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

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Message-ID: <20250320125827.GQ280585@kernel.org>
Date: Thu, 20 Mar 2025 12:58:27 +0000
From: Simon Horman <horms@...nel.org>
To: Matthieu Baerts <matttbe@...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>, Paolo Abeni <pabeni@...hat.com>,
	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 Wed, Mar 19, 2025 at 05:26:35PM +0100, Matthieu Baerts wrote:
> 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@...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.

Hi Matthieu,

Thanks for your detailed explanation.
With that in mind I agree that these seem appropriate for net.