Subject: Re: [GIT] Networking

From: Dan Williams <[email-masked]>
Date: Thu, 02 May 2013 11:27:36 -0500

On Thu, 2013-05-02 at 13:51 +0200, Bjørn Mork wrote:
> Ben Hutchings <[email-masked]> writes:
> > On Thu, 2013-05-02 at 05:17 -0400, David Miller wrote:
> >> From: Bjørn Mork <[email-masked]>
> >> Date: Thu, 02 May 2013 11:06:42 +0200
> >> 
> >> > Adding the new netdev features will make it go from 1 to 2:
> >> 
> >> We already had more than 31 feature bits before Patrick's
> >> changes, and I'm pretty sure this was the case when we added
> >> that ethtool API.
> >
> > It wasn't, but this should be OK.  Userland is supposed to query the
> > number of features using ETHTOOL_GSSET_INFO and then work out the number
> > of words/blocks using FEATURE_BITS_TO_BLOCKS().  
> 
> 
> Looking at
> http://cgit.freedesktop.org/NetworkManager/NetworkManager/tree/src/platform/nm-linux-platform.c#n1025
> there seems to be a couple of bugs in this area. This is certainly
> abusing the exported API, but it does mean that NM breaks if you ever
> move NETIF_F_VLAN_CHALLENGED (like the 802.1ad patch did):

NM doesn't actually use any of the code in src/platform/ yet, so it
wouldn't be affecting anything NM does at this time.  However, comments
like this are quite useful so we can fix it before NM does start to
depend on the code :)

Dan

> ----
> #define NETIF_F_VLAN_CHALLENGED (1 << 10)
> 
> static gboolean
> link_supports_vlans (NMPlatform *platform, int ifindex)
> {
> 	auto_nl_object struct rtnl_link *rtnllink = link_get (platform, ifindex);
> 	const char *name = nm_platform_link_get_name (ifindex);
> 	struct {
> 		struct ethtool_gfeatures features;
> 		struct ethtool_get_features_block features_block;
> 	} edata = { .features = { .cmd = ETHTOOL_GFEATURES, .size = 1 } };
> 
> 	/* Only ARPHRD_ETHER links can possibly support VLANs. */
> 	if (!rtnllink || rtnl_link_get_arptype (rtnllink) != ARPHRD_ETHER)
> 		return FALSE;
> 
> 	if (!name || !ethtool_get (name, &edata))
> 		return FALSE;
> 
> 	return !(edata.features.features[0].active & NETIF_F_VLAN_CHALLENGED);
> }
> ----
> 
> 
> Not that I see how this particular bug matters unless you need VLAN
> support in NM.  But there could be similar issues around.  I guess
> avoiding unnecessary renumbering of the NETIF_F bits can save us some
> trouble.  Although you can certainly argue that those bits never were
> intended to be part of the API, and that using them like this is a user
> application bug.
> 
> 
> 
> Bjørn
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