Subject: Re: [PATCH] net: tuntap: Fix tun_net_fix_features()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 17 May 2011 17:29:43 +0300

On Tue, May 17, 2011 at 10:19:54AM +0200, Michał Mirosław wrote:
> tun->set_features are meant to limit not force the features.
> 
> Signed-off-by: Michał Mirosław <[email-masked]>
> ---
>  drivers/net/tun.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 74e9405..f77c6d0 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -458,7 +458,7 @@ static u32 tun_net_fix_features(struct net_device *dev, u32 features)
>  {
>  	struct tun_struct *tun = netdev_priv(dev);
>  
> -	return (features & tun->set_features) | (features & ~TUN_USER_FEATURES);
> +	return features & (tun->set_features | ~TUN_USER_FEATURES);
>  }
>  
>  static const struct net_device_ops tun_netdev_ops = {
> -- 
> 1.7.2.5

One thing that this will do though: previously, if
ethtool disables offloads, then an application enables
them, the application will have the last say.
With this patch, the most conservative approach wins.
Right?
If we want to have the existing behaviour
I think the following would do this (untested). What do you think?

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 74e9405..1d6c7bc 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1199,6 +1199,8 @@ static int set_offload(struct tun_struct *tun, unsigned long arg)
 		return -EINVAL;
 
 	tun->set_features = features;
+	tun->dev->features &= TUN_USER_FEATURES;
+	tun->dev->features |= (features & TUN_USER_FEATURES);
 	netdev_update_features(tun->dev);
 
 	return 0;