RE: [PATCH] ALSA: opti9xx: shut up gcc-10 range warning

From: David Laight <David.Laight@ACULAB.COM>
To: 'Arnd Bergmann' <arnd@arndb.de>, Jaroslav Kysela <perex@perex.cz>,
	"Takashi Iwai" <tiwai@suse.com>
Cc: Takashi Iwai <tiwai@suse.de>,
	"alsa-devel@alsa-project.org" <alsa-devel@alsa-project.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH] ALSA: opti9xx: shut up gcc-10 range warning
Date: Thu, 30 Apr 2020 08:15:02 +0000	[thread overview]
Message-ID: <c513eb4c4a01470eb3c47d8134afbec1@AcuMS.aculab.com> (raw)
In-Reply-To: <20200429190216.85919-1-arnd@arndb.de>

From: Arnd Bergmann
> Sent: 29 April 2020 20:02
> gcc-10 points out a few instances of suspicious integer arithmetic
> leading to value truncation:
> 
> sound/isa/opti9xx/opti92x-ad1848.c: In function 'snd_opti9xx_configure':
> sound/isa/opti9xx/opti92x-ad1848.c:322:43: error: overflow in conversion from 'int' to 'unsigned char'
> changes value from '(int)snd_opti9xx_read(chip, 3) & -256 | 240' to '240' [-Werror=overflow]
>   322 |   (snd_opti9xx_read(chip, reg) & ~(mask)) | ((value) & (mask)))
>       |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
> sound/isa/opti9xx/opti92x-ad1848.c:351:3: note: in expansion of macro 'snd_opti9xx_write_mask'
>   351 |   snd_opti9xx_write_mask(chip, OPTi9XX_MC_REG(3), 0xf0, 0xff);
>       |   ^~~~~~~~~~~~~~~~~~~~~~
> sound/isa/opti9xx/miro.c: In function 'snd_miro_configure':
> sound/isa/opti9xx/miro.c:873:40: error: overflow in conversion from 'int' to 'unsigned char' changes
> value from '(int)snd_miro_read(chip, 3) & -256 | 240' to '240' [-Werror=overflow]
>   873 |   (snd_miro_read(chip, reg) & ~(mask)) | ((value) & (mask)))
>       |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
> sound/isa/opti9xx/miro.c:1010:3: note: in expansion of macro 'snd_miro_write_mask'
>  1010 |   snd_miro_write_mask(chip, OPTi9XX_MC_REG(3), 0xf0, 0xff);
>       |   ^~~~~~~~~~~~~~~~~~~
> 
> These are all harmless here as only the low 8 bit are passed down
> anyway. Change the macros to inline functions to make the code
> more readable and also avoid the warning.
> 
> Strictly speaking those functions also need locking to make the
> read/write pair atomic, but it seems unlikely that anyone would
> still run into that issue.
> 
> Fixes: 1841f613fd2e ("[ALSA] Add snd-miro driver")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  sound/isa/opti9xx/miro.c           | 9 ++++++---
>  sound/isa/opti9xx/opti92x-ad1848.c | 9 ++++++---
>  2 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/sound/isa/opti9xx/miro.c b/sound/isa/opti9xx/miro.c
> index e764816a8f7a..b039429e6871 100644
> --- a/sound/isa/opti9xx/miro.c
> +++ b/sound/isa/opti9xx/miro.c
> @@ -867,10 +867,13 @@ static void snd_miro_write(struct snd_miro *chip, unsigned char reg,
>  	spin_unlock_irqrestore(&chip->lock, flags);
>  }
> 
> +static inline void snd_miro_write_mask(struct snd_miro *chip,
> +		unsigned char reg, unsigned char value, unsigned char mask)
> +{
> +	unsigned char oldval = snd_miro_read(chip, reg);
> 
> -#define snd_miro_write_mask(chip, reg, value, mask)	\
> -	snd_miro_write(chip, reg,			\
> -		(snd_miro_read(chip, reg) & ~(mask)) | ((value) & (mask)))
> +	snd_miro_write(chip, reg, (oldval & ~mask) | (value & mask));
> +}

Isn't that likely to add additional masking with 0xff at the call sites?
You will probably get better code if the arguments are 'unsigned int'.

	David

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