CVE-2026-53013: macvlan: fix macvlan_get_size() not reserving space for IFLA_MACVLAN_BC_CUTOFF

In the Linux kernel, the following vulnerability has been resolved:

macvlan: fix macvlangetsize() not reserving space for IFLAMACVLANBCCUTOFF

macvlangetsize() does not account for IFLAMACVLANBCCUTOFF, but macvlanfillinfo() conditionally includes it when port->bccutoff != 1. This causes nlaputs32() to fail with -EMSGSIZE when the netlink skb runs out of space, triggering a WARNON in rtnetlink and preventing the interface from being dumped.

The bug can be reproduced with:

ip link add macvlan0 link eth0 type macvlan mode bridge ip link set macvlan0 type macvlan bccutoff 0 ip -d link show macvlan0 # fails with -EMSGSIZE

The bccutoff feature was added in commit 954d1fa1ac93 ("macvlan: Add netlink attribute for broadcast cutoff"), which added the nlaputs32() call in macvlanfillinfo() but missed adding the corresponding nlatotalsize(4) in macvlangetsize(). A follow-up commit 55cef78c244d ("macvlan: add forgotten nlapolicy for IFLAMACVLANBCCUTOFF") fixed the missing nlapolicy entry but still did not fix the size calculation.