[systemd-devel] udev - device nodes removed even if created/copied from /lib/udev/devices/

[Lennart Poettering]

On Mo, 08.04.19 15:59, Dr. Todor Dimitrov (dimitrov at technology.de) wrote:

> Hello,
>
> we are observing a weird problem with udev, where the nodes
> /dev/ttyUSB* are removed as soon as the corresponding devices are
> unplugged, although they have been statically created using
> /lib/udev/devices/. According to the documentation/change logs, this
> should not happen:

Support for /lib/dev/devices has been removed in systemd 183, see NEWS
file, i.e. in 2012, seven years ago.

Support for "ignore_remove" has been removed in udev 152, nine years
ago, see its announcement back then.

You can use tmpfiles.d/ snippets to create device nodes manually if
you like. if you do, they are created from userspace and then not
deleted by the kernel, since the kernel doesn't remove device nodes
created from userspace. Also, udev itself never deletes device nodes
either.

But do note that using tmpfiles.d/ for this is racy: if the device
already exists, tmpfiles.d won't delete it and reacreated it, hence
depending on initialization timing it sometimes is the kernel that
created the device node, and sometimes userspace, and hence you know.

Moreover, do note that device node major/minor are generally assumed
to be entirely dynamic these days, hence it's problematic to
pre-create most device nodes these days, in particular on hotpluggy
subsystems which operate with fully dynamic major/minors.

Hence, it's not really a great idea to do what you do. Interfering
from userspace into an allocation scheme and naming scheme owned by
the kernel is problematic. Yes, you can get away with a certain amount
of akwardness but it's ugly in any case. Doing the tmpfiles.d/ thing
to create device nodes is fully safe only on subsystems that never got
updated to the driver model properly (which I think is the lp0 driver
but nothing of relevance today, except nvidia drivers).

> Any help or suggestions are highly appreciated. We need the device
> nodes to be static since we are bind mounting them inside a
> container.

Why would you bind mount a device node that references a non-existing
device into a container?

Lennart

--
Lennart Poettering, Berlin