ip-monitor: Do not error out when RTNLGRP_STATS is not available

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, ignore the error 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.

v2:
* Silently ignore a failure to implicitly add the stats group, instead of
  printing a warning.

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>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

diff --git a/ip/ipmonitor.c b/ip/ipmonitor.c
index 8a72ea42db733b481597fe0b8f18734a7122ae16..d808369c2898e4c664d16822836c2c3354583e8f 100644 (file)
--- 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, failure to enable is an error */
+       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;
 
@@ -327,7 +330,8 @@ int do_ipmonitor(int argc, char **argv)
        }
 
        if (lmask & IPMON_LSTATS &&
-           rtnl_add_nl_group(&rth, RTNLGRP_STATS) < 0) {
+           rtnl_add_nl_group(&rth, RTNLGRP_STATS) < 0 &&
+           nmask & IPMON_LSTATS) {
                fprintf(stderr, "Failed to add stats group to list\n");
                exit(1);
        }