Re: [PATCH v4] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

From: "Geoffrey D. Bennett" <g@b4.vu>
To: Nathan Chancellor <nathan@kernel.org>
Cc: 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 v4] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config
Date: Sat, 26 Jun 2021 05:56:04 +0930	[thread overview]
Message-ID: <20210625202604.GB23780@m.b4.vu> (raw)
In-Reply-To: <20210625201150.1523987-1-nathan@kernel.org>

On Fri, Jun 25, 2021 at 01:11:51PM -0700, 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.

Your note about no bug which was added in v2 went missing in v3:

> > the loop does not iterate as expected. This is not a bug in practice
> > because count is not greater than one at the moment but this could
> > change in the future so this should be fixed.

> Replace the cast with a temporary variable of the proper type, which is
> less error prone and fixes the iteration. Do the same thing for the
> 'u8 *' below this if block.
> 
> Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1408
> Acked-by: Geoffrey D. Bennett <g@b4.vu>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
> 
> v1 -> v2:
> 
> * Use temporary variables of proper type rather than casting, as
>   requested by Takashi.
> 
> * Mention that there is not a bug at the moment per Geoffrey's comment.
> 
> v2 -> v3:
> 
> * Restrict scope of buf_16 more, as requested by Geoffrey.
> 
> * Add Geoffrey's ack.
> 
> v3 -> v4:
> 
> * Fix stray newline added below
> 
>   if (config_item->size >= 8) {
> 
>   leftover from buf_16's declaration.
> 
>  sound/usb/mixer_scarlett_gen2.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
> index fcba682cd422..161d832cafef 100644
> --- a/sound/usb/mixer_scarlett_gen2.c
> +++ b/sound/usb/mixer_scarlett_gen2.c
> @@ -1177,6 +1177,7 @@ static int scarlett2_usb_get_config(
>  	const struct scarlett2_config *config_item =
>  		&scarlett2_config_items[info->has_mixer][config_item_num];
>  	int size, err, i;
> +	u8 *buf_8;
>  	u8 value;
>  
>  	/* For byte-sized parameters, retrieve directly into buf */
> @@ -1185,9 +1186,12 @@ static int scarlett2_usb_get_config(
>  		err = scarlett2_usb_get(mixer, config_item->offset, buf, size);
>  		if (err < 0)
>  			return err;
> -		if (size == 2)
> -			for (i = 0; i < count; i++, (u16 *)buf++)
> -				*(u16 *)buf = le16_to_cpu(*(__le16 *)buf);
> +		if (size == 2) {
> +			u16 *buf_16 = buf;
> +
> +			for (i = 0; i < count; i++, buf_16++)
> +				*buf_16 = le16_to_cpu(*(__le16 *)buf_16);
> +		}
>  		return 0;
>  	}
>  
> @@ -1197,8 +1201,9 @@ static int scarlett2_usb_get_config(
>  		return err;
>  
>  	/* then unpack from value into buf[] */
> +	buf_8 = buf;
>  	for (i = 0; i < 8 && i < count; i++, value >>= 1)
> -		*(u8 *)buf++ = value & 1;
> +		*buf_8++ = value & 1;
>  
>  	return 0;
>  }
> 
> base-commit: 0cbbeaf370221fc469c95945dd3c1198865c5fe4
> -- 
> 2.32.0.93.g670b81a890
> 