net: update SOCK_MIN_RCVBUF

SOCK_MIN_RCVBUF current value is 256 bytes

It doesnt permit to receive the smallest possible frame, considering
socket sk_rmem_alloc/sk_rcvbuf account skb truesizes. On 64bit arches,
sizeof(struct sk_buff) is 240 bytes. Add the typical 64 bytes of
headroom, and we go over the limit.

With old kernels and 32bit arches, we were under the limit, if netdriver
was doing copybreak.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1558,7 +1558,11 @@ static inline void sk_wake_async(struct sock *sk, int how, int band)
 }
 
 #define SOCK_MIN_SNDBUF 2048
-#define SOCK_MIN_RCVBUF 256
+/*
+ * Since sk_rmem_alloc sums skb->truesize, even a small frame might need
+ * sizeof(sk_buff) + MTU + padding, unless net driver perform copybreak
+ */
+#define SOCK_MIN_RCVBUF (2048 + sizeof(struct sk_buff))
 
 static inline void sk_stream_moderate_sndbuf(struct sock *sk)
 {