linux-kernel - [PATCH v7 1/5] docs: proc: add documentation about mount restrictions

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Message-ID: <654e021422520a7b25df0cc8591b7643519585fa.1768295900.git.legion@kernel.org>
Date: Tue, 13 Jan 2026 10:20:33 +0100
From: Alexey Gladkov <legion@...nel.org>
To: Christian Brauner <brauner@...nel.org>,
	Dan Klishch <danilklishch@...il.com>
Cc: Al Viro <viro@...iv.linux.org.uk>,
	"Eric W . Biederman" <ebiederm@...ssion.com>,
	Kees Cook <keescook@...omium.org>,
	containers@...ts.linux-foundation.org,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v7 1/5] 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@...nel.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@...bold.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
 ==============================
 
-- 
2.52.0