> > > 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".
I don't like the window check on two variables.
However if ETH_HLEN and ETH_FCS_LEN are 'int' (not an unsigned type)
and 'new_mtu' is just below MAX_INT then the signed arithmetic
could overflow - generating an indeterminate value.
In which case 'max_frame' might not exceed MAX_JUMBO_FRAME_SIZE.
David
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