linux-kernel - Re: BPF selftest: mptcp subtest failing

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Message-ID: <da46ad00-910f-4eb1-9b74-14bd76fc8910@kernel.org>
Date: Thu, 7 Aug 2025 16:21:53 +0200
From: Matthieu Baerts <matttbe@...nel.org>
To: Harshvardhan Jha <harshvardhan.j.jha@...cle.com>,
 Mat Martineau <martineau@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
 Eduard Zingerman <eddyz87@...il.com>
Cc: Geliang Tang <geliang@...nel.org>, Mykola Lysenko <mykolal@...com>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Shuah Khan <shuah@...nel.org>,
 mptcp@...ts.linux.dev, bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 stable@...r.kernel.org
Subject: Re: BPF selftest: mptcp subtest failing

Hi Harshvardhan,

On 07/08/2025 05:50, Harshvardhan Jha wrote:
> Hi there,
> I have explicitly disabled mptpcp by default on my custom kernel and
> this seems to be causing the test case to fail. Even after enabling
> mtpcp via sysctl command or adding an entry to /etc/sysctl.conf this
> fails. I don't think this test should be failing and should account for
> cases where mptcp has not been enabled by default?

It looks like the test is failing because it expects MPTCP to be enabled
by default. Or, said differently, it doesn't expect the kernel to be
modified without adapting the corresponding tests :)

> This is the custom patch I had applied on the LTS v6.12.36 kernel and
> tested it:
> 
> diff --git a/net/mptcp/ctrl.c b/net/mptcp/ctrl.c
> index dd595d9b5e50c..bdcc4136e92ef 100644
> --- a/net/mptcp/ctrl.c
> +++ b/net/mptcp/ctrl.c
> @@ -89,7 +89,7 @@ const char *mptcp_get_scheduler(const struct net *net)
>  
>  static void mptcp_pernet_set_defaults(struct mptcp_pernet *pernet)
>  {
> -	pernet->mptcp_enabled = 1;
> +	pernet->mptcp_enabled = 0;
>  	pernet->add_addr_timeout = TCP_RTO_MAX;
>  	pernet->blackhole_timeout = 3600;
>  	atomic_set(&pernet->active_disable_times, 0);

First, I have the same question as the one I asked to RedHat devs: do
you still need to keep MPTCP disabled by default? If I remember well, on
RHEL side, they started to do that when they backported MPTCP on a
previous stable version, as an experimental feature. They left it like
that later mostly for internal process reasons I think. But honestly,
today, it no longer makes sense to do that and annoys users: all other
Linux distributions enable MPTCP by default without patching the kernel
like you did.

If you don't want to revert this patch, I guess you can modify the BPF
selftests in 'prog_tests/mptcp.c' to set 'sysctl net.mptcp.enabled=1' in
each netns created by the test. But again, not changing the default
kernel behaviour sounds like a better solution.

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