[Bug 2104380] Re: Build failure when CONFIG_NET_SWITCHDEV=n due to CVE-2024-26837 fix backport

** Tags removed: verification-needed-jammy-linux-kvm
** Tags added: verification-done-jammy-linux-kvm

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https://bugs.launchpad.net/bugs/2104380

Title:
Build failure when CONFIG_NET_SWITCHDEV=n due to CVE-2024-26837 fix
backport

Status in linux package in Ubuntu:
Invalid
Status in linux source package in Jammy:
In Progress

Bug description:
[ Impact ]

Since jammy:linux commit 0158242f01a0 ("net: bridge: switchdev: Skip
MDB replays of deferred events on offload"), the kernel build fails
when CONFIG_NET_SWITCHDEV=n, as observed in e.g. jammy:linux-kvm. The
error is as follows:

  /build/jammy/net/bridge/br_mdb.c: In function 'br_mdb_queue_one':
  /build/jammy/net/bridge/br_mdb.c:640:13: error: implicit declaration of function
  'switchdev_port_obj_act_is_deferred'; did you mean 'switchdev_port_obj_add'? [-Werror=implicit- function-declaration]
    640 | switchdev_port_obj_act_is_deferred(dev, action, &mdb.obj)) {
        | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        | switchdev_port_obj_add

[ Fix ]

Add a dummy implementation of switchdev_port_obj_act_is_deferred() for
the CONFIG_NET_SWITCHDEV=n case. This approach was chosen because it
is less risky and also requires minimal changes compared to the
alternatives:

* Backport 9776457c784f ("net: bridge: mdb: move all switchdev logic to br_switchdev.c"), which would involve resolving significant conflicts, or
* Revert the CVE fix, cherry-pick nearly all prerequisite patches from [1] and [2], and then reapply the CVE fix.

Note that register_switchdev_blocking_notifier() does nothing when
CONFIG_NET_SWITCHDEV=n. Therefore, nbp_switchdev_(un)sync_objs (the
sole caller of br_mdb_replay()) does not run in the first place.

[1] https://lore.kernel.org/all/20211026142743.1298877-1-vladimir.oltean@nxp.com/
[2] https://lore.kernel.org/all/20211027162119.2496321-1-vladimir.oltean@nxp.com/

[ Test Plan ]

Compile test with both CONFIG_NET_SWITCHDEV=y and
CONFIG_NET_SWITCHDEV=n.

[ Where problems could occur ]

Any regressions caused by this tiny fix-the-fix patch should be caught
during compilation.

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