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Subject[PATCH v11 6/8] docs: proc: add documentation for "hidepid=4" and "subset=pid" options and new mount behavior
Signed-off-by: Alexey Gladkov <>
Reviewed-by: Alexey Dobriyan <>
Reviewed-by: Kees Cook <>
Documentation/filesystems/proc.txt | 53 ++++++++++++++++++++++++++++++
1 file changed, 53 insertions(+)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 99ca040e3f90..bd0e0ab85048 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
@@ -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.
+ subset= Show only the specified subset of procfs.

hidepid=0 means classic mode - everybody may access all /proc/<pid>/ directories
@@ -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=4 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.
+subset=pid 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
+# 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
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