VNET: Revert "ifnet: make if_index global"
This reverts commit 91f44749c6feb50f39af8805dd803e860f0418f1.
Devirtualization of V_if_index and V_ifindex_table was rushed into
the tree lacking proper context, discussion, and declaration of intent,
so I'm backing it out as harmful to VNET on the following grounds:
- The change repurposed the decades-old and stable if_index KBI for
new, unclear goals which were omitted from the commit note.
- The change opened up a new resource exhaustion vector where any vnet
could starve the system of ifnet indices, including vnet0.
- To circumvent the newly introduced problem of separating ifnets
belonging to different vnets from the globalized ifindex_table, the
author introduced sysctl_ifcount() which does a linear traversal over
the (potentially huge) global ifnet list just to return a simple upper
bound on existing ifnet indices.
- The change effectively led to nonuniform ifnet index allocation
- The commit note clearly stated that the patch changed the implicit
if_index ABI contract where ifnet indices were assumed to be starting
from one. The commit note also included a correct observation that
holes in interface indices were always allowed, but failed to declare
that the userland-observable ifindex tables could now include huge
empty spans even under modest operating conditions.
- The author had an earlier proposal in the works which did not
affect per-vnet ifnet lists (D33265) but which he abandoned without
providing the rationale behind his decision to do so, at the expense
of sacrificing the vnet isolation contract and if_index ABI / KBI.
Furthermore, the author agreed to back out his changes himself and
to follow up with a proposal for a less intrusive alternative, but
later silently declined to act. Therefore, I decided to resolve the
status-quo by backing this out myself. This in no way precludes a
future proposal aiming to mitigate ifnet-removal related system
crashes or panics to be accepted, provided it would not unnecessarily
compromise the goal of as strict as possible isolation between vnets.
Obtained from: github.com/glebius/FreeBSD/commits/backout-ifindex