(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 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 a namespace via the
void *ns member of its
kernfs_node. If a directory entry is tagged,
kernfs_node->flags will have a flag between KOBJ_NS_TYPE_NONE
and KOBJ_NS_TYPES, and ns will point to the namespace to which it
Each sysfs superblock's kernfs_super_info contains an array
void *ns[KOBJ_NS_TYPES]. When 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
kernfs_node->ns pointers pointing to it.
Users of this interface:
define a type in the
call kobj_ns_type_register() with its
current_ns() which returns current's namespace
netlink_ns() which returns a socket's namespace
initial_ns() which returns the initial namespace
call kobj_ns_exit() when an individual tag is no longer valid