[PATCH 37/41] union-mount: Check read-only/read-write status of layers

[Valerie Aurora]

The top layer of a union mount must be writable (in order to support
readdir-triggered copyups) and the bottom layer must be read-only (to
avoid nasty races).

Thanks to Felix Fietkau <nbd@xxxxxxxxxxx> for a bug fix.

XXX - Add requirement that top layer is mounted only once

Signed-off-by: Valerie Aurora <vaurora@xxxxxxxxxx>
---
 fs/namespace.c |   73 +++++++++++++++++++++++++++++++++++++++++++++----------
 1 files changed, 59 insertions(+), 14 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 505974a..9b71743 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1462,6 +1462,61 @@ static int do_change_type(struct path *path, int flag)
 }
 
 /*
+ * Mount-time check of upper and lower layer file systems to see if we
+ * can union mount one on the other.
+ *
+ * Union mounts must follow these rules:
+ *
+ * - The lower layer must be read-only.  This avoids lots of nasty
+ *   unsolvable races where file system structures disappear suddenly.
+ *   XXX - Checking the vfsmnt for read-only is a temporary hack; the
+ *   file system could be mounted read-write elsewhere.  We need to
+ *   enforce read-only at the superblock level (patches coming).
+ *
+ * - The upper layer must be writable.  This isn't an absolute
+ *   requirement; right now we need it to make readdir() work since we
+ *   copy up directory entries to the top level.  A possible
+ *   workaround is to mount a tmpfs file system transparently over the
+ *   top.
+ *
+ * - The upper layer must support whiteouts and fallthrus (if it is
+ *   writeable).
+ *
+ * - The lower layer must not also be a union mount.  This is just to
+ *   make life simpler for now, there is no inherent limitation on the
+ *   number of layers.
+ *
+ * XXX - Check other mount flags for incompatibilities - I'm sure
+ * there are some.
+ */
+
+static int
+check_union_mnt(struct path *mntpnt, struct vfsmount *top_mnt, int mnt_flags)
+{
+	struct vfsmount *lower_mnt = mntpnt->mnt;
+
+	/* Is this even a union mount? */
+	if (!(mnt_flags & MNT_UNION))
+		return 0;
+
+	/* Lower layer must be read-only and not a union mount */
+	if (!(lower_mnt->mnt_sb->s_flags & MS_RDONLY) ||
+	    (lower_mnt->mnt_flags & MNT_UNION))
+		return -EBUSY;
+
+	/* Upper layer must be writable */
+	if (mnt_flags & MNT_READONLY)
+		return -EROFS;
+
+	/* Upper layer must support whiteouts and fallthrus */
+	if (!(top_mnt->mnt_sb->s_flags & MS_WHITEOUT))
+		return -EINVAL;
+
+	/* All good! */
+	return 0;
+}
+
+/*
  * do loopback mount.
  */
 static int do_loopback(struct path *path, char *old_name, int recurse,
@@ -1495,13 +1550,8 @@ static int do_loopback(struct path *path, char *old_name, int recurse,
 	if (!mnt)
 		goto out;
 
-	/*
-	 * Unions couldn't be writable if the filesystem doesn't know about
-	 * whiteouts
-	 */
-	err = -ENOTSUPP;
-	if ((mnt_flags & MNT_UNION) &&
-	    !(mnt->mnt_sb->s_flags & (MS_WHITEOUT|MS_RDONLY)))
+	err = check_union_mnt(path, mnt, mnt_flags);
+	if (err)
 		goto out;
 
 	if (mnt_flags & MNT_UNION)
@@ -1726,13 +1776,8 @@ int do_add_mount(struct vfsmount *newmnt, struct path *path,
 	if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
 		goto unlock;
 
-	/*
-	 * Unions couldn't be writable if the filesystem doesn't know about
-	 * whiteouts
-	 */
-	err = -ENOTSUPP;
-	if ((mnt_flags & MNT_UNION) &&
-	    !(newmnt->mnt_sb->s_flags & (MS_WHITEOUT|MS_RDONLY)))
+	err = check_union_mnt(path, newmnt, mnt_flags);
+	if (err)
 		goto unlock;
 
 	newmnt->mnt_flags = mnt_flags;
-- 
1.6.3.3

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