[PATCH 35/73] union-mount: Create prepare_mnt_union() andcleanup_mnt_union() [ver #2]

[David Howells]

From: Valerie Aurora <vaurora@xxxxxxxxxx>

prepare_mnt_union() ties together all the mount-time checks and setup
for union mounts.  It tests the layers for suitability and builds the
root union stack.

cleanup_mnt_union() unwinds everything prepare_mnt_union() does.

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 3355b99..261944d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1569,6 +1569,46 @@ out:
 	return err;
 }
 
+/**
+ * prepare_mnt_union - do setup necessary for a union mount
+ * @topmost_mnt: vfsmount of topmost layer
+ * @mntpnt: path of requested mountpoint
+ *
+ * We union every underlying file system that is mounted on the same mountpoint
+ * (well, pathname), read-only, and not shared.  If we get at least one layer,
+ * we don't return an error, although we will complain in the kernel log if we
+ * hit a mount that can't be unioned.
+ *
+ * Caller needs namespace_sem, but can't have vfsmount_lock.
+ */
+static int prepare_mnt_union(struct vfsmount *topmost_mnt, struct path *mntpnt)
+{
+	int err;
+
+	err = check_topmost_union_mnt(topmost_mnt, topmost_mnt->mnt_flags);
+	if (err)
+		return err;
+
+	err = clone_union_tree(real_mount(topmost_mnt), mntpnt);
+	if (err)
+		return err;
+
+	err = build_root_union(topmost_mnt);
+	if (err)
+		goto out;
+	return 0;
+
+out:
+	put_union_sb(topmost_mnt->mnt_sb);
+	return err;
+}
+
+static void cleanup_mnt_union(struct vfsmount *topmost_mnt)
+{
+	d_free_unions(topmost_mnt->mnt_root);
+	put_union_sb(topmost_mnt->mnt_sb);
+}
+
 /*
  *  @source_mnt : mount tree to be attached
  *  @nd         : place the mount tree @source_mnt is attached

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