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From:   Alexey Gladkov <gladkov.alexey@gmail.com>
To:     LKML <linux-kernel@vger.kernel.org>,
        "Eric W . Biederman" <ebiederm@xmission.com>
Cc:     Alexey Gladkov <legion@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Kees Cook <keescook@chromium.org>,
        Linux Containers <containers@lists.linux-foundation.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>
Subject: [PATCH v6 1/5] docs: proc: add documentation about mount restrictions
Date:   Fri, 12 Mar 2021 17:41:44 +0100
Message-Id: <e57f67df145fd79baabd4c8cbaa75c7d1269282a.1615567183.git.gladkov.alexey@gmail.com>
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Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
---
 Documentation/filesystems/proc.rst | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 2fa69f710e2a..5a1bb0e081fd 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -50,6 +50,7 @@ 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
 
@@ -2175,6 +2176,19 @@ 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.
 
+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
 ==============================
 
-- 
2.29.3