[PATCH v10 7/7] docs: proc: add documentation about mount restrictions

Alexey Gladkov
2026-04-27T08:27:12Z

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

Signed-off-by: Alexey Gladkov <legion@kernel.org>
---
Documentation/filesystems/proc.rst | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index b0c0d1b45b99..b836b725b35d 100644
--- a/Documentation/filesystems/proc.rst
+++ b/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

@@ -2401,7 +2402,9 @@ 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
@@ -2410,6 +2413,20 @@ 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
==============================

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