Re: [PATCH v3] mptcp: do not use Fast Open on MP_JOIN subflows

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] net/mptcp: unintentional disabling of TCP timestamps for MP_JOIN
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commit 2eba4e34ce837e7639b005fa4e73483919c7fa14
Author: Yilin Zhang <yilinzhang@moonshot.ai>

mptcp: do not use Fast Open on MP_JOIN subflows

This commit rejects Fast Open on MP_JOIN subflows since data on a subflow
requires a completed HMAC exchange. It falls back to the regular handshake
for TFO SYN packets and strips the Fast Open cookie from MP_JOIN SYN/ACKs.

> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index f0a6725d2c376..7bf94aea5f681 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -348,6 +348,10 @@ static void subflow_prep_synack(const struct sock *sk, struct request_sock *req,
>  	if (foc && foc->len > -1)
>  		ireq->tstamp_ok = 0;
>  
> +	/* MP_JOIN cannot use TFO, do not send a cookie in the SYN/ACK */
> +	if (foc && mptcp_subflow_rsk(req)->mp_join)
> +		foc->len = -1;
> +

[Severity: Medium]
Does this sequence unintentionally disable TCP timestamps for MP_JOIN requests
with TFO cookies?

In subflow_prep_synack(), if a remote client sends an MP_JOIN SYN with a TFO
cookie request, foc->len will initially be > -1, causing ireq->tstamp_ok to
be cleared to 0. 

Immediately following that, this new check sets foc->len to -1 to reject TFO.
While the TFO cookie is correctly dropped and not sent, tstamp_ok has already
been cleared and remains 0, leading to an unintended loss of TCP timestamps
on the subflow.

>  	if (synack_type == TCP_SYNACK_FASTOPEN)
>  		mptcp_fastopen_subflow_synack_set_params(subflow, req);
>  }

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Sashiko AI review · https://sashiko.dev/#/patchset/20260920061755.3575063-1-yilinzhang@moonshot.ai?part=1