ip: take care of last fragment in ip_append_data

While investigating a bit, I found ip_fragment() slow path was taken
because ip_append_data() provides following layout for a send(MTU +
N*(MTU - 20)) syscall :

- one skb with 1500 (mtu) bytes
- N fragments of 1480 (mtu-20) bytes (before adding IP header)
last fragment gets 17 bytes of trail data because of following bit:

	if (datalen == length + fraggap)
		alloclen += rt->dst.trailer_len;

Then esp4 adds 16 bytes of data (while trailer_len is 17... hmm...
another bug ?)

In ip_fragment(), we notice last fragment is too big (1496 + 20) > mtu,
so we take slow path, building another skb chain.

In order to avoid taking slow path, we should correct ip_append_data()
to make sure last fragment has real trail space, under mtu...

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 3 deletions

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index e42762023c27..3551b6dc7419 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -926,16 +926,19 @@ alloc_new_skb:
                             !(rt->dst.dev->features&NETIF_F_SG))
                                 alloclen = mtu;
                         else
-                                alloclen = datalen + fragheaderlen;
+                                alloclen = fraglen;
 
                         /* The last fragment gets additional space at tail.
                          * Note, with MSG_MORE we overallocate on fragments,
                          * because we have no idea what fragment will be
                          * the last.
                          */
-                        if (datalen == length + fraggap)
+                        if (datalen == length + fraggap) {
                                 alloclen += rt->dst.trailer_len;
-
+                                /* make sure mtu is not reached */
+                                if (datalen > mtu - fragheaderlen - rt->dst.trailer_len)
+                                        datalen -= ALIGN(rt->dst.trailer_len, 8);
+                        }
                         if (transhdrlen) {
                                 skb = sock_alloc_send_skb(sk,
                                                 alloclen + hh_len + 15,