bug#41755: feature/native-comp (master?): temacs crash in GC during mark phase

["Nicolas Bértolo" <nicolasbertolo@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 328 bytes --]

Hi,

Here's a new patch that copies the suffixes before building the heap-based list
in openp. I know this is not the solution I proposed, but I couldn't adapt the
code without increasing its complexity way too much for my liking. If you think
this is not an appropriate solution I will come up with another one.

Thanks, Nico.

[-- Attachment #2: 0001-Copy-suffixes-passed-to-openp-to-avoid-GC-crashes.-F.patch --]
[-- Type: application/octet-stream, Size: 2209 bytes --]

From 4b4fd526abe124c8a74bfa11209dd53c3a564cc7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nicol=C3=A1s=20B=C3=A9rtolo?= <nicolasbertolo@gmail.com>
Date: Mon, 8 Jun 2020 22:01:25 -0300
Subject: [PATCH] Copy suffixes passed to 'openp' to avoid GC crashes. Fixes
 bug#41755

In openp_add_middle_dir_to_suffixes we build a heap-based list from
the passed suffixes.  It is crucial that we don't create a heap-based
cons that points to a stack-based list.

* src/lread.c (openp_add_middle_dir_to_suffixes): Copy suffixes when
building a list of middle-dirs and suffixes.
---
 src/lread.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/lread.c b/src/lread.c
index c127d32eb17..65d84462c4c 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -1635,21 +1635,27 @@ openp_add_middle_dir_to_suffixes (Lisp_Object suffixes)
   Lisp_Object extended_suf = Qnil;
   FOR_EACH_TAIL_SAFE (tail)
     {
-#ifdef HAVE_NATIVE_COMP
+      /*  suffixes may be a stack-based cons pointing to stack-based
+          strings.  We must copy the suffix if we are putting it into
+          a heap-based cons to avoid a dangling reference.  This would
+          lead to crashes during the GC.  */
       CHECK_STRING_CAR (tail);
       char * suf = SSDATA (XCAR (tail));
+      Lisp_Object copied_suffix = build_string (suf);
+#ifdef HAVE_NATIVE_COMP
       if (strcmp (NATIVE_ELISP_SUFFIX, suf) == 0)
         {
           CHECK_STRING (Vcomp_native_path_postfix);
           /* Here we add them in the opposite order so that nreverse
              corrects it.  */
-          extended_suf = Fcons (Fcons (Qnil, XCAR (tail)), extended_suf);
-          extended_suf = Fcons (Fcons (Vcomp_native_path_postfix, XCAR (tail)),
+          extended_suf = Fcons (Fcons (Qnil, copied_suffix), extended_suf);
+          extended_suf = Fcons (Fcons (Vcomp_native_path_postfix,
+                                       copied_suffix),
                                 extended_suf);
         }
       else
 #endif
-	extended_suf = Fcons (Fcons (Qnil, XCAR (tail)), extended_suf);
+	extended_suf = Fcons (Fcons (Qnil, copied_suffix), extended_suf);
     }
 
   suffixes = Fnreverse (extended_suf);
-- 
2.25.1.windows.1