Re: [PATCH] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

From: Takashi Iwai <tiwai@suse.de>
To: Nathan Chancellor <nathan@kernel.org>
Cc: "Geoffrey D. Bennett" <g@b4.vu>, Jaroslav Kysela <perex@perex.cz>,
	Takashi Iwai <tiwai@suse.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	alsa-devel@alsa-project.org, linux-kernel@vger.kernel.org,
	clang-built-linux@googlegroups.com
Subject: Re: [PATCH] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config
Date: Fri, 25 Jun 2021 09:45:48 +0200	[thread overview]
Message-ID: <s5heecql74j.wl-tiwai@suse.de> (raw)
In-Reply-To: <20210624212048.1356136-1-nathan@kernel.org>

On Thu, 24 Jun 2021 23:20:48 +0200,
Nathan Chancellor wrote:
> 
> Clang warns:
> 
> sound/usb/mixer_scarlett_gen2.c:1189:32: warning: expression result
> unused [-Wunused-value]
>                         for (i = 0; i < count; i++, (u16 *)buf++)
>                                                     ^      ~~~~~
> 1 warning generated.
> 
> It appears the intention was to cast the void pointer to a u16 pointer
> so that the data could be iterated through like an array of u16 values.
> However, the cast happens after the increment because a cast is an
> rvalue, whereas the post-increment operator only works on lvalues, so
> the loop does not iterate as expected.
> 
> Replace the post-increment shorthand with the full expression so the
> cast can be added in the right place and the look works as expected.
> 
> Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1408
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  sound/usb/mixer_scarlett_gen2.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
> index fcba682cd422..c20c7f1ddc50 100644
> --- a/sound/usb/mixer_scarlett_gen2.c
> +++ b/sound/usb/mixer_scarlett_gen2.c
> @@ -1186,7 +1186,7 @@ static int scarlett2_usb_get_config(
>  		if (err < 0)
>  			return err;
>  		if (size == 2)
> -			for (i = 0; i < count; i++, (u16 *)buf++)
> +			for (i = 0; i < count; i++, buf = (u16 *)buf + 1)
>  				*(u16 *)buf = le16_to_cpu(*(__le16 *)buf);

That's still too error-prone.

Could you rather introduce another variable of u16 * type, and use it
instead?  Ditto for u8 access for the code after that, too.

thanks,

Takashi