xen-netback: Handle guests with too many frags

Xen network protocol had implicit dependency on MAX_SKB_FRAGS. Netback has to
handle guests sending up to XEN_NETBK_LEGACY_SLOTS_MAX slots. To achieve that:
- create a new skb
- map the leftover slots to its frags (no linear buffer here!)
- chain it to the previous through skb_shinfo(skb)->frag_list
- map them
- copy and coalesce the frags into a brand new one and send it to the stack
- unmap the 2 old skb's pages

It's also introduces new stat counters, which help determine how often the guest
sends a packet with more than MAX_SKB_FRAGS frags.

NOTE: if bisect brought you here, you should apply the series up until
"xen-netback: Timeout packets in RX path", otherwise malicious guests can block
other guests by not releasing their sent packets.

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 0 deletions

diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-netback/interface.c
index 44df8581b4d7..b646039e539b 100644
--- a/drivers/net/xen-netback/interface.c
+++ b/drivers/net/xen-netback/interface.c
@@ -253,6 +253,13 @@ static const struct xenvif_stat {
 		"tx_zerocopy_fail",
 		offsetof(struct xenvif, tx_zerocopy_fail)
 	},
+	/* Number of packets exceeding MAX_SKB_FRAG slots. You should use
+	 * a guest with the same MAX_SKB_FRAG
+	 */
+	{
+		"tx_frag_overflow",
+		offsetof(struct xenvif, tx_frag_overflow)
+	},
 };
 
 static int xenvif_get_sset_count(struct net_device *dev, int string_set)