tcp: fix false reordering signal in tcp_shifted_skb

When tcp_shifted_skb() shifts bytes from the skb that is currently
pointed to by 'highest_sack' then the increment of
TCP_SKB_CB(skb)->seq implicitly advances tcp_highest_sack_seq(). This
implicit advancement, combined with the recent fix to pass the correct
SACKed range into tcp_sacktag_one(), caused tcp_sacktag_one() to think
that the newly SACKed range was before the tcp_highest_sack_seq(),
leading to a call to tcp_update_reordering() with a degree of
reordering matching the size of the newly SACKed range (typically just
1 packet, which is a NOP, but potentially larger).

This commit fixes this by simply calling tcp_sacktag_one() before the
TCP_SKB_CB(skb)->seq advancement that can advance our notion of the
highest SACKed sequence.

Correspondingly, we can simplify the code a little now that
tcp_shifted_skb() should update the lost_cnt_hint in all cases where
skb == tp->lost_skb_hint.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 53c8ce4..ee42d42 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1403,8 +1403,16 @@ static int tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
 
        BUG_ON(!pcount);
 
-       /* Adjust hint for FACK. Non-FACK is handled in tcp_sacktag_one(). */
-       if (tcp_is_fack(tp) && (skb == tp->lost_skb_hint))
+       /* Adjust counters and hints for the newly sacked sequence
+        * range but discard the return value since prev is already
+        * marked. We must tag the range first because the seq
+        * advancement below implicitly advances
+        * tcp_highest_sack_seq() when skb is highest_sack.
+        */
+       tcp_sacktag_one(sk, state, TCP_SKB_CB(skb)->sacked,
+                       start_seq, end_seq, dup_sack, pcount);
+
+       if (skb == tp->lost_skb_hint)
                tp->lost_cnt_hint += pcount;
 
        TCP_SKB_CB(prev)->end_seq += shifted;
@@ -1430,12 +1438,6 @@ static int tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
                skb_shinfo(skb)->gso_type = 0;
        }
 
-       /* Adjust counters and hints for the newly sacked sequence range but
-        * discard the return value since prev is already marked.
-        */
-       tcp_sacktag_one(sk, state, TCP_SKB_CB(skb)->sacked,
-                       start_seq, end_seq, dup_sack, pcount);
-
        /* Difference in this won't matter, both ACKed by the same cumul. ACK */
        TCP_SKB_CB(prev)->sacked |= (TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS);