[BUG?] tcp: potential bug in tcp_is_sackblock_valid()

["Yan, Zheng" <zheng.z.yan@intel.com>]

Hi all,

I found a check in tcp_is_sackblock_valid() is suspicious. It against
its comment and RFC. I think the correct check should be:

---
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 385c470..a5d01b1 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1124,7 +1124,7 @@ static int tcp_is_sackblock_valid(struct tcp_sock *tp, int is_dsack,
                return 0;
 
        /* ...Then it's D-SACK, and must reside below snd_una completely */
-       if (!after(end_seq, tp->snd_una))
+       if (after(end_seq, tp->snd_una))
                return 0;
 
        if (!before(start_seq, tp->undo_marker))
---

I also checked /proc/net/netstat of my laptop, found TCPDSACKIgnoredOld
field is not zero. Maybe it's caused by the bug.

Regards
Yan, Zheng