[PATCH v2] netlink: Return prefix length for IPv6 addresses in nl_addr_get()

[Jon Maloy <jmaloy@redhat.com>]

nl_addr_get() was not setting the prefix_len output parameter for
IPv6 addresses, only for IPv4. This meant callers always got 0 for
IPv6, forcing them to use a hardcoded default (64).

Fix by assigning *prefix_len even in the IPv6 case.

We also add another functional change. We now check for if an AF_INET
address is link local, in which case we have to skip it. Although it
is conventional to set the scope of such addresses to RT_SCOPE_HOST,
this is not stated in any RFC, and we cannot trust it to have been set
correctly by the user, just as we cannot trust it to have been set
correctly for any other AF_INET address. We therefore add this check
explicitly on the address itself.

Signed-off-by: Jon Maloy <jmaloy@redhat.com>

---
v2: - Simplified according to feedback from S. Brivio
    - Added test fo AF_INET link local property
---
 netlink.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/netlink.c b/netlink.c
index 82a2f0c..f0e8fa7 100644
--- a/netlink.c
+++ b/netlink.c
@@ -752,7 +752,7 @@ int nl_addr_set_ll_nodad(int s, unsigned int ifi)
  * @ifi:	Interface index in outer network namespace
  * @af:		Address family
  * @addr:	Global address to fill
- * @prefix_len:	Mask or prefix length, to fill (for IPv4)
+ * @prefix_len:	Mask or prefix length, to fill
  * @addr_l:	Link-scoped address to fill (for IPv6)
  *
  * Return: 0 on success, negative error code on failure
@@ -777,6 +777,7 @@ int nl_addr_get(int s, unsigned int ifi, sa_family_t af,
 	nl_foreach_oftype(nh, status, s, buf, seq, RTM_NEWADDR) {
 		struct ifaddrmsg *ifa = (struct ifaddrmsg *)NLMSG_DATA(nh);
 		struct rtattr *rta;
+		bool link_local;
 		size_t na;
 
 		if (ifa->ifa_index != ifi || ifa->ifa_flags & IFA_F_DEPRECATED)
@@ -788,18 +789,14 @@ int nl_addr_get(int s, unsigned int ifi, sa_family_t af,
 			    (af == AF_INET6 && rta->rta_type != IFA_ADDRESS))
 				continue;
 
-			if (af == AF_INET && ifa->ifa_prefixlen > prefix_max) {
-				memcpy(addr, RTA_DATA(rta), RTA_PAYLOAD(rta));
+			link_local = (af == AF_INET6) ?
+				ifa->ifa_scope >= RT_SCOPE_LINK :
+				IN4_IS_ADDR_LINKLOCAL(RTA_DATA(rta));
 
-				prefix_max = *prefix_len = ifa->ifa_prefixlen;
-			} else if (af == AF_INET6 && addr &&
-				   ifa->ifa_scope < RT_SCOPE_LINK &&
-				   ifa->ifa_prefixlen > prefix_max) {
+			if (ifa->ifa_prefixlen > prefix_max && !link_local) {
 				memcpy(addr, RTA_DATA(rta), RTA_PAYLOAD(rta));
-
-				prefix_max = ifa->ifa_prefixlen;
+				prefix_max = *prefix_len = ifa->ifa_prefixlen;
 			}
-
 			if (addr_l &&
 			    af == AF_INET6 && ifa->ifa_scope == RT_SCOPE_LINK &&
 			    ifa->ifa_prefixlen > prefix_max_ll) {
-- 
2.52.0