[systemd-bugs] [Bug 89623] New: udev: floating net device number on devices with ro rootfs

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=89623

            Bug ID: 89623
           Summary: udev: floating net device number on devices with ro
                    rootfs
           Product: systemd
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: general
          Assignee: systemd-bugs at lists.freedesktop.org
          Reporter: vicamo at gmail.com
        QA Contact: systemd-bugs at lists.freedesktop.org

On devices with read-only rootfs, e.g. mobile phones, nic device number
(wlan<N>) may increase every time disabled and re-enabled. To be more
precisely, this happens only on devices when disabling a nic removes the
corresponding driver.

"/lib/udev/rules.d/75-persistent-net-generator.rules" checks whether NAME
attribute has been assigned to wlan<N> device: if yes, skip all the followed
steps, or, call to "/lib/udev/write_net_rules" to generate a persistent device
name rule file. That persistent file should be created under
"/etc/udev/rules.d" and named "70-persistent-net.rules", so it guarantees NAME
attribute should be assigned if available before being read. However, when
rootfs was previously mounted as read-only, a file
"/run/udev/tmp-ruiles--70-persistent-net.rules" is created instead. This
temporary file is supposed to be moved back into "/etc/udev/rules.d" by a
systemd service udev-finish right after the system finishes start-up chaos.
Again, if rootfs is still mounted as read-only, this move will certainly fail.
One last important thing, /run/udev is _NOT_ included in udev rules inclusion
paths, so any rules written here will not be taken into account when processing
uevents.

So, when wlan0 is probed for the first time on a device with read-only rootfs,
udev creates "/run/udev/tmp-ruiles--70-persistent-net.rules" and inserts one
rule for it. When wlan0 is disabled and re-enabled, since
"/run/udev/tmp-ruiles--70-persistent-net.rules" is not taken into account, its
NAME attribute will not be set, and udev recognize it as a new nic and tries to
write another rule for it again. However, in this time, "wlan0" has been taken
in the previously written temporary rules file, so "wlan1" is chosen instead,
and an exactly the same matching rule (except for NAME= part) is appended to
"/run/udev/tmp-ruiles--70-persistent-net.rules". When the device is again
disabled and re-enabled, "wlan2" will be assigned. And so on ....

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