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Subject[PATCH 37/41] union-mount: Check read-only/read-write status of layers
Date
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@openwrt.org> for a bug fix.

XXX - Add requirement that top layer is mounted only once

Signed-off-by: Valerie Aurora <vaurora@redhat.com>
---
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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