netdev - Re: [PATCH net-next 1/2] tcp: do not set a zero size receive buffer

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Message-ID: <CANn89i+baSpvbJM6gcbSjZMmWVyvwsFotvH1czui9ARVRjS5Bw@mail.gmail.com>
Date: Mon, 21 Jul 2025 05:30:49 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, Neal Cardwell <ncardwell@...gle.com>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, "David S. Miller" <davem@...emloft.net>, 
	David Ahern <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>, 
	Matthieu Baerts <matttbe@...nel.org>
Subject: Re: [PATCH net-next 1/2] tcp: do not set a zero size receive buffer

On Mon, Jul 21, 2025 at 3:50 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On 7/21/25 10:04 AM, Eric Dumazet wrote:
> > On Fri, Jul 18, 2025 at 10:25 AM Paolo Abeni <pabeni@...hat.com> wrote:
> >>
> >> The nipa CI is reporting frequent failures in the mptcp_connect
> >> self-tests.
> >>
> >> In the failing scenarios (TCP -> MPTCP) the involved sockets are
> >> actually plain TCP ones, as fallback for passive socket at 2whs
> >> time cause the MPTCP listener to actually create a TCP socket.
> >>
> >> The transfer is stuck due to the receiver buffer being zero.
> >> With the stronger check in place, tcp_clamp_window() can be invoked
> >> while the TCP socket has sk_rmem_alloc == 0, and the receive buffer
> >> will be zeroed, too.
> >>
> >> Pass to tcp_clamp_window() even the current skb truesize, so that
> >> such helper could compute and use the actual limit enforced by
> >> the stack.
> >>
> >> Fixes: 1d2fbaad7cd8 ("tcp: stronger sk_rcvbuf checks")
> >> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> >> ---
> >>  net/ipv4/tcp_input.c | 12 ++++++------
> >>  1 file changed, 6 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> >> index 672cbfbdcec1..c98de02a3c57 100644
> >> --- a/net/ipv4/tcp_input.c
> >> +++ b/net/ipv4/tcp_input.c
> >> @@ -610,24 +610,24 @@ static void tcp_init_buffer_space(struct sock *sk)
> >>  }
> >>
> >>  /* 4. Recalculate window clamp after socket hit its memory bounds. */
> >> -static void tcp_clamp_window(struct sock *sk)
> >> +static void tcp_clamp_window(struct sock *sk, int truesize)
> >
> >
> > I am unsure about this one. truesize can be 1MB here, do we want that
> > in general ?
>
> I'm unsure either. But I can't think of a different approach?!? If the
> incoming truesize is 1M the socket should allow for at least 1M rcvbuf
> size to accept it, right?

What I meant was :

This is the generic point, accepting skb->truesize as additional input
here would make us more vulnerable, or we could risk other
regressions.

The question is : why does MPTCP end up here in the first place.
Perhaps an older issue with an incorrectly sized sk_rcvbuf ?

Or maybe the test about the receive queue being empty (currently done
in tcp_data_queue()) should be moved to a more strategic place.