Re: [Xen-devel] [PATCH V4 5/7] xen-netfront: reduce gso_max_size to account for max TCP header

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.04.13 at 16:44, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Fri, 2013-04-12 at 15:24 +0100, Wei Liu wrote:
>> The maximum packet including header that can be handled by netfront / 
> netback
>> wire format is 65535. Reduce gso_max_size accordingly.
>> 
>> Drop skb and print warning when skb->len > 65535. This can 1) save the effort
>> to send malformed packet to netback, 2) help spotting misconfiguration of
>> netfront in the future.
>> 
>> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
>> ---
>>  drivers/net/xen-netfront.c |   14 +++++++++++++-
>>  1 file changed, 13 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
>> index 1bb2e20..9c2eca1 100644
>> --- a/drivers/net/xen-netfront.c
>> +++ b/drivers/net/xen-netfront.c
>> @@ -547,6 +547,16 @@ static int xennet_start_xmit(struct sk_buff *skb, 
> struct net_device *dev)
>>      unsigned int len = skb_headlen(skb);
>>      unsigned long flags;
>>  
>> +    /* If skb->len is too big for wire format, drop skb and alert
>> +     * user about misconfiguration.
>> +     */
>> +    if (unlikely(skb->len > (typeof(tx->size))(~0))) {
> 
> This seems rather a complicated way of saying... er, something!
> 
> Perhaps we could have a #define in netif.h to compare against?
> 
> I'm not 100% convinced that the case and the ~ are in the right order to
> have the behaviour you want unless perhaps sizeof(tx->size) <
> sizeof(int). In any case it's too obfuscated for me ;-)

Previously this was (u16)~0, which I complained to have no
correlation to where the restriction actually comes from. Hence
the request to use typeof() on the very field that doesn't
permit a larger value.

Jan


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