[Bug 2064508] Re: re-enable Ubuntu FAN in the Noble kernel

This -38.38 package fixed our cluster test that required FAN to be
working. Marked as verified, thanks!

# uname -a
Linux n1 6.8.0-38-generic #38-Ubuntu SMP PREEMPT_DYNAMIC Fri Jun 7 15:25:01 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux

** Tags removed: verification-needed-noble-linux
** Tags added: verification-done-noble-linux

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https://bugs.launchpad.net/bugs/2064508

Title:
re-enable Ubuntu FAN in the Noble kernel

Status in linux package in Ubuntu:
Invalid
Status in linux source package in Noble:
Fix Committed

Bug description:
[Impact]

In LP: #2063298, we have opted to deprecate Ubuntu FAN support because
of the maintenance overhead and the possibility of regressions /
conflicts with the new networking eBPF APIs in kernels >= 6.8.

However, we cannot disable this feature in HWE/backport kernels, so it
seems safer to adjust the Ubuntu FAN kernel patch set to ensure proper
co-existence with the updated vxlan policy requirements in newer 6.8
kernels.

The re-introduction of Ubuntu FAN should be considered as a temporary
measure, aimed at facilitating a smooth transition during its
deprecation without disrupting existing users (specifically Juju), and
enabling the backporting of 6.8 kernels to older releases.

The main plan is still to deprecate Ubuntu FAN in newer releases.

[Test case]

Rely on the specific Ubuntu FAN regression test to validate the proper
kernel support of this feature:

https://git.launchpad.net/~canonical-kernel-team/+git/autotest-client-
tests/tree/ubuntu_fan_smoke_test?h=autotest3

[Fix]

Re-apply the Ubuntu FAN patch set to the latest Noble kernel 6.8 and
integrate the IFLA_VXLAN_FAN_MAP attribute type in vxlan_policy to
satisfy the strict length validation check.

[Regression potential]

We may experience regressions with eBPF vxlan capabilities and
potentially specific use cases of the Ubuntu FAN technology (Juju
installations).

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