[PATCH v8 1/5] docs: proc: add documentation about mount restrictions

Alexey Gladkov
2026-02-13T10:44:48Z

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

Signed-off-by: Alexey Gladkov <legion@kernel.org>
---
Documentation/filesystems/proc.rst | 14 ++++++++++++++
1 file changed, 14 insertions(+)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 8256e857e2d7..c8864fcbdec7 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

@@ -2410,6 +2411,19 @@ 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.
+
+ a. It's 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 familty.
+
Chapter 5: Filesystem behavior
==============================

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