|(Taken almost verbatim from Eric Biederman's netns tagging patch
|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|- call kobj_ns_exit() when an individual tag is no longer valid