slsm: try to handle MNT_INTERNAL files

security/slsm/path.c

@@ -45,5 +45,18 @@ char *passt_get_path(const struct path *path, char **buf) {
 	*buf = kzalloc((size_t)buflen, GFP_KERNEL);
 	if (!*buf)
 		return ERR_PTR(-ENOMEM);
+	/* FDs that don't correspond to an actual file, e.g. pipes via /dev/fd,
+	 * aren't resolved to a path by d_absolute_path.
+	 * I *think* these can only be files already opened anyway */
+	if (path->mnt->mnt_flags & MNT_INTERNAL) {
+		/* cases we saw of internal FSs were always IS_ROOT, treating them like
+		 * / seems sensible enough */
+		if (IS_ROOT(path->dentry)) {
+			*buf[0] = '/';
+			return *buf;
+		}
+		/* no idea what this is */
+		return ERR_PTR(-EINVAL);
+	}
 	return d_absolute_path(path, *buf, buflen);
 }