ip: take care of last fragment in ip_append_data (59104f06) · Commits · 戴 / test

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>

net/ipv4/ip_output.c

+6 −3

Original line number	Original line	Diff line number	Diff line
	@@ -926,16 +926,19 @@ alloc_new_skb:
	!(rt->dst.dev->features&NETIF_F_SG))		!(rt->dst.dev->features&NETIF_F_SG))
	alloclen = mtu;		alloclen = mtu;
	else		else
	alloclen = datalen + fragheaderlen;		alloclen = fraglen;

	/* The last fragment gets additional space at tail.		/* The last fragment gets additional space at tail.
	* Note, with MSG_MORE we overallocate on fragments,		* Note, with MSG_MORE we overallocate on fragments,
	* because we have no idea what fragment will be		* because we have no idea what fragment will be
	* the last.		* the last.
	*/		*/
	if (datalen == length + fraggap)		if (datalen == length + fraggap) {
	alloclen += rt->dst.trailer_len;		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) {		if (transhdrlen) {
	skb = sock_alloc_send_skb(sk,		skb = sock_alloc_send_skb(sk,
	alloclen + hh_len + 15,		alloclen + hh_len + 15,

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