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

On Fri, 9 Sep 2011, Yan, Zheng wrote:

On 09/09/2011 09:54 AM, Herbert Xu wrote:

quoted

On Fri, Sep 09, 2011 at 09:45:52AM +0800, Yan, Zheng wrote:

quoted

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.

Hmm, some Ignored I was expecting to see even when nothing was wrong but I 
think it might have been the other counter... too long time already passed 
to remember all the details I've been thinking.

quoted

Yes this looks like a typo.  Please resend your patch with a
description and signed-off-by line.

I think the bug has a big influence on tcp DSACKs. It's better to
leave it to people who fully understand tcp code.

Indeed, it looks like a double negative error.

-- 
 i.