net: Extend address label support

IPv4 addresses can be tagged with label strings. Unlike IPv6 addrlabels, which are used for prioritization of IPv6 addresses, these "ip address labels" are simply tags that the userspace can assign to IP addresses arbitrarily.

IPv4 has had support for these tags since before Linux was tracked in GIT. However it has never been possible to change the label after it is once defined. This limits usefulness of this feature. A userspace that wants to change a label might drop and recreate the address, but that disrupts routing and is just impractical.

IPv6 addresses lack support for address labels (in the sense of address tags) altogether.

In this patchset, extend IPv4 to allow changing the label defined at an address (in patch #1). Then, in patches #2 and #3, extend IPv6 with a suite of address label operations fully analogous with those defined for IPv4. Then in patches #4 and #5 add selftest coverage for the feature.

An example session with the feature in action:

	# ip address add dev d 2001:db8:1::1/64 label foo
	# ip address show dev d
	4: d: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc [...]
	    link/ether 06:29:74:fd:1f:eb brd ff:ff:ff:ff:ff:ff
	    inet6 2001:db8:1::1/64 scope global foo <--
	    valid_lft forever preferred_lft forever
	    inet6 fe80::429:74ff:fefd:1feb/64 scope link d
	    valid_lft forever preferred_lft forever

	# ip address replace dev d 2001:db8:1::1/64 label bar
	# ip address show dev d
	4: d: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc [...]
	    link/ether 06:29:74:fd:1f:eb brd ff:ff:ff:ff:ff:ff
	    inet6 2001:db8:1::1/64 scope global bar <--
	    valid_lft forever preferred_lft forever
	    inet6 fe80::429:74ff:fefd:1feb/64 scope link d
	    valid_lft forever preferred_lft forever

	# ip address del dev d 2001:db8:1::1/64 label foo
	RTNETLINK answers: Cannot assign requested address
	# ip address del dev d 2001:db8:1::1/64 label bar

Petr Machata (5):
net: ipv4: Allow changing IPv4 labels
net: ipv6: addrconf: Support IPv6 address labels
net: ipv6: addrconf: Expose IPv6 address labels through netlink
selftests: rtnetlink: Make the set of tests to run configurable
selftests: rtnetlink: Add an address label test

 include/net/addrconf.h                   |   2 +
 include/net/if_inet6.h                   |   1 +
 net/ipv4/devinet.c                       |  10 +-
 net/ipv6/addrconf.c                      |  30 +++-
 tools/testing/selftests/net/rtnetlink.sh | 172 +++++++++++++++++------
 5 files changed, 169 insertions(+), 46 deletions(-)

2.39.0