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+Sysfs tagging
+(Taken almost verbatim from Eric Biederman's netns tagging patch
+commit msg)
+The problem. Network devices show up in sysfs and with the network
+namespace active multiple devices with the same name can show up in
+the same directory, ouch!
+To avoid that problem and allow existing applications in network
+namespaces to see the same interface that is currently presented in
+sysfs, sysfs now has tagging directory support.
+By using the network namespace pointers as tags to separate out the
+the sysfs directory entries we ensure that we don't have conflicts
+in the directories and applications only see a limited set of
+the network devices.
+Each sysfs directory entry may be tagged with zero or one
+namespaces. A sysfs_dirent is augmented with a void *s_ns. If a
+directory entry is tagged, then sysfs_dirent->s_flags will have a
+flag between KOBJ_NS_TYPE_NONE and KOBJ_NS_TYPES, and s_ns will
+point to the namespace to which it belongs.
+Each sysfs superblock's sysfs_super_info contains an array void
+*ns[KOBJ_NS_TYPES]. When a a task in a tagging namespace
+kobj_nstype first mounts sysfs, a new superblock is created. It
+will be differentiated from other sysfs mounts by having its
+s_fs_info->ns[kobj_nstype] set to the new namespace. Note that
+through bind mounting and mounts propagation, a task can easily view
+the contents of other namespaces' sysfs mounts. Therefore, when a
+namespace exits, it will call kobj_ns_exit() to invalidate any
+sysfs_dirent->s_ns pointers pointing to it.
+Users of this interface:
+- define a type in the kobj_ns_type enumeration.
+- call kobj_ns_type_register() with its kobj_ns_type_operations which has
+ - current_ns() which returns current's namespace
+ - netlink_ns() which returns a socket's namespace
+ - initial_ns() which returns the initial namesapce
+- call kobj_ns_exit() when an individual tag is no longer valid