[PATCH 16/31] ALSA: usb-audio: scarlett2: Add Gen 3 mixer support

[Geoffrey D. Bennett]

On Tue, Jun 22, 2021 at 09:34:25AM +0200, Takashi Iwai wrote:
> On Tue, 22 Jun 2021 09:07:20 +0200,
> Vladimir Sadovnikov wrote:
> > 
> > Hello Takashi!
> > 
> > Since Focusrite devices are too advanced in settings, the overall
> > amount of 256 controls is not enough for these devices (like 18i20).
> > I would like also to extend this constant up to 1024 or even more
> > since adding support of software configuration of the device also
> > can exceed the amount of 512 control elements.
> 
> This define isn't for the total number of mixer elements.  Instead,
> it's just a size of the bitmap table that contains the head of the
> linked list for each unit id (in the sense of USB mixer spec).
> So the number of mixer elements is unlimited.

Sorry I don't understand what's going on then. Am I calling
snd_usb_mixer_add_control() wrong? Because when I called it more than
MAX_ID_ELEMS times I got a buffer overflow in mixer->id_elems[] (from
memory, I can confirm tonight).

snd_usb_create_mixer() has:

        mixer->id_elems = kcalloc(MAX_ID_ELEMS, sizeof(*mixer->id_elems),
                                  GFP_KERNEL);

snd_usb_mixer_add_control() called from mixer_scarlett_gen2.c ends up
at snd_usb_mixer_add_list() which does:

        list->next_id_elem = mixer->id_elems[list->id];
        mixer->id_elems[list->id] = list;

And list->id was going over MAX_ID_ELEMS.