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

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Message-ID: <1d78b781-5cca-440c-b9d0-bdf40a410a3d@redhat.com>
Date: Mon, 21 Jul 2025 16:56:06 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Eric Dumazet <edumazet@...gle.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 7/21/25 3:52 PM, Eric Dumazet wrote:
> On Mon, Jul 21, 2025 at 6:32 AM Paolo Abeni <pabeni@...hat.com> wrote:
>> On 7/21/25 2:30 PM, Eric Dumazet wrote:
>>> 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.
>>
>> Understood, thanks for the clarification.
>>
>>> The question is : why does MPTCP end up here in the first place.
>>> Perhaps an older issue with an incorrectly sized sk_rcvbuf ?
>>
>> I collected a few more data. The issue happens even with plain TCP
>> sockets[1].
>>
>> The relevant transfer is on top of the loopback device. The scaling_rate
>> rapidly grows to 254 - that is `truesize` and `len` are very near.
>>
>> The stall happens when the received get in a packet with a slightly less
>> 'efficient' layout (in the experiment I have handy len is 71424,
>> truesize 72320) (almost) filling the receiver window.
>>
>> On such input, tcp_clamp_window() shrinks the receiver buffer to the
>> current rmem usage. The same happens on retransmissions until rcvbuf
>> becomes 0.
>>
>> I *think* that catching only the !sk_rmem_alloc case would avoid the
>> stall, but I think it's a bit 'late'.
> 
> A packetdrill test here would help understanding your concern.

I fear like a complete working script would take a lot of time, let me
try to sketch just the relevant part:

# receiver state is:
# rmem=110592 rcvbuf=174650 scaling_ratio=253 rwin=63232
# no OoO data, no memory pressure,

# the incoming packet is in sequence
+0 > P. 109297:172528(63232) ack 1

With just the 0 rmem check in tcp_prune_queue(), such function will
still invoke tcp_clamp_window() that will shrink the receive buffer to
110592.
tcp_collapse() can't make enough room and the incoming packet will be
dropped. I think we should instead accept such packet.

Side note: the above data are taken from an actual reproduction of the issue

Please LMK if the above clarifies a bit my doubt or if a full pktdrill
is needed.

Thanks,

Paolo