On Tue, Oct 01, 2013 at 11:47:21PM +0200, Hannes Frederic Sowa wrote:

quoted

The strange thing is that if I don't do the IPV6_MTU setsockopt I don't
get an oops.

This is incorrect, it just depends on the size of the writes and on the
interface mtu.

quoted

IPv4 seems to work without problems, too.

I also get kernel oopses from IPv4 now, too.


So, skb_is_gso is not accurate enough in the output and we have to check if we
already started to append to skb frags. The following diff does resolve this
issue in both the IPv4 and IPv6 non-page-append output path but I am not
confident if it is correct:

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 6d56840..3565450 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h

@@ -1308,6 +1308,11 @@ static inline int skb_pagelen(const struct sk_buff *skb)

	return len + skb_headlen(skb);
}

+static inline bool skb_has_frags(const struct sk_buff *skb)
+{
+	return skb_shinfo(skb)->nr_frags;
+}
+
/**
 * __skb_fill_page_desc - initialise a paged fragment in an skb
 * @skb: buffer containing fragment to be initialised

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 7d8357b..8dc3d8d 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c

@@ -836,7 +836,7 @@ static int __ip_append_data(struct sock *sk,

		csummode = CHECKSUM_PARTIAL;

	cork->length += length;
-	if (((length > mtu) || (skb && skb_is_gso(skb))) &&
+	if (((length > mtu) || (skb && skb_has_frags(skb))) &&
	    (sk->sk_protocol == IPPROTO_UDP) &&
	    (rt->dst.dev->features & NETIF_F_UFO) && !rt->dst.header_len) {
		err = ip_ufo_append_data(sk, queue, getfrag, from, length,

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index a54c45c..ded4f6f 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c

@@ -1227,7 +1227,7 @@ int ip6_append_data(struct sock *sk, int getfrag(void *from, char *to,

	skb = skb_peek_tail(&sk->sk_write_queue);
	cork->length += length;
	if (((length > mtu) ||
-	     (skb && skb_is_gso(skb))) &&
+	     (skb && skb_has_frags(skb))) &&
	    (sk->sk_protocol == IPPROTO_UDP) &&
	    (rt->dst.dev->features & NETIF_F_UFO)) {
		err = ip6_ufo_append_data(sk, getfrag, from, length,

Greetings,

 Hannes