lib: take into account the iff_lower_up flag

In Linux, a network driver can set the interface flags IFF_UP and
IFF_RUNNING although the IFF_LOWER_UP flag is down, which means the
interface is ready but the carrier is down:

> These values contain interface state:
>
> ifinfomsg::if_flags & IFF_UP:
>  Interface is admin up
> ifinfomsg::if_flags & IFF_RUNNING:
>  Interface is in RFC2863 operational state UP or UNKNOWN. This is for
>  backward compatibility, routing daemons, dhcp clients can use this
>  flag to determine whether they should use the interface.
> ifinfomsg::if_flags & IFF_LOWER_UP:
>  Driver has signaled netif_carrier_on()

However, FRR considers an interface is operational as soon it is up
(IFF_UP) and running (IFF_RUNNING), disregarding the IFF_LOWER_UP flag.
This can lead to a scenario where FRR starts adding routes through an
interface that is technically down at the carrier level, resulting in
kernel errors.

> Jan 02 18:07:18 dut-vm zebra[283731]: [WVJCK-PPMGD][EC 4043309093] netlink-dp (NS 0) error: Network is down, type=RTM_NEWNEXTHOP(104), seq=243, pid=3112881162
> Jan 02 18:07:18 dut-vm zebra[283731]: [X5XE1-RS0SW][EC 4043309074] Failed to install Nexthop (318[if 164]) into the kernel
> Jan 02 18:07:18 dut-vm zebra[283731]: [HSYZM-HV7HF] Extended Error: Carrier for nexthop device is down
> Jan 02 18:07:18 dut-vm zebra[283731]: [WVJCK-PPMGD][EC 4043309093] netlink-dp (NS 0) error: Network is down, type=RTM_NEWNEXTHOP(104), seq=245, pid=3112881162
> Jan 02 18:07:18 dut-vm zebra[283731]: [HSYZM-HV7HF] Extended Error: Nexthop id does not exist
> Jan 02 18:07:18 dut-vm zebra[283731]: [WVJCK-PPMGD][EC 4043309093] netlink-dp (NS 0) error: Invalid argument, type=RTM_NEWROUTE(24), seq=246, pid=3112881162
> Jan 02 18:07:18 dut-vm zebra[283731]: [X5XE1-RS0SW][EC 4043309074] Failed to install Nexthop (320[10.125.0.2 if 164]) into the kernel
> Jan 02 18:07:18 dut-vm zebra[283731]: [VYKYC-709DP] default(0:254):0.0.0.0/0: Route install failed

Consider an interface is operational when it has the IFF_UP, IFF_RUNNING
and IFF_LOWER_UP flags.

Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/networking/operstates.rst?h=v6.7-rc8#n29
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv4/nexthop.c?h=v6.7-rc8#n2886
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/netdevice.h?h=v6.7-rc8#n4198
Signed-off-by: Louis Scalbert <[email protected]>

lib/if.c

@@ -668,21 +668,26 @@ int if_is_running(const struct interface *ifp)
    if ptm checking is enabled, then ptm check has passed */
 int if_is_operative(const struct interface *ifp)
 {
-	return ((ifp->flags & IFF_UP)
-		&& (((ifp->flags & IFF_RUNNING)
-		     && (ifp->ptm_status || !ifp->ptm_enable))
-		    || !CHECK_FLAG(ifp->status,
-				   ZEBRA_INTERFACE_LINKDETECTION)));
+	return ((ifp->flags & IFF_UP) &&
+		(((ifp->flags & IFF_RUNNING)
+#ifdef IFF_LOWER_UP
+		  && (ifp->flags & IFF_LOWER_UP)
+#endif /* IFF_LOWER_UP */
+		  && (ifp->ptm_status || !ifp->ptm_enable)) ||
+		 !CHECK_FLAG(ifp->status, ZEBRA_INTERFACE_LINKDETECTION)));
 }
 
 /* Is the interface operative, eg. either UP & RUNNING
    or UP & !ZEBRA_INTERFACE_LINK_DETECTION, without PTM check */
 int if_is_no_ptm_operative(const struct interface *ifp)
 {
-	return ((ifp->flags & IFF_UP)
-		&& ((ifp->flags & IFF_RUNNING)
-		    || !CHECK_FLAG(ifp->status,
-				   ZEBRA_INTERFACE_LINKDETECTION)));
+	return ((ifp->flags & IFF_UP) &&
+		(((ifp->flags & IFF_RUNNING)
+#ifdef IFF_LOWER_UP
+		  && (ifp->flags & IFF_LOWER_UP)
+#endif /* IFF_LOWER_UP */
+			  ) ||
+		 !CHECK_FLAG(ifp->status, ZEBRA_INTERFACE_LINKDETECTION)));
 }
 
 /* Is this loopback interface ? */
@@ -744,6 +749,9 @@ const char *if_flag_dump(unsigned long flag)
 
 	strlcpy(logbuf, "<", BUFSIZ);
 	IFF_OUT_LOG(IFF_UP, "UP");
+#ifdef IFF_LOWER_UP
+	IFF_OUT_LOG(IFF_LOWER_UP, "LOWER_UP");
+#endif /* IFF_LOWER_UP */
 	IFF_OUT_LOG(IFF_BROADCAST, "BROADCAST");
 	IFF_OUT_LOG(IFF_DEBUG, "DEBUG");
 	IFF_OUT_LOG(IFF_LOOPBACK, "LOOPBACK");