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

[Paolo Abeni <pabeni@redhat.com>]

On 7/21/25 2:30 PM, Eric Dumazet wrote:
> On Mon, Jul 21, 2025 at 3:50 AM Paolo Abeni <pabeni@redhat.com> wrote:
>> On 7/21/25 10:04 AM, Eric Dumazet wrote:
>>> On Fri, Jul 18, 2025 at 10:25 AM Paolo Abeni <pabeni@redhat.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@redhat.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'. I'm unsure if we could
preventing/forbidding 'too high' values of scaling_rate? (also I'm
unsure where to draw the line exactly.

Cheers,

Paolo


[1] You can run the relevant test by adding '-t' on the mptcp_connect.sh
command line, but it will take a lot of time to run the 10-20 iterations
I need to observe the issue. To make it faster I manually trimmed the
not relevant test-cases.