docs: filesystems: convert sysfs-tagging.txt to ReST

   seq_file

   sharedsubtree

   sysfs-pci

   sysfs-tagging

   automount-support

.. SPDX-License-Identifier: GPL-2.0

=============

Sysfs tagging

-------------

=============

(Taken almost verbatim from Eric Biederman's netns tagging patch

commit msg)

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

``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

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

``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

- 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- call kobj_ns_exit() when an individual tag is no longer valid