vfs: more mnt_parent cleanups

a) mount --move is checking that ->mnt_parent is non-NULL before
looking if that parent happens to be shared; ->mnt_parent is never
NULL and it's not even an misspelled !mnt_has_parent()

b) pivot_root open-codes is_path_reachable(), poorly.

c) so does path_is_under(), while we are at it.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 27 insertions, 15 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index ec8512478b04..7aad258dcaf6 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1876,8 +1876,7 @@ static int do_move_mount(struct path *path, char *old_name)
         /*
          * Don't move a mount residing in a shared parent.
          */
-        if (old_path.mnt->mnt_parent &&
-            IS_MNT_SHARED(old_path.mnt->mnt_parent))
+        if (IS_MNT_SHARED(old_path.mnt->mnt_parent))
                 goto out1;
         /*
          * Don't move a mount tree containing unbindable mounts to a destination
@@ -2534,6 +2533,31 @@ out_type:
 }
 
 /*
+ * Return true if path is reachable from root
+ *
+ * namespace_sem or vfsmount_lock is held
+ */
+bool is_path_reachable(struct vfsmount *mnt, struct dentry *dentry,
+                         const struct path *root)
+{
+        while (mnt != root->mnt && mnt_has_parent(mnt)) {
+                dentry = mnt->mnt_mountpoint;
+                mnt = mnt->mnt_parent;
+        }
+        return mnt == root->mnt && is_subdir(dentry, root->dentry);
+}
+
+int path_is_under(struct path *path1, struct path *path2)
+{
+        int res;
+        br_read_lock(vfsmount_lock);
+        res = is_path_reachable(path1->mnt, path1->dentry, path2);
+        br_read_unlock(vfsmount_lock);
+        return res;
+}
+EXPORT_SYMBOL(path_is_under);
+
+/*
  * pivot_root Semantics:
  * Moves the root file system of the current process to the directory put_old,
  * makes new_root as the new root file system of the current process, and sets
@@ -2561,7 +2585,6 @@ out_type:
 SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
                 const char __user *, put_old)
 {
-        struct vfsmount *tmp;
         struct path new, old, parent_path, root_parent, root;
         int error;
 
@@ -2611,18 +2634,7 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
         if (!mnt_has_parent(new.mnt))
                 goto out4; /* not attached */
         /* make sure we can reach put_old from new_root */
-        tmp = old.mnt;
-        if (tmp != new.mnt) {
-                for (;;) {
-                        if (!mnt_has_parent(tmp))
-                                goto out4; /* already mounted on put_old */
-                        if (tmp->mnt_parent == new.mnt)
-                                break;
-                        tmp = tmp->mnt_parent;
-                }
-                if (!is_subdir(tmp->mnt_mountpoint, new.dentry))
-                        goto out4;
-        } else if (!is_subdir(old.dentry, new.dentry))
+        if (!is_path_reachable(old.mnt, old.dentry, &new))
                 goto out4;
         br_write_lock(vfsmount_lock);
         detach_mnt(new.mnt, &parent_path);