[PATCH] ASoC: mediatek: Check num_codecs is not zero to avoid panic during probe

[Fei Shao]

On Wed, Nov 27, 2024 at 4:10 AM Nícolas F. R. A. Prado
<nfraprado at collabora.com> wrote:
>
> Following commit 13f58267cda3 ("ASoC: soc.h: don't create dummy
> Component via COMP_DUMMY()"), COMP_DUMMY() became an array with zero
> length, and only gets populated with the dummy struct after the card is
> registered. Since the sound card driver's probe happens before the card
> registration, accessing any of the members of a dummy component during
> probe will result in undefined behavior.
>
> This can be observed in the mt8188 and mt8195 machine sound drivers. By
> omitting a dai link subnode in the sound card's node in the Devicetree,
> the default uninitialized dummy codec is used, and when its dai_name
> pointer gets passed to strcmp() it results in a null pointer dereference
> and a kernel panic.
>
> In addition to that, set_card_codec_info() in the generic helpers file,
> mtk-soundcard-driver.c, will populate a dai link with a dummy codec when
> a dai link node is present in DT but with no codec property.
>
> The result is that at probe time, a dummy codec can either be
> uninitialized with num_codecs = 0, or be an initialized dummy codec,
> with num_codecs = 1 and dai_name = "snd-soc-dummy-dai". In order to
> accommodate for both situations, check that num_codecs is not zero
> before accessing the codecs' fields but still check for the codec's dai
> name against "snd-soc-dummy-dai" as needed.
>
> While at it, also drop the check that dai_name is not null in the mt8192
> driver, introduced in commit 4d4e1b6319e5 ("ASoC: mediatek: mt8192:
> Check existence of dai_name before dereferencing"), as it is actually
> redundant given the preceding num_codecs != 0 check.
>
> Fixes: 13f58267cda3 ("ASoC: soc.h: don't create dummy Component via COMP_DUMMY()")
> Signed-off-by: Nícolas F. R. A. Prado <nfraprado at collabora.com>

Reviewed-by: Fei Shao <fshao at chromium.org>

I noticed the same issue on MT8188, so it's really nice to see this
fix. Many thanks!

Regards,
Fei