans de Goede <hdegoede@...hat.com>, Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>, 
	Maximilian Luz <luzmaximilian@...il.com>, Arnd Bergmann <arnd@...db.de>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, 
	Uwe Kleine-König <u.kleine-koenig@...gutronix.de>, 
	platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/9] surface3_power: avoid format string truncation warning

On Wed, Mar 27, 2024 at 1:05 AM Nathan Chancellor <nathan@...nel.org> wrote:
> On Tue, Mar 26, 2024 at 11:38:04PM +0100, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> >
> > clang warns about printing a pair of escaped strings into a buffer that is
> > too short:
> >
> > drivers/platform/surface/surface3_power.c:248:3: error: 'snprintf' will always be truncated; specified size is 10, but format string expands to at least 12 [-Werror,-Wformat-truncation-non-kprintf]
> >   248 |                 snprintf(bix->serial, ARRAY_SIZE(bix->serial), "%3pE%6pE", buf + 7, buf);
> >       |                 ^
> >
> > Change the format string two print two less bytes so it always fits. The string
> > is still truncated, so there is no change in behavior, but the compiler no
> > longer warns about it.

> > Fixes: 85f7582cd484 ("platform/surface: Move Surface 3 Power OpRegion driver to platform/surface")

Hmm... Is it really a move patch (which by title should not have
changed the contents)?
(I haven't looked into it, though.)

> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > ---
> > Not entirely sure about this one, as I've never used escaped strings, and
> > don't know if gcc is correct to warn here, or if the kernel defines it
> > differently from the standard.
>
> As far as I understand it, this is a false positive because clang does
> not understand the kernel's %p extensions.

Yes, %pE here is special. Btw, what has already been discussed a long
time is to have a validation plugin in the kernel to check those %p
extensions, but IIUC nobody committed to it.

That said, the patch is most likely incorrect.

> GCC does not warn for
> overflow or truncation when %p is involved but the clang developers
> chose to intentionally deviate from GCC in that aspect while sticking it
> under a separate diagnostic that we could disable. I sent a patch that
> did so some time ago but I guess Masahiro never applied it...
>
> https://lore.kernel.org/20231002-disable-wformat-truncation-overflow-non-kprintf-v1-1-35179205c8d9@kernel.org/
>
> Consider dropping the changes that fix non-kprintf warnings and
> including that patch as part of this series.

-- 
With Best Regards,
Andy Shevchenko