Fix crash backtraces on MS-Windows, broken by ASLR

* src/w32fns.c (DEFAULT_IMAGE_BASE): Define for 64-bit and 32-bit
MinGW builds.
(emacs_abort): Correct the callstack addresses for potential
relocation of the image base due to ASLR.  This makes 'addr2line'
be able to interpret emacs_backtrace.txt when ASLR is in effect,
which it is on every modern version of MS-Windows.  (Bug#63365)

* configure.ac (LD_SWITCH_SYSTEM_TEMACS) [mingw32]: Add comment
about keeping the image-base values in sync with w32fns.c.

* etc/DEBUG (How to disable ASLR): New section.

1 files changed, 23 insertions, 3 deletions

diff --git a/src/w32fns.c b/src/w32fns.c
index f8de45da7c9..f44460e52c0 100644
--- a/src/w32fns.c
+++ b/src/w32fns.c
@@ -11121,12 +11121,20 @@ my_exception_handler (EXCEPTION_POINTERS * exception_data)
     return prev_exception_handler (exception_data);
   return EXCEPTION_EXECUTE_HANDLER;
 }
-#endif
+#endif  /* !CYGWIN */
 
 typedef USHORT (WINAPI * CaptureStackBackTrace_proc) (ULONG, ULONG, PVOID *,
                                                       PULONG);
 
 #define BACKTRACE_LIMIT_MAX 62
+/* The below must be kept in sync with the value of the
+   -Wl,-image-base switch we use in LD_SWITCH_SYSTEM_TEMACS, see
+   configure.ac.  */
+#if defined MINGW_W64 && EMACS_INT_MAX > LONG_MAX
+# define DEFAULT_IMAGE_BASE (ptrdiff_t)0x400000000
+#else   /* 32-bit MinGW build */
+# define DEFAULT_IMAGE_BASE (ptrdiff_t)0x01000000
+#endif
 
 static int
 w32_backtrace (void **buffer, int limit)
@@ -11181,6 +11189,13 @@ emacs_abort (void)
       {
         void *stack[BACKTRACE_LIMIT_MAX + 1];
         int i = w32_backtrace (stack, BACKTRACE_LIMIT_MAX + 1);
+#ifdef CYGWIN
+        ptrdiff_t addr_offset = 0;
+#else   /* MinGW */
+        /* The offset below is zero unless ASLR is in effect.  */
+        ptrdiff_t addr_offset
+          = DEFAULT_IMAGE_BASE - (ptrdiff_t)GetModuleHandle (NULL);
+#endif  /* MinGW */
 
         if (i)
           {
@@ -11231,8 +11246,13 @@ emacs_abort (void)
               {
                 /* stack[] gives the return addresses, whereas we want
                    the address of the call, so decrease each address
-                   by approximate size of 1 CALL instruction.  */
-                sprintf (buf, "%p\r\n", (char *)stack[j] - sizeof(void *));
+                   by approximate size of 1 CALL instruction.  We add
+                   ADDR_OFFSET to account for ASLR which changes the
+                   base address of the program's image in memory,
+                   whereas 'addr2line' needs to see addresses relative
+                   to the fixed base recorded in the PE header.  */
+                sprintf (buf, "%p\r\n",
+                         (char *)stack[j] - sizeof(void *) + addr_offset);
                 if (stderr_fd >= 0)
                   write (stderr_fd, buf, strlen (buf));
                 if (errfile_fd >= 0)