tcp: diag: Dont report negative values for rx queue

Both netlink and /proc/net/tcp interfaces can report transient
negative values for rx queue.

ss ->
State   Recv-Q Send-Q  Local Address:Port  Peer Address:Port
ESTAB   -6     6       127.0.0.1:45956     127.0.0.1:3333 

netstat ->
tcp   4294967290      6 127.0.0.1:37784  127.0.0.1:3333 ESTABLISHED

This is because we dont lock socket while computing 
tp->rcv_nxt - tp->copied_seq,
and another CPU can update copied_seq before rcv_next in RX path.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 10 insertions, 2 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index fee9aabd5aa1..29002ab26e0d 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2318,6 +2318,7 @@ static void get_tcp4_sock(struct sock *sk, struct seq_file *f, int i, int *len)
         __be32 src = inet->inet_rcv_saddr;
         __u16 destp = ntohs(inet->inet_dport);
         __u16 srcp = ntohs(inet->inet_sport);
+        int rx_queue;
 
         if (icsk->icsk_pending == ICSK_TIME_RETRANS) {
                 timer_active    = 1;
@@ -2333,12 +2334,19 @@ static void get_tcp4_sock(struct sock *sk, struct seq_file *f, int i, int *len)
                 timer_expires = jiffies;
         }
 
+        if (sk->sk_state == TCP_LISTEN)
+                rx_queue = sk->sk_ack_backlog;
+        else
+                /*
+                 * because we dont lock socket, we might find a transient negative value
+                 */
+                rx_queue = max_t(int, tp->rcv_nxt - tp->copied_seq, 0);
+
         seq_printf(f, "%4d: %08X:%04X %08X:%04X %02X %08X:%08X %02X:%08lX "
                         "%08X %5d %8d %lu %d %p %lu %lu %u %u %d%n",
                 i, src, srcp, dest, destp, sk->sk_state,
                 tp->write_seq - tp->snd_una,
-                sk->sk_state == TCP_LISTEN ? sk->sk_ack_backlog :
-                                             (tp->rcv_nxt - tp->copied_seq),
+                rx_queue,
                 timer_active,
                 jiffies_to_clock_t(timer_expires - jiffies),
                 icsk->icsk_retransmits,