Merge tag 'docs-5.10' of git://git.lwn.net/linux

Pull documentation updates from Jonathan Corbet:
 "As hoped, things calmed down for docs this cycle; fewer changes and
  almost no conflicts at all. This includes:

   - A reworked and expanded user-mode Linux document

   - Some simplifications and improvements for submitting-patches.rst

   - An emergency fix for (some) problems with Sphinx 3.x

   - Some welcome automarkup improvements to automatically generate
     cross-references to struct definitions and other documents

   - The usual collection of translation updates, typo fixes, etc"

* tag 'docs-5.10' of git://git.lwn.net/linux: (81 commits)
  gpiolib: Update indentation in driver.rst for code excerpts
  Documentation/admin-guide: tainted-kernels: Fix typo occured
  Documentation: better locations for sysfs-pci, sysfs-tagging
  docs: programming-languages: refresh blurb on clang support
  Documentation: kvm: fix a typo
  Documentation: Chinese translation of Documentation/arm64/amu.rst
  doc: zh_CN: index files in arm64 subdirectory
  mailmap: add entry for <mstarovoitov@marvell.com>
  doc: seq_file: clarify role of *pos in ->next()
  docs: trace: ring-buffer-design.rst: use the new SPDX tag
  Documentation: kernel-parameters: clarify "module." parameters
  Fix references to nommu-mmap.rst
  docs: rewrite admin-guide/sysctl/abi.rst
  docs: fb: Remove vesafb scrollback boot option
  docs: fb: Remove sstfb scrollback boot option
  docs: fb: Remove matroxfb scrollback boot option
  docs: fb: Remove framebuffer scrollback boot option
  docs: replace the old User Mode Linux HowTo with a new one
  Documentation/admin-guide: blockdev/ramdisk: remove use of "rdev"
  Documentation/admin-guide: README & svga: remove use of "rdev"
  ...

2 files changed, 49 insertions, 0 deletions

diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst
index c29496fff81c..611e4b130c1e 100644
--- a/Documentation/networking/index.rst
+++ b/Documentation/networking/index.rst
@@ -95,6 +95,7 @@ Contents:
    seg6-sysctl
    strparser
    switchdev
+   sysfs-tagging
    tc-actions-env-rules
    tcp-thin
    team
diff --git a/Documentation/networking/sysfs-tagging.rst b/Documentation/networking/sysfs-tagging.rst
new file mode 100644
index 000000000000..83647e10c207
--- /dev/null
+++ b/Documentation/networking/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 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