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, 37 insertions, 0 deletions

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index fea2561d773b..fdda62e6115e 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -19,6 +19,28 @@ static const struct inode_operations proc_sys_inode_operations;
 static const struct file_operations proc_sys_dir_file_operations;
 static const struct inode_operations proc_sys_dir_operations;
 
+/* Support for permanently empty directories */
+
+struct ctl_table sysctl_mount_point[] = {
+        { }
+};
+
+static bool is_empty_dir(struct ctl_table_header *head)
+{
+        return head->ctl_table[0].child == sysctl_mount_point;
+}
+
+static void set_empty_dir(struct ctl_dir *dir)
+{
+        dir->header.ctl_table[0].child = sysctl_mount_point;
+}
+
+static void clear_empty_dir(struct ctl_dir *dir)
+
+{
+        dir->header.ctl_table[0].child = NULL;
+}
+
 void proc_sys_poll_notify(struct ctl_table_poll *poll)
 {
         if (!poll)
@@ -187,6 +209,17 @@ static int insert_header(struct ctl_dir *dir, struct ctl_table_header *header)
         struct ctl_table *entry;
         int err;
 
+        /* Is this a permanently empty directory? */
+        if (is_empty_dir(&dir->header))
+                return -EROFS;
+
+        /* Am I creating a permanently empty directory? */
+        if (header->ctl_table == sysctl_mount_point) {
+                if (!RB_EMPTY_ROOT(&dir->root))
+                        return -EINVAL;
+                set_empty_dir(dir);
+        }
+
         dir->header.nreg++;
         header->parent = dir;
         err = insert_links(header);
@@ -202,6 +235,8 @@ fail:
         erase_header(header);
         put_links(header);
 fail_links:
+        if (header->ctl_table == sysctl_mount_point)
+                clear_empty_dir(dir);
         header->parent = NULL;
         drop_sysctl_table(&dir->header);
         return err;
@@ -419,6 +454,8 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
                 inode->i_mode |= S_IFDIR;
                 inode->i_op = &proc_sys_dir_operations;
                 inode->i_fop = &proc_sys_dir_file_operations;
+                if (is_empty_dir(head))
+                        make_empty_dir_inode(inode);
         }
 out:
         return inode;