On Sat, May 08, 2021 at 01:39:45PM -0700, Linus Torvalds wrote:

+static inline int prepend_entries(struct prepend_buffer *b, const struct path *path, const struct path *root, struct mount *mnt)

If anything, s/path/dentry/, since vfsmnt here will be equal to &mnt->mnt all along.

+{
+	struct dentry *dentry = path->dentry;
+	struct vfsmount *vfsmnt = path->mnt;
+
+	while (dentry != root->dentry || vfsmnt != root->mnt) {
+		int error;
+		struct dentry * parent;
+
+		if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
+			struct mount *parent = READ_ONCE(mnt->mnt_parent);
+			struct mnt_namespace *mnt_ns;
+
+			/* Escaped? */
+			if (dentry != vfsmnt->mnt_root)
+				return 3;
+
+			/* Global root? */
+			if (mnt != parent) {
+				dentry = READ_ONCE(mnt->mnt_mountpoint);
+				mnt = parent;
+				vfsmnt = &mnt->mnt;
+				continue;
+			}
+			mnt_ns = READ_ONCE(mnt->mnt_ns);
+			/* open-coded is_mounted() to use local mnt_ns */
+			if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
+				return 1;	// absolute root
+
+			return 2;		// detached or not attached yet
+			break;

?

+		}
+		parent = dentry->d_parent;
+		prefetch(parent);
+		error = prepend_name(b, &dentry->d_name);
+		if (error)
+			break;

return error, surely?

+		dentry = parent;
+	}
+	return 0;
+}

FWIW, if we go that way, I would make that

	while (dentry != root->dentry || &mnt->mnt != root->mnt) {
		int error;
		struct dentry *parent = READ_ONCE(dentry->d_parent);

		if (unlikely(dentry == mnt->mnt.mnt_root)) {
			struct mount *m = READ_ONCE(mnt->mnt_parent);

			/* Global root? */
			if (unlikely(mnt == m) {
				struct mnt_namespace *mnt_ns;

				mnt_ns = READ_ONCE(mnt->mnt_ns);
				/* open-coded is_mounted() to use local mnt_ns */
				if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
					return 1;	// absolute root
				else
					return 2;	// detached or not attached yet
			}
			// cross into whatever it's mounted on
			dentry = READ_ONCE(mnt->mnt_mountpoint);
			mnt = m;
			continue;
		}

		/* Escaped? */
		if (unlikely(dentry == parent))
			return 3;

		prefetch(parent);
		error = prepend_name(b, &dentry->d_name);
		if (error)
			return error;
		dentry = parent;
	}
	return 0;