Re: [PATCH net 2/3] mptcp: sockopt: fix getting IPV6_V6ONLY

[Matthieu Baerts <matttbe@kernel.org>]

Hi Simon,

Thank you for your review!

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@vger.kernel.org
>> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/550
>> Reviewed-by: Mat Martineau <martineau@kernel.org>
>> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.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.

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.