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

[Su Hui]

On 2023/8/23 04:07, Christophe JAILLET wrote:
> 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?
Hi,

Really thanks for pointing this mistake.

this assignment is necessary and removing it may cause some problem.

So sorry for my mistake, I will send a patch to fix it right now.

Su Hui

> 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))