lsattr loading modules, changing permissions of device nodes

[Marc Haber]

On Tue, Mar 16, 2021 at 09:12:10PM +0100, Michael Biebl wrote:
> Am 16.03.21 um 21:08 schrieb Marc Haber:
> > And what creates the device node with root:root 600, which I see before
> > the lsattr call?
> 
> The kernel via devtmpfs

So the kernel is configured differently than udev? Can the kernel
configuration be influenced/set by local configuration or is that a
build-time thing? I'd like this stuff not changing after the system has
stabilized after bootup.

> > |root at testsid85:~# echo change > /sys/class/misc/btrfs-control/uevent
> > |-bash: /sys/class/misc/btrfs-control/uevent: No such file or directory
> 
> Weird. What kernel is this?

Linux testsid85 5.10.0-4-amd64 #1 SMP Debian 5.10.19-1 (2021-03-02)
x86_64 GNU/Linux

testsid85 is a machine that is deliberately VERY near to a stock Debian
install as it falls out of the Installer; I also have emptysid86 which
is one of "my" servers with a custom kernel and a bunch of ansibilisms.
That box has the same issue.

I can also confirm by now that the change is actually made by udev;
stopping udev and chmod 000 /bin/udevadm lets /dev/btrfs-control keep
its mode.

Do I see correctly that the root issue we're hunting for is udev not
applying its rules (trigger not working?) after the system has started
up and so the default values from the kernel stay around until the
device is actually _used_?

Greetings
Marc

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