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

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Date: Tue, 26 Mar 2024 16:05:11 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Arnd Bergmann <arnd@...nel.org>
Cc: 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>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/9] surface3_power: avoid format string truncation
 warning

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")
> 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. 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.

> ---
>  drivers/platform/surface/surface3_power.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/platform/surface/surface3_power.c b/drivers/platform/surface/surface3_power.c
> index 4c0f92562a79..72f904761fde 100644
> --- a/drivers/platform/surface/surface3_power.c
> +++ b/drivers/platform/surface/surface3_power.c
> @@ -245,7 +245,7 @@ static int mshw0011_bix(struct mshw0011_data *cdata, struct bix *bix)
>  		dev_err(&client->dev, "Error reading serial no: %d\n", ret);
>  		return ret;
>  	} else {
> -		snprintf(bix->serial, ARRAY_SIZE(bix->serial), "%3pE%6pE", buf + 7, buf);
> +		snprintf(bix->serial, ARRAY_SIZE(bix->serial), "%3pE%4pE", buf + 7, buf);
>  	}
>  
>  	/* get cycle count */
> -- 
> 2.39.2
>