attach: do not fail on non-existing namespaces

src/lxc/attach.c

@@ -281,8 +281,15 @@ static int in_same_namespace(pid_t pid1, pid_t pid2, const char *ns)
 	struct stat ns_st1, ns_st2;
 
 	ns_fd1 = lxc_preserve_ns(pid1, ns);
-	if (ns_fd1 < 0)
+	if (ns_fd1 < 0) {
+		/* The kernel does not support this namespace. This is not an
+		 * error.
+		 */
+		if (errno == ENOENT)
+			return -EINVAL;
+
 		goto out;
+	}
 
 	ns_fd2 = lxc_preserve_ns(pid2, ns);
 	if (ns_fd2 < 0)

src/lxc/utils.c

@@ -1967,8 +1967,9 @@ int lxc_preserve_ns(const int pid, const char *ns)
 	ret = snprintf(path, __NS_PATH_LEN, "/proc/%d/ns%s%s", pid,
 		       !ns || strcmp(ns, "") == 0 ? "" : "/",
 		       !ns || strcmp(ns, "") == 0 ? "" : ns);
+	errno = EFBIG;
 	if (ret < 0 || (size_t)ret >= __NS_PATH_LEN)
-		return -1;
+		return -EFBIG;
 
 	return open(path, O_RDONLY | O_CLOEXEC);
 }