[PATCH 31/73] union-mount: Create check_topmost_union_mnt() [ver #2]

[David Howells]

From: Valerie Aurora <vaurora@xxxxxxxxxx>

check_topmost_union_mnt() checks that the topmost layer of a proposed
union mount is read-write, supports fallthrus and whiteouts, and isn't
mounted elsewhere.

Original-author: Valerie Aurora <vaurora@xxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 fs/namespace.c |   40 ++++++++++++++++++++++++++++++++++++++++
 1 files changed, 40 insertions(+), 0 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 33aa310..bed9ccd 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1410,6 +1410,46 @@ static int invent_group_ids(struct mount *mnt, bool recurse)
 	return 0;
 }
 
+/**
+ * check_topmost_union_mnt - mount-time checks for union mount
+ * @topmost_mnt: vfsmount of the topmost union filed system
+ * @mnt_flags: mount flags for the topmost mount
+ *
+ * Our readdir() solution of copying up directory entries requires
+ * that the topmost layer be writeable and support whiteouts and
+ * fallthrus.  The topmost file system can't be mounted elsewhere
+ * because it's Too Hard(tm).
+ */
+static int check_topmost_union_mnt(struct vfsmount *topmost_mnt, int mnt_flags)
+{
+	struct super_block *sb = topmost_mnt->mnt_sb;
+
+#ifndef CONFIG_UNION_MOUNT
+	printk(KERN_INFO "union mount: not supported by the kernel\n");
+	return -EINVAL;
+#else
+	if (mnt_flags & MNT_READONLY)
+		return -EROFS;
+
+	if (atomic_read(&sb->s_active) != 1) {
+		printk(KERN_INFO "union mount: topmost fs mounted elsewhere\n");
+		return -EBUSY;
+	}
+
+	if (!(sb->s_flags & MS_WHITEOUT)) {
+		printk(KERN_INFO "union mount: whiteouts not supported by fs\n");
+		return -EINVAL;
+	}
+
+	if (!(sb->s_flags & MS_FALLTHRU)) {
+		printk(KERN_INFO "union mount: fallthrus not supported by fs\n");
+		return -EINVAL;
+	}
+
+	return 0;
+#endif
+}
+
 /*
  *  @source_mnt : mount tree to be attached
  *  @nd         : place the mount tree @source_mnt is attached

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