docs: proc: add documentation about mount restrictions

procfs has a number of mounting restrictions that are not documented
anywhere.

Signed-off-by: Alexey Gladkov <legion@kernel.org>
Link: https://patch.msgid.link/e7cb804df3c1759ee17cf9df1dc4c211d63d7a5f.1777278334.git.legion@kernel.org
Reviewed-by: Aleksa Sarai <aleksa@amutable.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Documentation/filesystems/proc.rst

@@ -52,6 +52,7 @@ fixes/update part 1.1  Stefani Seibold <stefani@seibold.net>    June 9 2009
 
   4	Configuring procfs
   4.1	Mount options
+  4.2	Mount restrictions
 
   5	Filesystem behavior
 
@@ -2425,7 +2426,9 @@ prohibited by hidepid=.  If you use some daemon like identd which needs to learn
 information about processes information, just add identd to this group.
 
 subset=pid hides all top level files and directories in the procfs that
-are not related to tasks.
+are not related to tasks. This option cannot be changed on an existing
+procfs instance because overmounts that existed before the change could
+otherwise remain reachable after the top level procfs entries are hidden.
 
 pidns= specifies a pid namespace (either as a string path to something like
 `/proc/$pid/ns/pid`, or a file descriptor when using `FSCONFIG_SET_FD`) that
@@ -2434,6 +2437,20 @@ will use the calling process's active pid namespace. Note that the pid
 namespace of an existing procfs instance cannot be modified (attempting to do
 so will give an `-EBUSY` error).
 
+4.2	Mount restrictions
+--------------------------
+
+If user namespaces are in use, the kernel additionally checks the instances of
+procfs available to the mounter and will not allow procfs to be mounted if:
+
+  1. This mount is not fully visible unless the new procfs is going to be
+     mounted with subset=pid option.
+
+     a. Its root directory is not the root directory of the filesystem.
+     b. If any file or non-empty procfs directory is hidden by another mount.
+
+  2. A new mount overrides the readonly option or any option from atime family.
+
 Chapter 5: Filesystem behavior
 ==============================