[pulseaudio-discuss] Accessing audio as root
gmane at colin.guthr.ie
Thu Nov 26 07:15:10 PST 2009
'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
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
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), 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!
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