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Date: Wed, 25 Dec 2019 13:51:51 +0100
From: Alexey Gladkov <gladkov.alexey@...il.com>
To: LKML <linux-kernel@...r.kernel.org>,
	Kernel Hardening <kernel-hardening@...ts.openwall.com>,
	Linux API <linux-api@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linux Security Module <linux-security-module@...r.kernel.org>
Cc: Akinobu Mita <akinobu.mita@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Alexey Gladkov <gladkov.alexey@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Daniel Micay <danielmicay@...il.com>,
	Djalal Harouni <tixxdz@...il.com>,
	"Dmitry V . Levin" <ldv@...linux.org>,
	"Eric W . Biederman" <ebiederm@...ssion.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"J . Bruce Fields" <bfields@...ldses.org>,
	Jeff Layton <jlayton@...chiereds.net>,
	Jonathan Corbet <corbet@....net>,
	Kees Cook <keescook@...omium.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Solar Designer <solar@...nwall.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: [PATCH v6 10/10] docs: proc: add documentation for "hidepid=3" and "pidonly" options and new mount behavior

Signed-off-by: Alexey Gladkov <gladkov.alexey@...il.com>
---
 Documentation/filesystems/proc.txt | 53 ++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 99ca040e3f90..6a62ae20a181 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -50,6 +50,8 @@ Table of Contents
   4	Configuring procfs
   4.1	Mount options
 
+  5	Filesystem behavior
+
 ------------------------------------------------------------------------------
 Preface
 ------------------------------------------------------------------------------
@@ -2021,6 +2023,7 @@ The following mount options are supported:
 
 	hidepid=	Set /proc/<pid>/ access mode.
 	gid=		Set the group authorized to learn processes information.
+	pidonly=	Show only task related subset of procfs.
 
 hidepid=0 means classic mode - everybody may access all /proc/<pid>/ directories
 (default).
@@ -2042,6 +2045,56 @@ information about running processes, whether some daemon runs with elevated
 privileges, whether other user runs some sensitive program, whether other users
 run any program at all, etc.
 
+hidepid=3 means that procfs should only contain /proc/<pid>/ directories
+that the caller can ptrace.
+
 gid= defines a group authorized to learn processes information otherwise
 prohibited by hidepid=.  If you use some daemon like identd which needs to learn
 information about processes information, just add identd to this group.
+
+The pidonly=1 hides all top level files and directories in the procfs that
+are not related to tasks.
+
+------------------------------------------------------------------------------
+5 Filesystem behavior
+------------------------------------------------------------------------------
+
+Originally, before the advent of pid namepsace, procfs was a global file
+system. It means that there was only one procfs instance in the system.
+
+When pid namespace was added, a separate procfs instance was mounted in
+each pid namespace. So, procfs mount options are global among all
+mountpoints within the same namespace.
+
+# grep ^proc /proc/mounts
+proc /proc proc rw,relatime,hidepid=2 0 0
+
+# strace -e mount mount -o hidepid=1 -t proc proc /tmp/proc
+mount("proc", "/tmp/proc", "proc", 0, "hidepid=1") = 0
++++ exited with 0 +++
+
+# grep ^proc /proc/mounts
+proc /proc proc rw,relatime,hidepid=2 0 0
+proc /tmp/proc proc rw,relatime,hidepid=2 0 0
+
+and only after remounting procfs mount options will change at all
+mountpoints.
+
+# mount -o remount,hidepid=1 -t proc proc /tmp/proc
+
+# grep ^proc /proc/mounts
+proc /proc proc rw,relatime,hidepid=1 0 0
+proc /tmp/proc proc rw,relatime,hidepid=1 0 0
+
+This behavior is different from the behavior of other filesystems.
+
+The new procfs behavior is more like other filesystems. Each procfs mount
+creates a new procfs instance. Mount options affect own procfs instance.
+It means that it became possible to have several procfs instances
+displaying tasks with different filtering options in one pid namespace.
+
+# mount -o hidepid=2 -t proc proc /proc
+# mount -o hidepid=1 -t proc proc /tmp/proc
+# grep ^proc /proc/mounts
+proc /proc proc rw,relatime,hidepid=2 0 0
+proc /tmp/proc proc rw,relatime,hidepid=1 0 0
-- 
2.24.1

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