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Date: Sun, 26 Oct 2008 18:41:47 +0100
From: Florian Westphal <fw@...len.de>
To: David Miller <davem@...emloft.net>
Cc: agl@...erialviolet.org, netdev@...r.kernel.org
Subject: Re: suspicious logic in tcp_make_synack()
David Miller <davem@...emloft.net> wrote:
>
> I've been debugging something over the past few days and noticed
> something I can't explain in your change:
>
> commit 33ad798c924b4a1afad3593f2796d465040aadd5
> Author: Adam Langley <agl@...erialviolet.org>
> Date: Sat Jul 19 00:04:31 2008 -0700
>
> tcp: options clean up
>
> specifically this hunk of the change:
>
> @@ -2244,18 +2295,7 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
> TCP_SKB_CB(skb)->when = cookie_init_timestamp(req);
> else
> #endif
> - TCP_SKB_CB(skb)->when = tcp_time_stamp;
> - tcp_syn_build_options((__be32 *)(th + 1), dst_metric(dst, RTAX_ADVMSS), ireq->tstamp_ok,
> - ireq->sack_ok, ireq->wscale_ok, ireq->rcv_wscale,
> - TCP_SKB_CB(skb)->when,
> - req->ts_recent,
> - (
> -#ifdef CONFIG_TCP_MD5SIG
> - md5 ? &md5_hash_location :
> -#endif
> - NULL)
> - );
> -
> + tcp_options_write((__be32 *)(th + 1), tp, &opts, &md5_hash_location);
>
> the code right above this stuff reads:
>
> #ifdef CONFIG_SYN_COOKIES
> if (unlikely(req->cookie_ts))
> TCP_SKB_CB(skb)->when = cookie_init_timestamp(req);
> else
> #endif
>
> The old code unconditionally built TCP options for the SYNACK, but now
> it doesn't build those options if req->cookie_ts, that call falls into
> the "else" branch, is that intentional?
I hope not 8-)
It essentially disables 4dfc2817025965a2fc78a18c50f540736a6b5c24
('[Syncookies]: Add support for TCP options via timestamps.'), thanks
for catching this.
> That req->cookie_ts test is meant to only control how
> TCP_SKB_CB(skb)->when gets calculated.
Yes. Moving the #ifdef CONFIG_SYN_COOKIES block allows syncookies
to use a tcp option subset again:
From: Florian Westphal <fw@...len.de>
Date: Sun, 26 Oct 2008 16:25:11 +0100
Subject: [PATCH] syncookies: fix inclusion of tcp options in syn-ack
David Miller noticed that commit
33ad798c924b4a1afad3593f2796d465040aadd5 '(tcp: options clean up')
did not move the req->cookie_ts check.
This essentially disabled commit 4dfc2817025965a2fc78a18c50f540736a6b5c24
'[Syncookies]: Add support for TCP options via timestamps.'.
This restores the original logic.
Signed-off-by: Florian Westphal <fw@...len.de>
---
net/ipv4/tcp_output.c | 10 +++++-----
1 files changed, 5 insertions(+), 5 deletions(-)
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index e4c5ac9..ba85d88 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2279,6 +2279,11 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
}
memset(&opts, 0, sizeof(opts));
+#ifdef CONFIG_SYN_COOKIES
+ if (unlikely(req->cookie_ts))
+ TCP_SKB_CB(skb)->when = cookie_init_timestamp(req);
+ else
+#endif
TCP_SKB_CB(skb)->when = tcp_time_stamp;
tcp_header_size = tcp_synack_options(sk, req, mss,
skb, &opts, &md5) +
@@ -2304,11 +2309,6 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
/* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
th->window = htons(min(req->rcv_wnd, 65535U));
-#ifdef CONFIG_SYN_COOKIES
- if (unlikely(req->cookie_ts))
- TCP_SKB_CB(skb)->when = cookie_init_timestamp(req);
- else
-#endif
tcp_options_write((__be32 *)(th + 1), tp, &opts, &md5_hash_location);
th->doff = (tcp_header_size >> 2);
TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTSEGS);
--
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