Re: [PATCH] netlink: Fix selection of template interface

On 20/03/2024 06:33, David Gibson wrote:
...
  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(a)gibson.dropbear.id.au>
 ---
   conf.c    |  4 ++--
   netlink.c | 62 ++++++++++++++++++++++++++++++++++---------------------
   2 files changed, 40 insertions(+), 26 deletions(-)

 diff --git a/conf.c b/conf.c
 index 644752cc..9e0318a5 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;
   	}
   
 diff --git a/netlink.c b/netlink.c
 index 632304c1..c0a5f158 100644
 --- a/netlink.c
 +++ b/netlink.c
 @@ -254,8 +254,8 @@ unsigned int nl_get_ext_if(int s, sa_family_t af)
   		.rtm.rtm_type	 = RTN_UNICAST,
   		.rtm.rtm_family	 = af,
   	};
 -	bool default_only = true;
 -	unsigned int ifi = 0;
 +	unsigned defifi = 0, anyifi = 0;
 +	unsigned ndef = 0, nany = 0;
   	struct nlmsghdr *nh;
   	struct rtattr *rta;
   	char buf[NLBUFSIZ];
 @@ -263,7 +263,6 @@ unsigned int nl_get_ext_if(int s, sa_family_t af)
   	uint32_t seq;
   	size_t na;
   
 -again:
   	/* Look for an interface with a default route first, failing that, look
   	 * for any interface with a route, and pick it only if it's the only
   	 * interface with a route.
 @@ -271,46 +270,61 @@ again:
   	seq = nl_send(s, &req, RTM_GETROUTE, NLM_F_DUMP, sizeof(req));
   	nl_foreach_oftype(nh, status, s, buf, seq, RTM_NEWROUTE) {
   		struct rtmsg *rtm = (struct rtmsg *)NLMSG_DATA(nh);
 +		unsigned thisifi = 0;
   
 -		if (default_only) {
 -			if (ifi || rtm->rtm_dst_len || rtm->rtm_family != af)
 -				continue;
 -		} else {
 -			if (rtm->rtm_family != af)
 -				continue;
 -		}
 +		if (rtm->rtm_family != af)
 +			continue;
   
   		for (rta = RTM_RTA(rtm), na = RTM_PAYLOAD(nh); RTA_OK(rta, na);
   		     rta = RTA_NEXT(rta, na)) {
   			if (rta->rta_type == RTA_OIF) {
 -				if (!default_only && ifi &&
 -				    ifi != *(unsigned int *)RTA_DATA(rta))
 -					return 0;
 -
 -				ifi = *(unsigned int *)RTA_DATA(rta);
 +				thisifi = *(unsigned int *)RTA_DATA(rta);
   			} else if (rta->rta_type == RTA_MULTIPATH) {
   				const struct rtnexthop *rtnh;
   
   				rtnh = (struct rtnexthop *)RTA_DATA(rta);
 +				thisifi = rtnh->rtnh_ifindex;
 +			}
 +		}
   
 -				if (!default_only && ifi &&
 -				    (int)ifi != rtnh->rtnh_ifindex)
 -					return 0;
 +		if (!thisifi)
 +			continue; /* No interface for this route */
   
 -				ifi = rtnh->rtnh_ifindex;
 -			}
 +		if (rtm->rtm_dst_len == 0) {
 +			/* Default route */
 +			ndef++;
 +			if (!defifi)
 +				defifi = thisifi;
 +		} else {
 +			/* Non-default route */
 +			nany++;
 +			if (!anyifi)
 +				anyifi = thisifi;
   		}
   	}
   
   	if (status < 0)
   		warn("netlink: RTM_GETROUTE failed: %s", strerror(-status));
   
 -	if (!ifi && default_only) {
 -		default_only = false;
 -		goto again;
 +	if (defifi) {
 +		if (ndef > 1)
 +			info("Multiple default %s routes, picked first",
 +			     af == AF_INET ? "IPv4" : "IPv6");
 +		return defifi;
   	}
   
 -	return ifi;
 +	if (anyifi) {
 +		if (nany == 1)
 +			return anyifi;
 +
 +		warn("Multiple interfaces with %s routes, use -i to select one",
 +		     af == AF_INET ? "IPv4" : "IPv6"); 
This should not be a warning, for me this always triggers because I have 
two interfaces with link local addresses and no global ipv6 route as I 
do not have any ipv6 connection.

Or maybe the correct fix is to never consider ipv6 link local routes for 
this logic? At least I cannot see the purpose of using a interface with 
only a link local route.

...
  +	}
 +
 +	if (!nany)
 +		warn("No interfaces with %s routes", af == AF_INET ? "IPv4" :
"IPv6");
 +
 +	return 0;
   }
   
   /** 
I reproduced by having a second interface and confirm this patch fixes it.