[PATCH] ALSA: control: expand limitation on the number of user-defined control element set per card
Takashi Sakamoto
o-takashi at sakamocchi.jp
Sun Jan 24 06:52:25 CET 2021
Hi,
On Sat, Jan 23, 2021 at 10:10:20AM +0100, Takashi Iwai wrote:
> 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.
The memory object for data of Type-Length-Value style is underestimate in
your calculation for the worst case. For user-defined control element set,
the size is (1024 * 128) per control element set[1].
Of cource, it's possible to judge that usual userspace application don't
use data of Type-Length-Value style so much. However, we are assuming
the worst case now.
```
Objects linearly increased according to the number of user-defined control
element sets:
* struct snd_kcontrol (144 bytes)
* struct user_element (320 bytes)
* data of TLV ((1024 * 128) bytes as maximum)
Objects linearly increased according to the number of control elements:
* data of values (max. 1024 bytes in System V ABI with LP64 data type)
* data of snd_kcontrol_volatile (16 bytes)
Memory consumption under current limitation:
(144 + 320 + 1024 * 128) * 32 + (1024 + 16) * 1028 = 5,278,272
Scenario for the worst case when appying the patch:
* adding 32,896 control element sets
* for elements: (1024 + 16) * 1028 * 32 = 34,211,840
* for element sets: (144 + 320 + 1024 * 128) * 32896 = 4,327,008,256
Scenario for the worst case when applying my patch:
* adding 150 control element sets
* for elements: (1024 + 16) * 1028 * 150 = 160,368,000
* for element sets: (144 + 320 + 1024 * 128) * 150 = 19,730,400
```
[1] https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/tree/sound/core/control.c#n1299
Regards
Takashi Sakamoto
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