netlink: Fix selection of template interface

Since f919dc7a4b1c ("conf, netlink: Don't require a default route to
start"), if there is only one host interface with routes, we will pick that
as the template interface, even if there are no default routes for an IP
version.  Unfortunately this selection had a serious flaw: in some cases
it would 'return' in the middle of an nl_foreach() loop, meaning we
wouldn't consume all the netlink responses for our query.  This could cause
later netlink operations to fail as we read leftover responses from the
aborted query.

Rewrite the interface detection to avoid this problem.  While we're there:
  * Perform detection of both default and non-default routes in a single
    pass, avoiding an ugly goto
  * Give more detail on error and working but unusual paths about the
    situation (no suitable interface, multiple possible candidates, etc.).

Fixes: f919dc7a4b1c ("conf, netlink: Don't require a default route to start")
Link: https://bugs.passt.top/show_bug.cgi?id=83
Link: https://github.com/containers/podman/issues/22052
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2270257
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
[sbrivio: Use info(), not warn() for somewhat expected cases where one
 IP version has no default routes, or no routes at all]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/conf.c b/conf.c
index 644752c..9e0318a 100644
--- a/conf.c
+++ b/conf.c
@@ -584,7 +584,7 @@ static unsigned int conf_ip4(unsigned int ifi,
 		ifi = nl_get_ext_if(nl_sock, AF_INET);
 
 	if (!ifi) {
-		info("No interface with a route for IPv4: disabling IPv4");
+		info("Couldn't pick external interface: disabling IPv4");
 		return 0;
 	}
 
@@ -656,7 +656,7 @@ static unsigned int conf_ip6(unsigned int ifi,
 		ifi = nl_get_ext_if(nl_sock, AF_INET6);
 
 	if (!ifi) {
-		info("No interface with a route for IPv6: disabling IPv6");
+		info("Couldn't pick external interface: disabling IPv6");
 		return 0;
 	}