linux-kernel - Re: [PATCH net 2/3] mptcp: sockopt: fix getting IPV6

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Message-ID: <1811bd9a-846a-48f4-95f3-6badc3e063f0@redhat.com>
Date: Thu, 20 Mar 2025 13:55:36 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Matthieu Baerts <matttbe@...nel.org>, Simon Horman <horms@...nel.org>
Cc: mptcp@...ts.linux.dev, Mat Martineau <martineau@...nel.org>,
 Geliang Tang <geliang@...nel.org>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Florian Westphal <fw@...len.de>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH net 2/3] mptcp: sockopt: fix getting IPV6_V6ONLY

On 3/19/25 5:26 PM, Matthieu Baerts wrote:
> On 19/03/2025 16:38, Simon Horman wrote:
>> On Fri, Mar 14, 2025 at 09:11:32PM +0100, Matthieu Baerts (NGI0) wrote:
>>> When adding a socket option support in MPTCP, both the get and set parts
>>> are supposed to be implemented.
>>>
>>> IPV6_V6ONLY support for the setsockopt part has been added a while ago,
>>> but it looks like the get part got forgotten. It should have been
>>> present as a way to verify a setting has been set as expected, and not
>>> to act differently from TCP or any other socket types.
>>>
>>> Not supporting this getsockopt(IPV6_V6ONLY) blocks some apps which want
>>> to check the default value, before doing extra actions. On Linux, the
>>> default value is 0, but this can be changed with the net.ipv6.bindv6only
>>> sysctl knob. On Windows, it is set to 1 by default. So supporting the
>>> get part, like for all other socket options, is important.
>>>
>>> Everything was in place to expose it, just the last step was missing.
>>> Only new code is added to cover this specific getsockopt(), that seems
>>> safe.
>>>
>>> Fixes: c9b95a135987 ("mptcp: support IPV6_V6ONLY setsockopt")
>>> Cc: stable@...r.kernel.org
>>> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/550
>>> Reviewed-by: Mat Martineau <martineau@...nel.org>
>>> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@...nel.org>
>>
>> Hi Matthieu, all,
>>
>> TBH, I would lean towards this being net-next material rather than a fix
>> for net. But that notwithstanding this looks good to me.
> I understand. This patch and the next one target "net" because, with
> MPTCP, we try to mimic TCP when interacting with the userspace.
> 
> Not supporting "getsockopt(IPV6_V6ONLY)" breaks some legacy apps forced
> to use MPTCP instead of TCP. These apps apparently "strangely" check
> this "getsockopt(IPV6_V6ONLY)" before changing the behaviour with
> "setsockopt(IPV6_V6ONLY)" which is supported for a long time. The "get"
> part should have been added from the beginning, and I don't see this
> patch as a new feature. Because it simply sets an integer like most
> other "get" options, it seems better to target net and fix these apps
> ASAP rather than targeting net-next and delay this "safe" fix.
> 
> If that's OK, I would then prefer if these patches are applied in "net".
> Or they can be applied in "net-next" if we can keep their "Cc: stable"
> and "Fixes" tags, but that looks strange.

As per off-line discussion I'm going to apply only the first patch in
this series to net, and leave the other for net-next.

/P