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

[Takashi Iwai <tiwai@suse.de>]

On Tue, 22 Aug 2023 22:07:40 +0200,
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?

It's misleading, yeah.  Basically all callers are with non-NULL, hence
we took rather make it mandatory.  Maybe it should have been with
WARN_ON() to catch the NULL argument for an out-of-tree stuff. 

> 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?

Yes, we need the NULL initialization.
Care to submit an additional fix patch?


thanks,

Takashi

> 
> 
> 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@nfschina.com>
> > ---
> >   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))
>