netdev - Re: [PATCH iproute2-next 06/11] ipstats: Add a group "link"

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Date:   Sun, 1 May 2022 21:56:16 -0700
From:   David Ahern <dsahern@...il.com>
To:     Ido Schimmel <idosch@...sch.org>, Petr Machata <petrm@...dia.com>
Cc:     netdev@...r.kernel.org,
        Stephen Hemminger <stephen@...workplumber.org>,
        Ido Schimmel <idosch@...dia.com>
Subject: Re: [PATCH iproute2-next 06/11] ipstats: Add a group "link"

On 5/1/22 8:52 AM, Ido Schimmel wrote:
> When I tested this on 5.15 / 5.16 everything was fine, but now I get:
> 
> $ ip stats show dev lo group link
> 1: lo: group link
> Error: attribute payload too short
> 

...

> 
> Note the difference in size of IFLA_STATS_LINK_64 which carries struct
> rtnl_link_stats64: 196 bytes vs. 204 bytes
> 
> The 8 byte difference is most likely from the addition of
> rx_otherhost_dropped at the end:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=794c24e9921f32ded4422833a990ccf11dc3c00e
> 
> I guess it worked for me because I didn't have this member in my copy of
> the uAPI file, but it's now in iproute2-next:
> 
> https://git.kernel.org/pub/scm/network/iproute2/iproute2-next.git/commit/?id=bba95837524d09ee2f0efdf6350b83a985f4b2f8
> 
> I was under the impression that such a size increase in a uAPI struct is
> forbidden, which is why we usually avoid passing structs over netlink.

extending structs at the end is typically allowed. The ABI breakage is
when an entry is added in the middle.


> 
>> +		return -EINVAL;
>> +	}
>> +
>> +	open_json_object("stats64");
>> +	print_stats64(stdout, stats, NULL, NULL);
>> +	close_json_object();
>> +	return 0;
>> +}