[alsa-devel] multiseat and alsa audio systems (usb headsets and usb audio adapters)

Jelle de Jong jelledejong at powercraft.nl
Wed Oct 22 22:37:07 CEST 2008

Sean McNamara wrote:
> On Wed, Oct 22, 2008 at 1:48 PM, Jelle de Jong
> <jelledejong at powercraft.nl> wrote:
>> Hello everybody,
>> I have a four to eighth muliseat debian linux system here and it is
>> missing sound for the users. I was hoping the alsa team can help me with
>> this issue.
> All sound issues on Linux are extremely application-specific. Getting
> sound to work without any specific application in mind is asking for
> trouble, because there is no single configuration of the sound stack
> (which, if you expand into historical sound APIs and servers, contains
> about 20 different ways to play sound) that will satisfy any
> application without further configuration.
> Since this is not a really well-formed question, I will proceed with
> the assumption that you want to be able to run, for example,
> aplay /usr/share/sounds/pop.wav
> in the console, and have it produce sound in the "appropriate"
> headset. Here's how you proceed:
> 1. Each person on the multiseat box must have their own UNIX user account.
> 2. Each user account must have an ~/.asoundrc file.
> 3. Each ~/.asoundrc file must redefine the "default" device. How you
> redefine it depends on a lot of things:
> (a) Do you want _direct_ hardware access to the sound card, i.e. one
> application at a time?
> (b) Do you want to use ALSA's built-in software mixing, i.e. dmix?
> (c) How many channels?
> (d) Do you want to use PulseAudio?
> (e) Do you want to use JACK?
> (f) For each USB headset, what is its corresponding "raw device
> string"? This will be something like hw:n, where n is a number
> starting from 0, depending on the order in which the USB drivers
> initialize each USB controller.
> All of the above questions, and others, will determine exactly how
> each user configures their ~/.asoundrc file.
> The *extremely naive* (not recommended) example of an ~/.asoundrc is like:
> pcm.!default {
> card 3
> }
> to have ALSA clients play sound, by default, to the fourth sound
> device that happened to be initialized.
>> I have usb headsets and usb audio devices and all users have there own
>> usb hub. There are no internal audio cards in the multiseat server.
>> How can we configure an system that the user can listen to audio only to
>> the devices connected to his usb hub.
> Well, this is guaranteed by design, isn't it? Without physical access
> to someone else's headset, how would I get access to the audio streams
> being played to that headset? I don't think this really expresses what
> your problem is.
> If we flip your question around, it might be a little more
> interesting: "How can we configure a system so that each user's
> applications will only play audio to that user's corresponding
> headset?"
> You can interpret this question in two ways:
> 1. "How can we _prevent_ users from playing sound to each other's
> sound cards?" [mutually untrust[ed/ing] users]
> or
> 2. "How can we configure the default settings so that, if untampered
> with, a user's applications will play sound to that user's own headset
> and not anyone else's?" [mutually trust[ed/ing] users]
> The lack of hardware mixing actually comes back to being a security
> benefit to user space control in this kind of situation. Recall that
> if I run
> aplay --device=hw:0 /usr/share/sounds/pop.wav
> then as long as the aplay program is playing sound to hw:0, it is
> *impossible* (short of some really devious hacking in alsa-lib or the
> kernel) for another application -- from this user, or another -- to
> also play sound to hw:0 at the same time. I'll call this claim "No
> Native SW Mixing".
> Now, let's look at what is required to satisfy the question in each of
> the two above cases.
> Trusted:
> 1. We know that users aren't going to maliciously try and hurt one
> another's ears by setting someone else's mixer really loud and playing
> static to them.
> 2. We know that users won't try to tamper with eachother's processes
> or configuration files; once things are configured correctly, it'll
> stay that way.
> 3. By 'No Native SW Mixing', we know that users can *theoretically*
> hog another user's sound device whenever that user has zero
> applications currently using it.
> 4. Since users trust each other, they won't do this.
> 5. We can use something like dmix, or just use hw:0 if users don't
> need simultaneous app output, and things will be fine.
> 6. dmix is a viable option to implement software mixing.
> 7. PulseAudio is also a viable option to implement software mixing.
> Untrusted:
> 1. Users may want to maliciously try and hurt one another's ears by
> setting someone else's mixer really loud and playing static to them.
> 2. Users may try to tamper with eachother's processes or configuration
> files; once things are configured correctly, it could get messed up
> again. [There's not much you can do to prevent this ... at least not
> based on existing open source tools.]
> 3. By 'No Native SW Mixing', we know that users can *theoretically*
> hog another user's sound device whenever that user has zero
> applications currently using it.
> 4. Since users do not trust each other, there is potential for 3 to happen.
> 5. We must use 'No Native SW Mixing' to our advantage, to create a
> walled garden around each user's sound experience. We need a process
> that, once started by a user, will permanently "hog" the sound card
> that user wants to use. This process should, ideally, allow other
> processes run by that user (and by that user only) to access their USB
> headset for output or input. We need a "gatekeeper" that is sensitive
> to the context in which an app is being run.
> 6. dmix is a NOT viable option to implement software mixing, because:
> (a) Once all dmix applications close their streams, the audio device
> hw:n is available, and some malicious user can decide to acquire it,
> abuse it, and prevent its rightful user from accessing it with his own
> applications.
> (b) dmix does not restrict users from outputting sound based on their
> UID or session. It is NOT a gatekeeper, just an open gate.
> 7. PulseAudio is [perhaps the best] viable solution to implement
> software mixing for each respective user. It is also a viable solution
> to preventing users from taking control of one another's sound
> devices.
> Configuring pulseaudio for a multi-soundcard multi-user system is out
> of the scope of this mailing list, but suffice it to say that it can
> be done. You will want to run one PulseAudio daemon per user, and each
> daemon will hold exclusive control over exactly one USB headset. Each
> daemon will only accept clients' requests for audio I/O from the user
> who started the daemon. Do make sure that you disable
> module-suspend-on-idle, or PulseAudio will actually give up the sound
> device after a period of inactivity, making it insecure in an
> untrusted environment.
> BTW, if this project is for a commercial interest, you usually pay
> someone for this kind of in-depth analysis. If you're that someone,
> you might want to read up on some of the underlying technologies so
> that you can develop this sort of reasoning on your own. I can
> envision a very real situation for both the trusted and the untrusted
> environment, so there is definitely a demand for this kind of
> specialist. Trusted environments are popular at workplaces, and
> untrusted multi-seat boxes are popular at computer labs, public
> libraries, etc.
>> Any help is really appreciated.
> Please let me know if this information was helpful -- it will help me
> gauge, in turn, whether I should spend my time writing up these sorts
> of answers on the ML. FWIW, I searched around for existing articles
> touching this subject, but I couldn't find anyone who addresses this
> specific issue accurately or in enough detail.
> Sean
> P.S. - I once spoke to a user on IRC who was having intermittent
> problems with multiple USB headsets. It turns out that the USB
> controllers in the computer didn't have enough bandwidth, or power (or
> both) to allow all of the headsets to play at once. If, after
> configuring, you run into issues where only so many users can play
> sound at once, then you should invest in additional PCI or
> (preferably) PCI-Express USB controllers. Note that adding hubs
> upstream of a USB controller does absolutely nothing to lessen the
> load on the USB controllers that are integrated into your motherboard.
>> Thank in advance,
>> Jelle

Thank you Sean, this is by far the most extensive response i received in
months with an "open" question on an mailinglist, this really is a good
character, that I will reward if you sent an paypal address :-p.

I have extensive experience configuring alsa and usb headset, little new
info was delivered for me but i think others will really like your
response too.

My main issue is how to start alsa when there are no audio cards on the
system using debian sid? so i can use bluetooth or hotplug usb audio

Then I think it will become an udev issue to regulate a fixed group and
permissions for usb audio devices. I was kind of hoping somebody had
done this kind of udev ruling for usb audio devices and can provide some
examples this will save me time...anybody?

PS. I have also noticed usb audio problems (also posted them to irc
maybe 1 year ago) in the past with usb hubs and other kinds of things,
these things can brake an design or make it:-p

So thank you very much for this great response, and keep up doing this
kind of work.

Kind regards,


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