linux-kernel - Re: [PATCH v5] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get

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Date:   Thu, 01 Jul 2021 19:12:01 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     "Geoffrey D. Bennett" <g@...vu>, Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        clang-built-linux@...glegroups.com, alsa-devel@...a-project.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

On Sun, 27 Jun 2021 07:12:03 +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. 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@...vu>
> Signed-off-by: Nathan Chancellor <nathan@...nel.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.
> 
> v4 -> v5 (or how many times does it take Nathan to get a patch right):
> 
> * Re-add note about no bug that was dropped in v3 by accident, as
>   noticed by Geoffrey. My apologies for the multiple revisions.

Thanks, applied now.


Takashi