netlink: Don't try to get further datagrams in nl_route_dup() on NLMSG_DONE

Martin reports that, with Fedora Linux kernel version
kernel-core-6.9.0-0.rc0.20240313gitb0546776ad3f.4.fc41.x86_64,
including commit 87d381973e49 ("genetlink: fit NLMSG_DONE into same
read() as families"), pasta doesn't exit once the network namespace
is gone.

Actually, pasta is completely non-functional, at least with default
options, because nl_route_dup(), which duplicates routes from the
parent namespace into the target namespace at start-up, is stuck on
a second receive operation for RTM_GETROUTE.

However, with that commit, the kernel is now able to fit the whole
response, including the NLMSG_DONE message, into a single datagram,
so no further messages will be received.

It turns out that commit 4d6e9d0816 ("netlink: Always process all
responses to a netlink request") accidentally relied on the fact that
we would always get at least two datagrams as a response to
RTM_GETROUTE.

That is, the test to check if we expect another datagram, is based
on the 'status' variable, which is 0 if we just parsed NLMSG_DONE,
but we'll also expect another datagram if NLMSG_OK on the last
message is false. But NLMSG_OK with a zero length is always false.

The problem is that we don't distinguish if status is zero because
we got a NLMSG_DONE message, or because we processed all the
available datagram bytes.

Introduce an explicit check on NLMSG_DONE. We should probably
refactor this slightly, for example by introducing a special return
code from nl_status(), but this is probably the least invasive fix
for the issue at hand.

Reported-by: Martin Pitt <mpitt@redhat.com>
Link: https://github.com/containers/podman/issues/22052
Fixes: 4d6e9d0816 ("netlink: Always process all responses to a netlink request")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Tested-by: Paul Holzinger <pholzing@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

netlink.c

@@ -525,7 +525,8 @@ int nl_route_dup(int s_src, unsigned int ifi_src,
 		}
 	}
 
-	if (!NLMSG_OK(nh, status) || status > 0) {
+	if (nh->nlmsg_type != NLMSG_DONE &&
+	    (!NLMSG_OK(nh, status) || status > 0)) {
 		/* Process any remaining datagrams in a different
 		 * buffer so we don't overwrite the first one.
 		 */