On Fri, Sep 13, 2013 at 09:58:25AM +0100, David Laight wrote:
> > If new_mtu is very large then "new_mtu + ETH_HLEN + ETH_FCS_LEN" can
> > wrap and the check on the next line can underflow. This is one of those
> > bugs which can be triggered by the user if you have namespaces
> > configured.
> >
> > This is a static checker fix and I'm not sure what the impact is.
> >
> > Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> >
> > diff --git a/drivers/net/ethernet/intel/igbvf/netdev.c b/drivers/net/ethernet/intel/igbvf/netdev.c
> > index 95d5430..24e3883 100644
> > --- a/drivers/net/ethernet/intel/igbvf/netdev.c
> > +++ b/drivers/net/ethernet/intel/igbvf/netdev.c
> > @@ -2342,7 +2342,7 @@ static struct net_device_stats *igbvf_get_stats(struct net_device *netdev)
> > static int igbvf_change_mtu(struct net_device *netdev, int new_mtu)
> > {
> > struct igbvf_adapter *adapter = netdev_priv(netdev);
> > - int max_frame = new_mtu + ETH_HLEN + ETH_FCS_LEN;
> > + unsigned int max_frame = new_mtu + ETH_HLEN + ETH_FCS_LEN;
> >
> > if ((new_mtu < 68) || (max_frame > MAX_JUMBO_FRAME_SIZE)) {
> > dev_err(&adapter->pdev->dev, "Invalid MTU setting\n");
>
> It is safer to check:
> if ((new_mtu < 68) || (new_mtu > MAX_JUMBO_FRAME_SIZE - ETH_HLEN - ETH_FCS_LEN)) {
>
I believe my fix is already 100% safe... Where is the bug in my code?
Your fix harder to read because of the additional math and because it's
checking "new_mtu" when we care about "max_frame".
regards,
dan carpenter
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