Some virtual if drivers has (ab)used ifa ifa_rtrequest hook to enforce
route MTU to be not bigger that interface MTU. While ifa_rtrequest hooking
might be an option in some situation, it is not feasible to do MTU checks
there: generic (or per-domain) routing code is perfectly capable of doing
this.
We currrently have 3 places where MTU is altered:
1) route addition.
In this case domain overrides radix _addroute callback (in[6]_addroute)
and all necessary checks/fixes are/can be done there.
2) route change (especially, GW change).
In this case, there are no explicit per-domain calls, but one can
override rte by setting ifa_rtrequest hook to domain handler
(inet6 does this).
3) ifconfig ifaceX mtu YYYY
In this case, we have no callbacks, but ip[6]_output performes runtime
checks and decreases rt_mtu if necessary.
Generally, the goals are to be able to handle all MTU changes in
control plane, not in runtime part, and properly deal with increased
interface MTU.
This commit changes the following:
* removes hooks setting MTU from drivers side
* adds proper per-doman MTU checks for case 1)
* adds generic MTU check for case 2)
* The latter is done by using new dom_ifmtu callback since
if_mtu denotes L3 interface MTU, e.g. maximum trasmitted _packet_ size.
However, IPv6 mtu might be different from if_mtu one (e.g. default 1280)
for some cases, so we need an abstract way to know maximum MTU size
for given interface and domain.
* moves rt_setmetrics() before MTU/ifa_rtrequest hooks since it copies
user-supplied data which must be checked.
* removes RT_LOCK_ASSERT() from other ifa_rtrequest hooks to be able to
use this functions on new non-inserted rte.
More changes will follow soon.
MFC after: 1 month
Sponsored by: Yandex LLC
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