kk Blog —— 通用基础

commit 2cd0d743b05e87445c54ca124a9916f22f16742e
Author: Neal Cardwell <ncardwell@google.com>
Date:   Wed Jun 18 21:15:03 2014 -0400

	tcp: fix tcp_match_skb_to_sack() for unaligned SACK at end of an skb

	If there is an MSS change (or misbehaving receiver) that causes a SACK
	to arrive that covers the end of an skb but is less than one MSS, then
	tcp_match_skb_to_sack() was rounding up pkt_len to the full length of
	the skb ("Round if necessary..."), then chopping all bytes off the skb
	and creating a zero-byte skb in the write queue.

	This was visible now because the recently simplified TLP logic in
	bef1909ee3ed1c ("tcp: fixing TLP's FIN recovery") could find that 0-byte
	skb at the end of the write queue, and now that we do not check that
	skb's length we could send it as a TLP probe.

	Consider the following example scenario:

	 mss: 1000
	 skb: seq: 0 end_seq: 4000  len: 4000
	 SACK: start_seq: 3999 end_seq: 4000

	The tcp_match_skb_to_sack() code will compute:

	 in_sack = false
	 pkt_len = start_seq - TCP_SKB_CB(skb)->seq = 3999 - 0 = 3999
	 new_len = (pkt_len / mss) * mss = (3999/1000)*1000 = 3000
	 new_len += mss = 4000

	Previously we would find the new_len > skb->len check failing, so we
	would fall through and set pkt_len = new_len = 4000 and chop off
	pkt_len of 4000 from the 4000-byte skb, leaving a 0-byte segment
	afterward in the write queue.

	With this new commit, we notice that the new new_len >= skb->len check
	succeeds, so that we return without trying to fragment.

	Fixes: adb92db857ee ("tcp: Make SACK code to split only at mss boundaries")
	Reported-by: Eric Dumazet <edumazet@google.com>
	Signed-off-by: Neal Cardwell <ncardwell@google.com>
	Cc: Eric Dumazet <edumazet@google.com>
	Cc: Yuchung Cheng <ycheng@google.com>
	Cc: Ilpo Jarvinen <ilpo.jarvinen@helsinki.fi>
	Acked-by: Eric Dumazet <edumazet@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 40661fc..b5c2375 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1162,7 +1162,7 @@ static int tcp_match_skb_to_sack(struct sock *sk, struct sk_buff *skb,
          unsigned int new_len = (pkt_len / mss) * mss;
          if (!in_sack && new_len < pkt_len) {
              new_len += mss;
-             if (new_len > skb->len)
+             if (new_len >= skb->len)
                  return 0;
          }
          pkt_len = new_len;

mss = skb->gso_size
len = len/mss * mss

|---|-------|-------|
 mss    |
        V
|---|---|

len = (packets - cnt) * mss

|--------|--|--|
   mss   |
         V
|--------|--------|

new_len = (pkt_len/mm)*mss
in_sack = 1
pkt_len = new_len

|---|-------|-------|
 mss    |
        V
|---|---|

skb_shinfo(skb)->gso_size = 0;
skb_shinfo(skb)->gso_segs = 0;
skb_shinfo(skb)->gso_type = 0;

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 8c8de27..4ff3b6d 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1141,11 +1141,9 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
  sk_mem_uncharge(sk, len);
  sock_set_flag(sk, SOCK_QUEUE_SHRUNK);
- /* Any change of skb->len requires recalculation of tso
-  * factor and mss.
-  */
+ /* Any change of skb->len requires recalculation of tso factor. */
  if (tcp_skb_pcount(skb) > 1)
-     tcp_set_skb_tso_segs(sk, skb, tcp_current_mss(sk));
+     tcp_set_skb_tso_segs(sk, skb, tcp_skb_mss(skb));
  return 0;
 }