Subject: bug#78738: (signal nil 5) crashes Emacs

From: Pip Cet <[email-masked]>
Date: Tue, 10 Jun 2025 06:22:49 +0000

"Daniel Colascione" <[email-masked]> writes:

> (signal nil 5) crashes Emacs

Good catch!

This would fix it, right?

diff --git a/src/eval.c b/src/eval.c
index fbb881d682d..be6d1a0a9ac 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1888,7 +1888,7 @@ DEFUN ("signal", Fsignal, Ssignal, 2, 2, 0,
   (Lisp_Object error_symbol, Lisp_Object data)
 {
   /* If they call us with nonsensical arguments, produce "peculiar error".  */
-  if (NILP (error_symbol) && NILP (data))
+  if (NILP (error_symbol) && !CONSP (data))
     error_symbol = Qerror;
   signal_or_quit (error_symbol, data, false);
   eassume (false);

Note that there are currently many ways to crash Emacs by executing
dodgy Elisp; I think there should be fewer, but others appear to
disagree.  We specifically added an eassert to crash Emacs when a Lisp
function returns invalid data, for example.

Pip