[PATCH 09/16] devpts: Fail early (if appropriate) on overmount

["Eric W. Biederman" <ebiederm@xmission.com>]

Update the vfs with a may_overmount superblock operation, allowing
devpts to fail early if the primary mount of devpts is going to be
mounted on top of itself.

This change is in preparation for each mount of devpts being distinct
from every other mount of devpts.  To maintain a backward compatible
notion of a primary mount of devpts we need overmounts of the mount to
fail (as they do now), which requires a little vfs support so the case
can be detected.

Cause failed over mounts of devpts to go through the devpts remount
path.  This already happens as overmounts have previously been detected
late, and it looks like CentOS 6 may actually depend on this behavior
to allow changing devpts mount options by placing them in /etc/fstab.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 fs/devpts/inode.c  | 15 +++++++++++++++
 fs/namespace.c     |  8 ++++++++
 include/linux/fs.h |  1 +
 3 files changed, 24 insertions(+)

diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index 1266abd3251e..051a2657be36 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -339,10 +339,25 @@ static int devpts_show_options(struct seq_file *seq, struct dentry *root)
 	return 0;
 }
 
+bool devpts_may_overmount(struct super_block *sb,
+			  int flags, const char *dev_name, void *data)
+{
+	if ((sb == devpts_mnt->mnt_sb) &&
+	    (current_user_ns() == &init_user_ns) &&
+	    !parse_newinstance(data)) {
+		down_write(&sb->s_umount);
+		devpts_remount(sb, &flags, data);
+		up_write(&sb->s_umount);
+		return false;
+	}
+	return true;
+}
+
 static const struct super_operations devpts_sops = {
 	.statfs		= simple_statfs,
 	.remount_fs	= devpts_remount,
 	.show_options	= devpts_show_options,
+	.may_overmount	= devpts_may_overmount,
 };
 
 static void *new_pts_fs_info(void)
diff --git a/fs/namespace.c b/fs/namespace.c
index 278ecb7b8e8e..01ad3207c3a2 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2401,6 +2401,7 @@ static int do_new_mount(struct path *path, const char *fstype, int flags,
 {
 	struct file_system_type *type;
 	struct user_namespace *user_ns = current->nsproxy->mnt_ns->user_ns;
+	struct super_block *path_sb;
 	struct vfsmount *mnt;
 	int err;
 
@@ -2429,6 +2430,13 @@ static int do_new_mount(struct path *path, const char *fstype, int flags,
 		}
 	}
 
+	path_sb = path->mnt->mnt_sb;
+	if ((path_sb->s_type == type) &&
+	    (path->mnt->mnt_root == path->dentry) &&
+	    path_sb->s_op->may_overmount &&
+	    !path_sb->s_op->may_overmount(path_sb, flags, name, data))
+		return -EBUSY;
+
 	mnt = vfs_kern_mount(type, flags, name, data);
 	if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
 	    !mnt->mnt_sb->s_subtype)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 14a97194b34b..aade033bed49 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1755,6 +1755,7 @@ struct super_operations {
 				  struct shrink_control *);
 	long (*free_cached_objects)(struct super_block *,
 				    struct shrink_control *);
+	bool (*may_overmount)(struct super_block *, int, const char *, void *);
 };
 
 /*
-- 
2.8.1