Re: [PATCH v2] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

Geoffrey D. Bennett
2021-06-25T18:43:47Z

On Fri, Jun 25, 2021 at 10:54:19AM -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. 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
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>
> v1 -> v2:
>
> * Use temporary variables of proper type rather than casting, as
> requested by Takashi. I did not include Geoffrey's ack for this
> reason.
>
> * Mention that there is not a bug at the moment per Geoffrey's comment.
>
> sound/usb/mixer_scarlett_gen2.c | 14 ++++++++++----
> 1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
> index fcba682cd422..b13903bed330 100644
> --- a/sound/usb/mixer_scarlett_gen2.c
> +++ b/sound/usb/mixer_scarlett_gen2.c
> @@ -1177,17 +1177,22 @@ 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 */
> if (config_item->size >= 8) {
> + u16 *buf_16;
> +

I would prefer that the u16 *buf_16 declaration above be removed from
there...

> size = config_item->size / 8 * count;
> 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) {
> + buf_16 = buf;

...and combined with the assignment here, like: u16 *buf_16 = buf;

Regardless:

Acked-by: Geoffrey D. Bennett <g@b4.vu>

And, thanks again!

> + for (i = 0; i < count; i++, buf_16++)
> + *buf_16 = le16_to_cpu(*(__le16 *)buf_16);
> + }
> return 0;
> }
>
> @@ -1197,8 +1202,9 @@ 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
>