tcp: do not set a zero size receive buffer

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.

Check for the critical condition in tcp_prune_queue() and just
drop the packet without shrinking the receiver buffer.

Fixes: 1d2fbaad7cd8 ("tcp: stronger sk_rcvbuf checks")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20c18165d3f848e1c5c1b782d88c1a5ab38b3f70.1753118029.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 672cbfbdcec1de22a5b1494d365863303271d222..81b6d37708120632d16a50892442ea04779cc3a4 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5549,6 +5549,10 @@ static int tcp_prune_queue(struct sock *sk, const struct sk_buff *in_skb)
 {
        struct tcp_sock *tp = tcp_sk(sk);
 
+       /* Do nothing if our queues are empty. */
+       if (!atomic_read(&sk->sk_rmem_alloc))
+               return -1;
+
        NET_INC_STATS(sock_net(sk), LINUX_MIB_PRUNECALLED);
 
        if (!tcp_can_ingest(sk, in_skb))