Re: [PATCH iproute2-net] mptcp: limits: fix JSON support

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 29 Oct 2026 14:17:42 +0100
"Matthieu Baerts (NGI0)" <matttbe@kernel.org> wrote:

> print_mptcp_limit() was trying to open a JSON sub-object and closing it,
> but no JSON object was actually created before.
> 
> Because of that, JSON support was working with the MPTCP limits:
> 
>   $ ip -j mptcp limits show
>   add_addr_accepted 0 subflows 2
> 
> Create a plain object before calling print_mptcp_limit: an array is not
> needed here because all limits have a different name.
> 
> After this modification, we now have:
> 
>   $ ip -j mptcp limits show | jq
>   {
>     "add_addr_accepted": 0,
>     "subflows": 2
>   }
> 
> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
> ---

Should be the regular json_obj variants, not the _plain ones
here.

The AI response was:

On Thu, 29 Oct 2026 14:17:42 +0100, Matthieu Baerts (NGI0) wrote:
> print_mptcp_limit() was trying to open a JSON sub-object and closing it,
> but no JSON object was actually created before.

Right, and that part of the diagnosis is correct -- the open_json_object()
and close_json_object() calls in print_mptcp_limit() were silent no-ops
because no writer existed. Thanks for catching it.

> Create a plain object before calling print_mptcp_limit: an array is not
> needed here because all limits have a different name.

This is where I'd push back. An array isn't needed in the sense that there
is only ever one object, but it is what every other "ip" subcommand emits,
including the other three JSON sites in this same file:

	ip/ipmptcp.c:317	new_json_obj(json)	mptcp_addr_dump()
	ip/ipmptcp.c:345	new_json_obj(json)	mptcp_addr_show()
	ip/ipmptcp.c:611	new_json_obj(json)	mptcp_monitor()

So after this patch "ip mptcp" is inconsistent with itself:

	$ ip -j mptcp endpoint show
	[]
	$ ip -j mptcp limits show
	{"add_addr_accepted":0,"subflows":2}

The _plain variant exists for the standalone utilities that have their own
output conventions -- dcb, devlink, dpll, nstat, ifstat, lnstat, vdpa. It
is not used anywhere else under ip/, and I'd rather it stayed that way.

> +		new_json_obj_plain(json);
>  		ret = print_mptcp_limit(answer, stdout);
> +		delete_json_obj_plain();

Please use the regular pair instead:

		new_json_obj(json);
		ret = print_mptcp_limit(answer, stdout);
		delete_json_obj();

which gives

	$ ip -j mptcp limits show
	[{"add_addr_accepted":0,"subflows":2}]
	$ ip -j -p mptcp limits show
	[
	    {
	        "add_addr_accepted": 0,
	        "subflows": 2
	    }
	]

No other change is needed: print_mptcp_limit() already calls
open_json_object(NULL), which is required either way -- under an array it
creates the element object, under _plain it created the root object. Plain
text output is unchanged in both versions.

Worth doing now rather than later: JSON output for the limits never worked,
so nothing can be depending on the current shape yet. I checked the mptcp
selftests as the likely consumer and they parse text only --
pm_netlink.sh:71 does

	ip -n "${ns1}" mptcp limits | awk '{ print $2" "$4 }'

with no -j or --json use anywhere under selftests/net/mptcp/ (the only jq
there parses nft output in mptcp_sockopt.sh). So the shape is still free to
change; once it ships in a release it isn't.

One nit in the commit message while you're respinning:

> Because of that, JSON support was working with the MPTCP limits:

Missing a negation -- "was not working". The example output just below makes
the intent clear, so this alone wouldn't be worth a v2.