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

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 26 Oct 2022 07:29:09 +0900
Benjamin Poirier <bpoirier@nvidia.com> wrote:

> 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)

You still end up warning on older kernels. My version is simpler.
All needs to not include lstats