[PATCH 1/4] net: add name_assign_type netdev attribute

[David Herrmann <dh.herrmann@gmail.com>]

The name_assign_type attribute gives hints where the interface name of a
given net-device comes from. Three different values are currently defined:
  NET_NAME_ENUM:
    This is the default. The ifname is provided by the kernel with an
    enumerated suffix. Names may be reused and unstable.
  NET_NAME_USER:
    The ifname was provided by user-space during net-device setup.
  NET_NAME_RENAMED:
    The net-device has been renamed via RTNL. Once this type is set, it
    cannot change again.

This attribute comes in handy for reliable net-device names. If an ifname
is provided by user-space, we can safely assume that the naming-policy
avoids reuse and is stable. Only if it was set by the kernel, the
interfaces might need to be renamed.

The NET_NAME_RENAMED value allows us to detect whether some-one else
already renamed the device, in which case we shouldn't touch it again. The
NET_NAME_USER value allows us to detect whether some other naming policy
created the device, in which case there's no need to rename it.

The most significant use-case is to detect virtual wifi-P2P devices, which
are named by wpa_supplicant et al. We shouldn't rename them as wpas
already provides a proper naming-policy.

Note that this patch only provides the core infrastructure. The different
net-dev types need to be manually fixed to use NET_NAME_USER instead of
the default (NET_NAME_ENUM). NET_NAME_ENUM is the least restrictive,
though, so it seems safe to use it as fallback for non-converted net-dev
types.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
---
 include/linux/netdevice.h | 6 ++++++
 net/core/dev.c            | 7 +++++++
 net/core/net-sysfs.c      | 2 ++
 net/core/rtnetlink.c      | 2 ++
 4 files changed, 17 insertions(+)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index e8eeebd..d3a11c8 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -74,6 +74,11 @@ void netdev_set_default_ethtool_ops(struct net_device *dev,
 #define NET_RX_SUCCESS		0	/* keep 'em coming, baby */
 #define NET_RX_DROP		1	/* packet dropped */
 
+/* interface name assignment types */
+#define NET_NAME_ENUM		0	/* enumerated by kernel (default) */
+#define NET_NAME_USER		1	/* provided by user-space */
+#define NET_NAME_RENAMED	2	/* renamed by user-space */
+
 /*
  * Transmit return codes: transmit return codes originate from three different
  * namespaces:
@@ -1165,6 +1170,7 @@ struct net_device {
 	 * of the interface.
 	 */
 	char			name[IFNAMSIZ];
+	unsigned char		name_assign_type; /* name assignment type */
 
 	/* device name hash chain, please keep it close to name[] */
 	struct hlist_node	name_hlist;
diff --git a/net/core/dev.c b/net/core/dev.c
index b1b0c8d..3608fb2 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1083,6 +1083,7 @@ static int dev_get_valid_name(struct net *net,
 int dev_change_name(struct net_device *dev, const char *newname)
 {
 	char oldname[IFNAMSIZ];
+	unsigned char old_assign_type;
 	int err = 0;
 	int ret;
 	struct net *net;
@@ -1109,10 +1110,14 @@ int dev_change_name(struct net_device *dev, const char *newname)
 		return err;
 	}
 
+	old_assign_type = dev->name_assign_type;
+	dev->name_assign_type = NET_NAME_RENAMED;
+
 rollback:
 	ret = device_rename(&dev->dev, dev->name);
 	if (ret) {
 		memcpy(dev->name, oldname, IFNAMSIZ);
+		dev->name_assign_type = old_assign_type;
 		write_seqcount_end(&devnet_rename_seq);
 		return ret;
 	}
@@ -1141,6 +1146,8 @@ rollback:
 			write_seqcount_begin(&devnet_rename_seq);
 			memcpy(dev->name, oldname, IFNAMSIZ);
 			memcpy(oldname, newname, IFNAMSIZ);
+			dev->name_assign_type = old_assign_type;
+			old_assign_type = NET_NAME_RENAMED;
 			goto rollback;
 		} else {
 			pr_err("%s: name change rollback failed: %d\n",
diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index 9388624..72a8ce5 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -104,6 +104,7 @@ static ssize_t netdev_store(struct device *dev, struct device_attribute *attr,
 }
 
 NETDEVICE_SHOW_RO(dev_id, fmt_hex);
+NETDEVICE_SHOW_RO(name_assign_type, fmt_dec);
 NETDEVICE_SHOW_RO(addr_assign_type, fmt_dec);
 NETDEVICE_SHOW_RO(addr_len, fmt_dec);
 NETDEVICE_SHOW_RO(iflink, fmt_dec);
@@ -375,6 +376,7 @@ static struct attribute *net_class_attrs[] = {
 	&dev_attr_dev_id.attr,
 	&dev_attr_iflink.attr,
 	&dev_attr_ifindex.attr,
+	&dev_attr_name_assign_type.attr,
 	&dev_attr_addr_assign_type.attr,
 	&dev_attr_addr_len.attr,
 	&dev_attr_link_mode.attr,
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 1a0dac2..e10baa0 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -1962,6 +1962,8 @@ replay:
 		}
 
 		dev->ifindex = ifm->ifi_index;
+		if (tb[IFLA_IFNAME])
+			dev->name_assign_type = NET_NAME_USER;
 
 		if (ops->newlink) {
 			err = ops->newlink(net, dev, tb, data);
-- 
1.9.0