[RFC PATCH 3/8] vfs - add path_has_submounts()

[Ian Kent]

From: Ian Kent <ikent@xxxxxxxxxx>

d_mountpoint() can only be used reliably to establish if a dentry is
not mounted in any namespace. It isn't aware of the possibility there
may be multiple mounts using the given dentry, possibly in a different
namespace.

Add function, path_has_submounts(), that checks is a struct path contains
mounts (or is a mountpoint itself) to handle this case.

Signed-off-by: Ian Kent <raven@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Omar Sandoval <osandov@xxxxxxxxxxx>
---
 fs/dcache.c            |   35 +++++++++++++++++++++++++++++++++++
 include/linux/dcache.h |    1 +
 2 files changed, 36 insertions(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index 5c7cc95..872f04e 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1306,6 +1306,41 @@ int have_submounts(struct dentry *parent)
 }
 EXPORT_SYMBOL(have_submounts);
 
+struct check_mount {
+	struct vfsmount *mnt;
+	unsigned int mounted;
+};
+
+static enum d_walk_ret path_check_mount(void *data, struct dentry *dentry)
+{
+	struct check_mount *info = data;
+	struct path path = { .mnt = info->mnt, .dentry = dentry };
+
+	if (path_is_mountpoint(&path)) {
+		info->mounted = 1;
+		return D_WALK_QUIT;
+	}
+	return D_WALK_CONTINUE;
+}
+
+/**
+ * path_has_submounts - check for mounts over a dentry in the
+ *                      current namespace.
+ * @parent: path to check.
+ *
+ * Return true if the parent or its subdirectories contain
+ * a mount point in the current namespace.
+ */
+int path_has_submounts(struct path *parent)
+{
+	struct check_mount data = { .mnt = parent->mnt, .mounted = 0 };
+
+	d_walk(parent->dentry, &data, path_check_mount, NULL);
+
+	return data.mounted;
+}
+EXPORT_SYMBOL(path_has_submounts);
+
 /*
  * Called by mount code to set a mountpoint and check if the mountpoint is
  * reachable (e.g. NFS can unhash a directory dentry and then the complete
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index ad2df92..5c5d197 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -255,6 +255,7 @@ extern void d_prune_aliases(struct inode *);
 
 /* test whether we have any submounts in a subdir tree */
 extern int have_submounts(struct dentry *);
+extern int path_has_submounts(struct path *);
 
 /*
  * This adds the entry to the hash queues.