[Bug 1922117] Re: URFkill fails to read RFKILL events with latest kernel

[William Wilson]

This bug will not exist in hwe kernels 5.11 or 5.13. Both of those
kernels use a separate rfkill_event and rfkill_event_ext struct with the
following explanation:

/**
 * DOC: Extensibility
 *
 * Originally, we had planned to allow backward and forward compatible
 * changes by just adding fields at the end of the structure that are
 * then not reported on older kernels on read(), and not written to by
 * older kernels on write(), with the kernel reporting the size it did
 * accept as the result.
 *
 * This would have allowed userspace to detect on read() and write()
 * which kernel structure version it was dealing with, and if was just
 * recompiled it would have gotten the new fields, but obviously not
 * accessed them, but things should've continued to work.
 *
 * Unfortunately, while actually exercising this mechanism to add the
 * hard block reasons field, we found that userspace (notably systemd)
 * did all kinds of fun things not in line with this scheme:
 *
 * 1. treat the (expected) short writes as an error;
 * 2. ask to read sizeof(struct rfkill_event) but then compare the
 *    actual return value to RFKILL_EVENT_SIZE_V1 and treat any
 *    mismatch as an error.
 *
 * As a consequence, just recompiling with a new struct version caused
 * things to no longer work correctly on old and new kernels.
 *
 * Hence, we've rolled back &struct rfkill_event to the original version
 * and added &struct rfkill_event_ext. This effectively reverts to the
 * old behaviour for all userspace, unless it explicitly opts in to the
 * rules outlined here by using the new &struct rfkill_event_ext.
 *
 * Userspace using &struct rfkill_event_ext must adhere to the following
 * rules
 *
 * 1. accept short writes, optionally using them to detect that it's
 *    running on an older kernel;
 * 2. accept short reads, knowing that this means it's running on an
 *    older kernel;
 * 3. treat reads that are as long as requested as acceptable, not
 *    checking against RFKILL_EVENT_SIZE_V1 or such.
 */


This preserves backwards compatibility and therefore does not result in the bug seen previously.

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https://bugs.launchpad.net/bugs/1922117

Title:
  URFkill fails to read RFKILL events with latest kernel

Status in urfkill package in Ubuntu:
  Fix Released

Bug description:
  The kernel has added a byte to the rfkill event struct to account for
  a possible reason field. See:
  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=14486c82612a177cb910980c70ba900827ca0894

  urfkill is checking to make sure the rfkill event is exactly 8 bytes,
  and therefore failing when the reason field is included.

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