vfs: add path_has_submounts()

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.

Link: http://lkml.kernel.org/r/20161011053403.27645.55242.stgit@pluto.themaw.net
Signed-off-by: Ian Kent <raven@themaw.net>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Omar Sandoval <osandov@osandov.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 39 insertions, 0 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 5c7cc953ac81..8515875854b6 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1306,6 +1306,45 @@ 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 (likely(!d_mountpoint(dentry)))
+                return D_WALK_CONTINUE;
+        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(const struct path *parent)
+{
+        struct check_mount data = { .mnt = parent->mnt, .mounted = 0 };
+
+        read_seqlock_excl(&mount_lock);
+        d_walk(parent->dentry, &data, path_check_mount, NULL);
+        read_sequnlock_excl(&mount_lock);
+
+        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