sysctl: Allow creating permanently empty directories that serve as mountpoints.

commit f9bd6733d3f11e24f3949becf277507d422ee1eb upstream.

Add a magic sysctl table sysctl_mount_point that when used to
create a directory forces that directory to be permanently empty.

Update the code to use make_empty_dir_inode when accessing permanently
empty directories.

Update the code to not allow adding to permanently empty directories.

Update /proc/sys/fs/binfmt_misc to be a permanently empty directory.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 1 insertions, 7 deletions

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 2082b1a88fb9..c3eee4c6d6c1 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1531,12 +1531,6 @@ static struct ctl_table vm_table[] = {
         { }
 };
 
-#if defined(CONFIG_BINFMT_MISC) || defined(CONFIG_BINFMT_MISC_MODULE)
-static struct ctl_table binfmt_misc_table[] = {
-        { }
-};
-#endif
-
 static struct ctl_table fs_table[] = {
         {
                 .procname       = "inode-nr",
@@ -1690,7 +1684,7 @@ static struct ctl_table fs_table[] = {
         {
                 .procname       = "binfmt_misc",
                 .mode           = 0555,
-                .child          = binfmt_misc_table,
+                .child          = sysctl_mount_point,
         },
 #endif
         {