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diff --git a/Documentation/filesystems/sysfs-tagging.rst b/Documentation/filesystems/sysfs-tagging.rst new file mode 100644 index 000000000000..8888a05c398e --- /dev/null +++ b/Documentation/filesystems/sysfs-tagging.rst @@ -0,0 +1,48 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============= +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 a namespace via the +``void *ns member`` of its ``kernfs_node``. If a directory entry is tagged, +then ``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 +belongs. + +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 ``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namespace + - initial_ns() which returns the initial namesapce + +- call kobj_ns_exit() when an individual tag is no longer valid |