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Re: [Xen-devel] [PATCH 2/4] xen-netfront: drop skb when skb->len > 65535

On 09.04.13 16:45, Ben Hutchings wrote:
On Tue, 2013-04-09 at 15:30 +0100, Ian Campbell wrote:
On Tue, 2013-03-19 at 21:28 +0000, Ben Hutchings wrote:
On Tue, 2013-03-19 at 21:24 +0000, Ben Hutchings wrote:
On Mon, 2013-03-18 at 15:07 +0000, Ian Campbell wrote:
On Mon, 2013-03-18 at 15:04 +0000, Wei Liu wrote:
On Mon, 2013-03-18 at 14:54 +0000, Ian Campbell wrote:
On Mon, 2013-03-18 at 14:40 +0000, Wei Liu wrote:
On Mon, 2013-03-18 at 11:42 +0000, Ian Campbell wrote:
On Mon, 2013-03-18 at 10:35 +0000, Wei Liu wrote:
The `size' field of Xen network wire format is uint16_t, anything bigger than
65535 will cause overflow.

Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
  drivers/net/xen-netfront.c |   12 ++++++++++++
  1 file changed, 12 insertions(+)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 5527663..8c3d065 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -547,6 +547,18 @@ static int xennet_start_xmit(struct sk_buff *skb, struct 
net_device *dev)
        unsigned int len = skb_headlen(skb);
        unsigned long flags;

+       /*
+        * wire format of xen_netif_tx_request only supports skb->len
+        * < 64K, because size field in xen_netif_tx_request is
+        * uint16_t.

Is there some field we can set e.g. in struct ethernet_device which
would stop this from happening?

struct ethernet_device? I could not find it.

And for struct net_device,

I meant struct net_device.

  there is no field for this AFAICT.

Interesting. Are hardware devices expected to cope with arbitrary sized
GSO skbs then I wonder.

No idea. But there is a macro called GSO_MAX_SIZE (65536) in struct
net_device. :-)

But aren't we seeing skb's bigger than that?

Maybe this is just a historical bug in some older guests?

GSO_MAX_SIZE is the maximum payload length, not the maximum total length
of an skb.

...and it's actually just the default value assigned to
dev->gso_max_size.  You'll want to change it to your actual maximum
(65535 - maximum length of headers) before registering your net devices.


"maximum length of headers" might be a bit tricky to determine
generically :-(.

Well you don't need to be generic, you need to know the maximum length
of headers that might appear in a TSO skb.

Ethernet + VLAN tag + IPv6 + TCP + timestamp option = 90 bytes, but I'm
not sure whether there can be other IP or TCP options in a TSO skb.  I'd
really like to get the TSO requirements clearly documented somewhere.

What about encapsulated IPSEC, IP-in-IP-tunnels, etc. ?


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