Improve XDG_RUNTIME_DIR diagnostic

* lib-src/emacsclient.c (set_local_socket):
If there appears to be an XDG runtime directory for the user
but XDG_RUNTIME_DIR is unset, suggest setting it while warning
about potential security issues (Bug#35300).

1 files changed, 23 insertions, 5 deletions

diff --git a/lib-src/emacsclient.c b/lib-src/emacsclient.c
index f4768408980..5871a18ce69 100644
--- a/lib-src/emacsclient.c
+++ b/lib-src/emacsclient.c
@@ -1357,6 +1357,7 @@ set_local_socket (char const *server_name)
   int tmpdirlen = -1;
   int socknamelen = -1;
   uid_t uid = geteuid ();
+  bool tmpdir_used = false;
 
   if (strchr (server_name, '/')
       || (ISSLASH ('\\') && strchr (server_name, '\\')))
@@ -1389,6 +1390,7 @@ set_local_socket (char const *server_name)
             }
           socknamelen = local_sockname (sockname, socknamesize, tmpdirlen,
                                         uid, server_name);
+          tmpdir_used = true;
         }
     }
 
@@ -1462,11 +1464,27 @@ set_local_socket (char const *server_name)
   if (sock_status < 0)
     message (true, "%s: Invalid socket owner\n", progname);
   else if (sock_status == ENOENT)
-    message (true,
-             ("%s: can't find socket; have you started the server?\n"
-              "%s: To start the server in Emacs,"
-              " type \"M-x server-start\".\n"),
-             progname, progname);
+    {
+      if (tmpdir_used)
+        {
+          uintmax_t id = uid;
+          char sockdirname[socknamesize];
+          int sockdirnamelen = snprintf (sockdirname, sizeof sockdirname,
+                                         "/run/user/%"PRIuMAX, id);
+          if (0 <= sockdirnamelen && sockdirnamelen < sizeof sockdirname
+              && euidaccess (sockdirname, X_OK) == 0)
+            message
+              (true,
+               ("%s: Should XDG_RUNTIME_DIR='%s' be in the environment?\n"
+                "%s: (Be careful: XDG_RUNTIME_DIR is security-related.)\n"),
+               progname, sockdirname, progname);
+        }
+      message (true,
+               ("%s: can't find socket; have you started the server?\n"
+                "%s: To start the server in Emacs,"
+                " type \"M-x server-start\".\n"),
+               progname, progname);
+    }
   else
     message (true, "%s: can't stat %s: %s\n",
              progname, sockname, strerror (sock_status));