[Bug 1796748] Re: regression in 'ip --family bridge neigh' since linux v4.12

** Also affects: linux (Ubuntu Cosmic)
Importance: Undecided
Status: New

** Changed in: linux (Ubuntu Cosmic)
Importance: Undecided => Medium

** Changed in: linux (Ubuntu Cosmic)
Status: New => Fix Committed

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https://bugs.launchpad.net/bugs/1796748

Title:
regression in 'ip --family bridge neigh' since linux v4.12

Status in linux package in Ubuntu:
Triaged
Status in linux source package in Bionic:
Fix Committed
Status in linux source package in Cosmic:
Fix Committed

Bug description:
[Impact]

 * Netlink RTM_GETNEIGH requests for PF_BRIDGE are broken since linux
v4.12.

 * Users, tools (e.g., iproute2), and libraries (e.g., go netlink) that use
   such request/family currently receive nothing back in the kernel response.

 * The upstream fix resolves the breakage in the userspace-kernel interface
   by explicitly checking for the old/broken request to ensure it's replied.

[Test Case]

 * The command 'ip --family bridge neigh' returns nothing on broken kernels,
   and matches 'bridge fdb show' on fixed kernels.

 * Before:

    $ ip --family bridge neigh
    $

 * After:

    $ ip --family bridge neigh
    dev ens3 lladdr 33:33:00:00:00:01 PERMANENT
    dev ens3 lladdr 01:00:5e:00:00:01 PERMANENT
    dev ens3 lladdr 33:33:ff:e9:9d:60 PERMANENT

 * Reference:

    $ bridge fdb show
    33:33:00:00:00:01 dev ens3 self permanent
    01:00:5e:00:00:01 dev ens3 self permanent
    33:33:ff:e9:9d:60 dev ens3 self permanent

[Regression Potential]

 * Low, for three reasons:

 * The fix is fairly contained (RTM_GETNEIGH request for PF_BRIDGE
family).

 * The checks introduced by the fix are conservative, based on the size
   of the old request (the size of the old/new requests are different),
   and it does nothing different in case the (old) size doesn't match.

 * Given the above, only applications with message length and contents
   specially hand-crafted (and likely not valid nor useful) might fail.
   To the best of my knowledge, this is not the common case out there.

[Other Info]

 * The patch is only applicable to v4.12+ (so not Trusty nor Xenial).

 * The patch is the same for Bionic, Cosmic, and unstable.

 * Upstream commit: bd961c9bc664 ("rtnetlink: fix rtnl_fdb_dump() for ndmsg header")
   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bd961c9bc66497f0c63f4ba1d02900bb85078366

 * I'll submit the patch shortly to the kernel-team mailing list.

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