Subject: Re: bug#62009: 29.0.60; Emacs crashes on setf symbol-name

From: Robert Pluim <[email-masked]>
Date: Fri, 10 Mar 2023 12:30:48 +0100

>>>>> On Fri, 10 Mar 2023 12:09:59 +0100, Daniel Mendler <[email-masked]> said:

    Daniel> One could check if the string is located in read-only memory. Or one
    Daniel> could add a flag bit to the string data structure (and possibly to other
    Daniel> data structures too). Freezing data structures such that they become
    Daniel> read-only is a generally useful feature. There won't be any performance
    Daniel> overhead of the check since a branch not taken is fast thanks to the
    Daniel> branch predictor.

We already have such a flag:

      /* Number of characters in string; MSB is used as the mark bit.  */
      ptrdiff_t size;
      /* If nonnegative, number of bytes in the string (which is multibyte).
	 If negative, the string is unibyte:
	 -1 for data normally allocated
	 -2 for data in rodata (C string constants)
	 -3 for data that must be immovable (used for bytecode)  */
      ptrdiff_t size_byte;

Try this:

diff --git a/src/lisp.h b/src/lisp.h
index 1276285e2f2..80bbb047824 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -1685,6 +1685,8 @@ SREF (Lisp_Object string, ptrdiff_t index)
 INLINE void
 SSET (Lisp_Object string, ptrdiff_t index, unsigned char new)
 {
+  if (XSTRING (string)->u.s.size_byte == -2)
+    Fsignal (Qsetting_constant, string);
   SDATA (string)[index] = new;
 }
 INLINE ptrdiff_t

Robert
--