sysfs-tagging.rst 1.9 KB

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  1. .. SPDX-License-Identifier: GPL-2.0
  2. =============
  3. Sysfs tagging
  4. =============
  5. (Taken almost verbatim from Eric Biederman's netns tagging patch
  6. commit msg)
  7. The problem. Network devices show up in sysfs and with the network
  8. namespace active multiple devices with the same name can show up in
  9. the same directory, ouch!
  10. To avoid that problem and allow existing applications in network
  11. namespaces to see the same interface that is currently presented in
  12. sysfs, sysfs now has tagging directory support.
  13. By using the network namespace pointers as tags to separate out
  14. the sysfs directory entries we ensure that we don't have conflicts
  15. in the directories and applications only see a limited set of
  16. the network devices.
  17. Each sysfs directory entry may be tagged with a namespace via the
  18. ``void *ns member`` of its ``kernfs_node``. If a directory entry is tagged,
  19. then ``kernfs_node->flags`` will have a flag between KOBJ_NS_TYPE_NONE
  20. and KOBJ_NS_TYPES, and ns will point to the namespace to which it
  21. belongs.
  22. Each sysfs superblock's kernfs_super_info contains an array
  23. ``void *ns[KOBJ_NS_TYPES]``. When a task in a tagging namespace
  24. kobj_nstype first mounts sysfs, a new superblock is created. It
  25. will be differentiated from other sysfs mounts by having its
  26. ``s_fs_info->ns[kobj_nstype]`` set to the new namespace. Note that
  27. through bind mounting and mounts propagation, a task can easily view
  28. the contents of other namespaces' sysfs mounts. Therefore, when a
  29. namespace exits, it will call kobj_ns_exit() to invalidate any
  30. kernfs_node->ns pointers pointing to it.
  31. Users of this interface:
  32. - define a type in the ``kobj_ns_type`` enumeration.
  33. - call kobj_ns_type_register() with its ``kobj_ns_type_operations`` which has
  34. - current_ns() which returns current's namespace
  35. - netlink_ns() which returns a socket's namespace
  36. - initial_ns() which returns the initial namesapce
  37. - call kobj_ns_exit() when an individual tag is no longer valid