Avoid crashes due to objects read with the #n=object form

* src/lread.c (read1): Use Fcons for 'placeholder', not AUTO_CONS,
because elements of the list in 'read_objects' cannot be allocated
off the stack.  (Bug#24640)

1 files changed, 12 insertions, 1 deletions

diff --git a/src/lread.c b/src/lread.c
index ef58b20070d..8a368806e15 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -2845,7 +2845,18 @@ read1 (Lisp_Object readcharfun, int *pch, bool first_in_list)
                   if (c == '=')
                     {
                       /* Make a placeholder for #n# to use temporarily.  */
-                      AUTO_CONS (placeholder, Qnil, Qnil);
+                      /* Note: We used to use AUTO_CONS to allocate
+                         placeholder, but that is a bad idea, since it
+                         will place a stack-allocated cons cell into
+                         the list in read_objects, which is a
+                         staticpro'd global variable, and thus each of
+                         its elements is marked during each GC.  A
+                         stack-allocated object will become garbled
+                         when its stack slot goes out of scope, and
+                         some other function reuses it for entirely
+                         different purposes, which will cause crashes
+                         in GC.  */
+                      Lisp_Object placeholder = Fcons (Qnil, Qnil);
                       Lisp_Object cell = Fcons (make_number (n), placeholder);
                       read_objects = Fcons (cell, read_objects);