Problems with Ubuntu 16.04 kernels and wireguard 1.0.20200611

[Gregory ORIOL]

Hi,

Since commit e24c9a9265af40781fa27b5de11dd5b78925c5be to
wireguard-linux-compat with a fix for some Ubuntu versions, we are
experiencing a problem with some older, but still LTS, versions of
Ubuntu 16.04: older kernels 4.4.0-148, 4.4.0-166 fail to build with
wireguard 1.0.20200611

So, any system running an (or still having an installed) "older" kernel
and doing an apt upgrade to install wireguard 1.0.20200611 would fail
during the wireguard dkms step, while trying to build wireguard for all
the kernels available.

The problem gets more problematic when a newer kernel 4.4.0-184 gets
installed with the same apt upgrade: then, trying to downgrade wireguard
also fails; none of the 1.0.20200611 or 1.0.20200520 versions work
anymore with this combination of old/new kernels...

To recap :
# wireguard 1.0.20200520
 - ok with kernels 4.4.0-148, 4.4.0-166
 - fails with kernel 4.4.0-184
# wireguard 1.0.20200611
 - fails with kernels 4.4.0-148, 4.4.0-166
 - ok with kernel 4.4.0-184

(nb: we see it now with -184 but it could have started with an earlier
version)

We could partially fix this by manually getting each deb/src and doing
dkms install:
dkms install wireguard/1.0.20200520 -k 4.4.0-148-generic
dkms install wireguard/1.0.20200611 -k 4.4.0-184-generic
...
But apt upgrade is still broken.

While we could boot onto the newer kernel and remove the older ones to
get rid of the problem, this situation would prevent from having a
"previous working" kernel on the system, which is not very safe.

Could there be a fix for this made to wireguard-linux-compat for those
versions?

Best regards,
Greg