zfs-dkms on Focal still allows building on Linux kernel version 5.10+

== SRU update, zfs-dkms, Focal ==

[Impact]

Due to a mistake in the BUILD_EXCLUSIVE_KERNEL regex in dkms.conf, dkms will still consider zfs-dkms to be buildable on kernel version 5.10+. This can happen if one installs a mainline kernel from the mainline PPA for one reason or another.

Fixing this will stop users from installing incompatible newer versions of the kernel and hence stop dkms build failures on known incompatible kernel versions. The fix is also trivial.

[The Fix]

Correctly escape . in regex so that the final sed'd string gets modified to produce the correct regex pattern, as follows:

        sed -ie '/^PACKAGE_VERSION/a BUILD_EXCLUSIVE_KERNEL="^(4\\.[0-9]+|5\\.[01234])\\."' \
                '$(CURDIR)/scripts/zfs-dkms.dkms'

[Test Plan]

Without the fix, install kernels > than versions 5.4. ZFS dkms will attempt to be built against these and fail with broken builds. Installing earlier kernels such as 4.20, 5.0, 5.4 should succeed.

With the fix, installing kernels > 5.4 will skip the ZFS dkms build phase and report a meaningful error:

Error! The /var/lib/dkms/zfs/0.8.3/5.5.19-050519-generic/x86_64/dkms.conf for module zfs includes a BUILD_EXCLUSIVE directive which
does not match this kernel/arch. This indicates that it should not be built

Test kernels for the test from https://kernel.ubuntu.com/~kernel-ppa/mainline/?C=N;O=D can be used to test this out.

[Where problems could occur]

The change is to a regex pattern match. If this is incorrect then kernels such a 5.4 will not be allowed to build the dkms zfs driver. This will clearly show up as a ZFS dkms build denied failure with an error such as:

Error! The /var/lib/dkms/zfs/0.8.3/5.5.19-050519-generic/x86_64/dkms.conf for module zfs includes a BUILD_EXCLUSIVE directive which
does not match this kernel/arch. This indicates that it should not be built

This fix is relatively low-risk, it fixes am issue with an earlier fix to the regex which wasn't quite working for kernels > 5.4 and yet didn't cause any major issues blocking legitimate users from using known working 5.4 kernels with zfs dkms.

-----------------------------------------------------------------------

Due to a mistake in the BUILD_EXCLUSIVE_KERNEL regex in dkms.conf, dkms will still consider zfs-dkms to be buildable on kernel version 5.10+. This can happen if one installs a mainline kernel from the mainline PPA for one reason or another [1].

The following patch to dkms.conf should fix the issue:

--- ./dkms.conf.orig 2021-03-16 04:15:55.077568974 +0700
+++ /usr/src/zfs-0.8.3/dkms.conf 2021-03-16 04:17:16.598219797 +0700
@@ -1,6 +1,6 @@
 PACKAGE_NAME="zfs"
 PACKAGE_VERSION="0.8.3"
-BUILD_EXCLUSIVE_KERNEL="^(4.[0-9]+|5.[01234])."
+BUILD_EXCLUSIVE_KERNEL='^(4.[0-9]+|5.[01234])\.'
 PACKAGE_CONFIG="/etc/sysconfig/zfs"
 PRE_BUILD="configure
   --prefix=/usr

Although it's not convenient for me at the moment to create a debdiff.

Related: LP: #1902701

[1] For me it's to get a complete fix for LP: #1887190, as Focal's HWE kernel doesn't seem to work.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: zfs-dkms 0.8.3-1ubuntu12.6
Uname: Linux 5.11.6-051106-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
Date: Tue Mar 16 04:26:08 2021
InstallationDate: Installed on 2021-03-15 (0 days ago)
InstallationMedia: Ubuntu 20.04.2.0 LTS "Focal Fossa" - Release amd64 (20210209.1)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=th_TH.UTF-8
 SHELL=/bin/bash
SourcePackage: zfs-linux
UpgradeStatus: No upgrade log present (probably fresh install)