[pulseaudio-discuss] Accessing audio as root

[Colin Guthrie]

'Twas brillig, and Markus Rechberger at 26/11/09 14:50 did gyre and gimble:
> I don't know how the permission stuff is handled, but root should be
> an exception for this and
> be allowed by default.

The exception to the rule is not necessarily the problem (the concept 
itself is valid enough), but to think about this problem generally you 
need to understand how audio works and, more importantly, what are the 
underlying limitations.

Firstly, PulseAudio handles software mixing for you. This is because 
most hardware does not support hardware mixing. This means that (at the 
lowest level) only one application is able to use the sound card at any 
given time. This clearly sucks, but obviously a software mixer solves 
this problem.

Keeping in mind that the on a multi user system, the active user can 
flip around, with each real user being denied and allowed access to the 
audio hardware as appropriate, that it is not a nice idea to let root 
use the current active user's PA as this can change later, denying root 
the sound access by extension.

So really there are only two solutions here:
  1. Bypass pulse and access the audio directly.
  2. Run roots very own PA process and use it.
  3. Run system-wide PA.
  4. Run PA on top of some lower level mixer e.g. dmix.

Now all of these have major flaws and disadvantages. 1 and 2 require 
hardware mixing which is not all that common these days so is totally 
out of the question. 3. Is nasty, requiring access rules to be pushed 
into user administration (e.g. adding users to the pulse-access 
group)[0], and also breaking SHM usage in IPC which adds huge overhead. 
4. Adds lots of latency and totally breaks glitch free control of the 
sound hardware which has huge knock on effects for power savings and 
other important aspects.

So really the problem is not all that simple. When considering all the 
various things that the old approach did not allow, but pulseaudio 
permits, this regression is really not a major one. Like I said 
previously you have to ask yourself some very serious questions when you 
are using a root process to interact with sound anyway? Why should any 
root process be doing that? Root is evil and should be avoided except 
when absolutely necessary. Added to that, the 99% use case of "root 
requireing sound" is while working under X11 and your regular user 
becomes root (su/sudo) to display a GUI app, sound will work just fine, 
so this is not a problem. The other marginal cases really don't present 
me with a burning problem that keeps me awake at night!

Col






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Colin Guthrie
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http://colin.guthr.ie/

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