How are permissions on /dev devices set at boot time?

faentur · 02-12-2009, 12:11 PM

Specifically I'm looking at devices like /dev/dsp, /dev/mixer, etc - all the sound devices. On boot they're set to 0660: after login the pam_console process changes them to 0666, as defined in /etc/security/console.perms.

I'd like to have them set to 0666 at boot time - *before* the first user logs in - so sound can be played using remote terminals. I think it might be pam_console that sets the original perms at boot time, but I can't figure out how from the man pages - or if I can change it.

for the moment I've thrown a 'chmod' command into /etc/X11/xdm/Xsetup_0 so that those files are all changed when X starts, but that's a hack: I'm looking for a more elegant solution.

thx
faentur

ilikejam · 02-13-2009, 05:50 AM

Hi.

Take a look in /etc/security/console.perms[.d] and all should be revealed.

Dave

faentur · 02-17-2009, 01:35 PM

Thanks, Dave, I looked at /etc/security/console.perms, but as I said in my initial update I want to modify permissions before the first user logs in.

Console.perms only takes effect after the first user logs in at the gui console. But there's definitely a mechanism that resets those permissions at boot time: I've tried rebooting the machine, ssh'ing in and changing the permissions, and then rebooting again without anyone ever logging in at the gui. The permissions revert to their original state.

ilikejam · 02-17-2009, 02:00 PM

Apologies, must have read your post in haste.

The devices are created at boot, so you're getting the defaults for your sound devices. The permissions can be set in the udev rules.
On RedHat/Fedora, the sound device rules are in /etc/udev/rules.d/40-alsa.rules , but as the file says, it'll be overwritten on update.
Probably best to create a 99-my.rules in that directory to munge the permissions once the devices have been created (udev runs through all the files in there in numerical order). On my system, I'd use something like:

Code:

KERNEL=="controlC[0-9]*",	MODE="0666"
KERNEL=="hwC[D0-9]*",		MODE="0666"
KERNEL=="pcmC[D0-9cp]*",	MODE="0666"
KERNEL=="midiC[D0-9]*",		MODE="0666"
KERNEL=="timer",		MODE="0666"
KERNEL=="seq",			MODE="0666"
KERNEL=="mixer0",		MODE="0666"
KERNEL=="dsp0",			MODE="0666"
KERNEL=="audio0",		MODE="0666"

Haven't actually tried this, but I think it should work OK. If not, just edit the udev rule file that creates the devices, and add the ', MODE="0666"'.

Dave

nx5000 · 02-17-2009, 02:03 PM

udev creates devices on modern systems
Look in /etc/udev/rules
Here it is 91-permissions.rules

faentur · 02-19-2009, 07:06 PM

Well, those are great suggestions... and they totally make sense, but they didn't quite work for me. I tried both creating a new file, /etc/udev/rules.d/99-sounddev.rules

Code:

KERNEL="dsp0",          MODE="0666"
KERNEL="audio0",        MODE="0666"
KERNEL="midi[CD0-9]*",  MODE="0666"
KERNEL="mixer0",        MODE="0666"

etc., etc.

I also tried editing /etc/udev/rules.d/50-udev.rules:

Code:

# alsa devices
KERNEL="seq",            NAME="snd/%k", MODE="0666"
KERNEL="audio0",        SYMLINK="audio",        MODE="0666"
KERNEL="dsp0",          SYMLINK="dsp",  MODE="0666"
KERNEL="mixer0",        SYMLINK="mixer",        MODE="0666"

etc., etc.

Rebooted after each change, of course, and saw no changes. However, I did keep researching and found the solution: looking at /etc/udev/udev.conf I spied the following:

Quote:

# udev_permissions - The name and location of the udev permission file
udev_permissions="/etc/udev/permissions.d/"

# default_mode - set the default mode for all nodes that have no
# explicit match in the permissions file
default_mode="0600"

Ah-Ha! I took a look at /etc/udev/permissions.d/50-udev.permissions and found:

Code:

# audio devices
dsp*:root:root:0660
audio*:root:root:0660
midi*:root:root:0660
mixer*:root:root:0660
sequencer*:root:root:0660
sound/*:root:root:0660
snd/*:root:root:0660
beep:root:root:0660
admm*:root:root:0660
adsp*:root:root:0660
aload*:root:root:0660
amidi*:root:root:0660
dmfm*:root:root:0660
dmmidi*:root:root:0660
sndstat:root:root:0660

Yup, that's it. Changed those to 0666 and that solved the problem.

Thanks for your help, guys, couldn't have figured it out without the helpful boot in the right direction!