Re: [PATCH] ALSA: ac97: Fix possible NULL dereference in snd_ac97_mixer

[Christophe JAILLET]

Le 15/06/2023 à 04:17, Su Hui a écrit :
> smatch error:
> sound/pci/ac97/ac97_codec.c:2354 snd_ac97_mixer() error:
> we previously assumed 'rac97' could be null (see line 2072)
>
> remove redundant assignment, return error if rac97 is NULL.

Hi,

why is the assigment redundant?

Should an error occur, the 'struct snd_ac97 **' parameter was garanted 
to be set to NULL, now it is left as-is.

I've checked all callers and apparently this is fine because the probes 
fail if snd_ac97_mixer() returns an error.

However, some drivers with several mixers seem to rely on the value 
being NULL in case of error.

See [1] as an example of such code that forces a NULL value on its own, 
to be sure.

So, wouldn't it be safer to leave a "*rac97 = NULL;" just after the 
added sanity check?


CJ


[1]: 
https://elixir.bootlin.com/linux/v6.5-rc7/source/sound/pci/atiixp.c#L1438

> Fixes: da3cec35dd3c ("ALSA: Kill snd_assert() in sound/pci/*")
> Signed-off-by: Su Hui <suhui@xxxxxxxxxxxx>
> ---
>   sound/pci/ac97/ac97_codec.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/sound/pci/ac97/ac97_codec.c b/sound/pci/ac97/ac97_codec.c
> index 9afc5906d662..80a65b8ad7b9 100644
> --- a/sound/pci/ac97/ac97_codec.c
> +++ b/sound/pci/ac97/ac97_codec.c
> @@ -2069,8 +2069,8 @@ int snd_ac97_mixer(struct snd_ac97_bus *bus, struct snd_ac97_template *template,
>   		.dev_disconnect =	snd_ac97_dev_disconnect,
>   	};
>   
> -	if (rac97)
> -		*rac97 = NULL;
> +	if (!rac97)
> +		return -EINVAL;
>   	if (snd_BUG_ON(!bus || !template))
>   		return -EINVAL;
>   	if (snd_BUG_ON(template->num >= 4))