[vazub]

Problem

With keventd handling input devices, every rule-created /dev/input/by-id/usb-* link was broken, and the names were missing the USB interface number:

usb-X-event-kbd     -> ../../event17    # resolves to /dev/event17, which does not exist
usb-X-if-event-kbd                      # should be ...-if02-event-kbd
usb-X-event-if                          # should be ...-event-if01

Two independent bugs were causing this, fixed with this combined PR.

Issue 1

symlink_add() built the target as one ../ per / in the link name followed by the basename of the node. That is only right for nodes directly in /dev (disk/by-id/x -> ../../sda). For SYMLINK+="input/by-id/..." and the node /dev/input/event17 it gave ../../event17. Any rule-created link to a node in a subdirectory (input/, dri/, snd/, ...) was dangling.

The target is now computed relative to the link's directory. Nodes directly in /dev come out exactly as before. A path that does not fit the buffer is reported instead of truncated.

Issue 2

The installed 60-persistent-input.rules builds the name with

ATTRS{bInterfaceNumber}=="?*", ...
    SYMLINK+="input/by-id/...-if$attr{bInterfaceNumber}-event-..."

In udev, $attr{} and %s{} read the attribute from the event device and, if it has none, from the parent selected by a preceding KERNELS/SUBSYSTEMS/DRIVERS/ATTRS == key. Here match_parent_chain() found the USB interface but discarded it, and subst_value() only looked at the event device, so the number expanded to nothing.

The syspath of the matched parent is now remembered and used as a fallback. It is cleared at the start of every rule and again after the rule has been applied, so it cannot leak into the next rule or into the deferred RUN+= expansion.

Testing

Device	Before	After
interface 02, keyboard	usb-X-if-event-kbd	usb-X-if02-event-kbd
interface 01, no class	usb-X-event-if	usb-X-event-if01
interface 00, mouse	usb-X-event-mouse	unchanged