[PATCH iproute2] ip-monitor: Do not error out when RTNLGRP_STATS is not available

[Benjamin Poirier <bpoirier@nvidia.com>]

Following commit 4e8a9914c4d4 ("ip-monitor: Include stats events in default
and "all" cases"), `ip monitor` fails to start on kernels which do not
contain linux.git commit 5fd0b838efac ("net: rtnetlink: Add UAPI toggle for
IFLA_OFFLOAD_XSTATS_L3_STATS") because the netlink group RTNLGRP_STATS
doesn't exist:

 $ ip monitor
 Failed to add stats group to list

When "stats" is not explicitly requested, change the error to a warning so
that `ip monitor` and `ip monitor all` continue to work on older kernels.

Note that the same change is not done for RTNLGRP_NEXTHOP because its value
is 32 and group numbers <= 32 are always supported; see the comment above
netlink_change_ngroups() in the kernel source. Therefore
NETLINK_ADD_MEMBERSHIP 32 does not error out even on kernels which do not
support RTNLGRP_NEXTHOP.

Reported-by: Stephen Hemminger <stephen@networkplumber.org>
Fixes: 4e8a9914c4d4 ("ip-monitor: Include stats events in default and "all" cases")
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
---
 ip/ipmonitor.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/ip/ipmonitor.c b/ip/ipmonitor.c
index 8a72ea42..45e4e8f1 100644
--- a/ip/ipmonitor.c
+++ b/ip/ipmonitor.c
@@ -195,6 +195,8 @@ static int accept_msg(struct rtnl_ctrl_data *ctrl,
 int do_ipmonitor(int argc, char **argv)
 {
 	unsigned int groups = 0, lmask = 0;
+	/* "needed" mask */
+	unsigned int nmask;
 	char *file = NULL;
 	int ifindex = 0;
 
@@ -253,6 +255,7 @@ int do_ipmonitor(int argc, char **argv)
 	ipneigh_reset_filter(ifindex);
 	ipnetconf_reset_filter(ifindex);
 
+	nmask = lmask;
 	if (!lmask)
 		lmask = IPMON_L_ALL;
 
@@ -328,8 +331,11 @@ int do_ipmonitor(int argc, char **argv)
 
 	if (lmask & IPMON_LSTATS &&
 	    rtnl_add_nl_group(&rth, RTNLGRP_STATS) < 0) {
+		if (!(nmask & IPMON_LSTATS))
+			fprintf(stderr, "Warning: ");
 		fprintf(stderr, "Failed to add stats group to list\n");
-		exit(1);
+		if (nmask & IPMON_LSTATS)
+			exit(1);
 	}
 
 	if (listen_all_nsid && rtnl_listen_all_nsid(&rth) < 0)
-- 
2.37.2