[PATCH] ALSA: control: expand limitation on the number of user-defined control element set per card

[Takashi Iwai]

On Sat, 23 Jan 2021 04:10:25 +0100,
Takashi Sakamoto wrote:
> 
> Hi,
> 
> On Fri, Jan 22, 2021 at 03:04:56PM +0100, Takashi Iwai wrote:
> > On Fri, 22 Jan 2021 09:20:32 +0100,
> > Takashi Sakamoto wrote:
> > > 
> > > ALSA control core allows usespace application to register control element
> > > set by call of ioctl(2) with SNDRV_CTL_IOCTL_ELEM_ADD request. The added
> > > control elements are called as 'user-defined'. Currently sound card has
> > > limitation on the number of the user-defined control element set up
> > > to 32.
> > > 
> > > The limitation is inconvenient to drivers in ALSA firewire stack since
> > > the drivers expect userspace applications to implement function to
> > > control device functionalities such as mixing and routing. As the
> > > userspace application, snd-firewire-ctl-services project starts:
> > > https://github.com/alsa-project/snd-firewire-ctl-services/
> > > 
> > > The project supports many devices supported by ALSA firewire stack. The
> > > limitation is mostly good since routing and mixing controls can be
> > > represented by control element set, which includes multiple control element
> > > with the same parameters. Nevertheless, it's actually inconvenient to
> > > device which has many varied functionalities. For example, plugin effect
> > > such as channel strip and reverb has many parameters. For the case, many
> > > control elements are required to configure the parameters and control
> > > element set cannot aggregates them for the parameters. At present, the
> > > implementations for below models requires more control element sets
> > > than 32:
> > > 
> > >  * snd-bebob-ctl-service
> > >    * Apogee Ensemble (31 sets for 34 elements)
> > >  * snd-dice-ctl-service
> > >    * TC Electronic Konnekt 24d (78 sets for 94 elements)
> > >    * TC Electronic Studio Konnekt 48 (98 sets for 114 elements)
> > >    * TC Electronic Konnekt Live (88 sets for 104 elements)
> > >    * TC Electronic Impact Twin (70 sets for 86 elements)
> > >    * Focusrite Liquid Saffire 56 (37 sets for 52 elements)
> > > 
> > > This commit expands the limitation according to requirement from the above
> > > applications. As a result, userspace applications can add control element
> > > sets up to 150 per sound card. It results in 154,200 user-defined control
> > > elements as maximum since one control element set can include 1028 control
> > > elements.
> > 
> > Thinking of this change again after reading your description, I find
> > that a more flexible and safer approach would be to limit the number
> > of total elements.  That is, count the number of items in each
> > element, and set the max to (32 * MAX_CONTROL_COUNT).  This will keep
> > the same max as the current implementation can achieve, while it
> > allows more elements as long as they contain lower number of items.
> > 
> > So, something like below (totally untested).
> > 
> > 
> > thanks,
> > 
> > Takashi
> > 
> > --- a/sound/core/control.c
> > +++ b/sound/core/control.c
> > @@ -18,10 +18,11 @@
> >  #include <sound/info.h>
> >  #include <sound/control.h>
> >  
> > -/* max number of user-defined controls */
> > -#define MAX_USER_CONTROLS	32
> >  #define MAX_CONTROL_COUNT	1028
> >  
> > +/* max number of user-defined controls */
> > +#define MAX_USER_CONTROLS	(32 * MAX_CONTROL_COUNT)
> > +
> >  struct snd_kctl_ioctl {
> >  	struct list_head list;		/* list of all ioctls */
> >  	snd_kctl_ioctl_func_t fioctl;
> > @@ -520,6 +521,7 @@ static int snd_ctl_remove_user_ctl(struct snd_ctl_file * file,
> >  	struct snd_card *card = file->card;
> >  	struct snd_kcontrol *kctl;
> >  	int idx, ret;
> > +	int count;
> >  
> >  	down_write(&card->controls_rwsem);
> >  	kctl = snd_ctl_find_id(card, id);
> > @@ -536,10 +538,11 @@ static int snd_ctl_remove_user_ctl(struct snd_ctl_file * file,
> >  			ret = -EBUSY;
> >  			goto error;
> >  		}
> > +	count = kctl->count;
> >  	ret = snd_ctl_remove(card, kctl);
> >  	if (ret < 0)
> >  		goto error;
> > -	card->user_ctl_count--;
> > +	card->user_ctl_count -= count;
> >  error:
> >  	up_write(&card->controls_rwsem);
> >  	return ret;
> > @@ -1435,18 +1438,18 @@ static int snd_ctl_elem_add(struct snd_ctl_file *file,
> >  			return err;
> >  	}
> >  
> > +	/* Check the number of elements for this userspace control. */
> > +	count = info->owner;
> > +	if (count == 0)
> > +		count = 1;
> > +
> >  	/*
> >  	 * The number of userspace controls are counted control by control,
> >  	 * not element by element.
> >  	 */
> > -	if (card->user_ctl_count + 1 > MAX_USER_CONTROLS)
> > +	if (card->user_ctl_count + count > MAX_USER_CONTROLS)
> >  		return -ENOMEM;
> >  
> > -	/* Check the number of elements for this userspace control. */
> > -	count = info->owner;
> > -	if (count == 0)
> > -		count = 1;
> > -
> >  	/* Arrange access permissions if needed. */
> >  	access = info->access;
> >  	if (access == 0)
> > @@ -1535,7 +1538,7 @@ static int snd_ctl_elem_add(struct snd_ctl_file *file,
> >  	 * which locks the element.
> >  	 */
> >  
> > -	card->user_ctl_count++;
> > +	card->user_ctl_count += count;
> >  
> >   unlock:
> >  	up_write(&card->controls_rwsem);
> 
> I have no objection to any change as long as it allows the service programs
> to add control elements enough for target device. However, it's unclear
> what is flexible and safe in the above patch.
> 
> When adding user-defined control element set, some objects are allocated
> for below structures with some variable-length members:
>  * struct snd_kcontrol (in include/sound/control.h)
>  * struct user_element (in sound/core/control.h)
> 
> Current implementation is to avoid too much allocation for the above
> against userspace applications with bugs or mis-programming. It's
> reasonable to limit the allocation according to count of added control
> element set for the purpose.
> 
> On the other hand, when counting the number of added control element for
> the limitation, the above becomes loose. In the worst case, the patch
> allows 32,896 sets to be allocated and against comments in my previous
> patch.

OK, my previous patch was too simplified (I forgot to take the
private_data into account), but the point is that what we want is to
cap the worst case memory consumption.

If I calculate correctly, user_element is 320 bytes, and the value is
up to 512 bytes for each item, and snd_kcontrol is 144 bytes, and
snd_kcontrol_volatile is 16 bytes per item.  And each element may
contain 1028 items.  So, the worst case scenario of the memory
consumption is:
  (320 + 512*1028 + 144 + 16*1024) * 32 = 17383936
That is, currently we allow 16MB at most.

By increasing MAX_USER_CONTROLS to 150, it'll become 77MB.

And, think what if you'd need to increase more in future, e.g. 512
elements.  The max consumption also increases linearly.

OTOH, imagine that we cap the memory consumption to 16MB instead of
limiting only the MAX_USER_CONTROLS.  This lets user still allow to
allocate more elements with smaller number of items (that is the
common use case).  In this way, we don't take a risk of higher memory
consumption while user can deploy the user elements more flexibly.


Takashi