Subject: bug#18140: [PATCH] macros.c: CHECK_VECTOR_OR_STRING invokes wrong_type_argument for Qnil instead of return 0

From: Jan Chaloupka <[email-masked]>
Date: Tue, 29 Jul 2014 07:01:58 +0200

In function Fstart_kbd_macro (macros.c), Vlast_kbd_macro of current_kboard is Qnil for the first invocation.
If NILP (append) is false, current_kboard->kbd_macro_ptr has random value (in our case 0x5353535353535353),
which after CHECK_VECTOR_OR_STRING failure (invocation of wrong_type_argument) results in garbage collecting.
During gc, marking of objects is processed and mark_kboards (keyboard.c) is invoked. Following for loop is fired:

for (p = kb->kbd_macro_buffer; p < kb->kbd_macro_ptr; p++)
                   mark_object (*p);

Since kb->kbd_macro_ptr is set to 0x5353535353535353, mark_object (*p) is trying to mark object on address
out of memory space (or memory that cannot be accessed). Thus resulting in SIGSEGV signal.

Solution is to check for Qnil before calling CHECK_VECTOR_OR_STRING and set len to 0 if Qnil occurs.

https://bugzilla.redhat.com/show_bug.cgi?id=1104012

Signed-off-by: Jan Chaloupka <[email-masked]>
---
  src/macros.c |   18 +++++++++++++++++-
  1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/macros.c b/src/macros.c
index 4730a8b..219eb39 100644
--- a/src/macros.c
+++ b/src/macros.c
@@ -85,7 +85,23 @@ macro before appending to it.  */)
        bool cvt;
  
        /* Check the type of last-kbd-macro in case Lisp code changed it.  */
-      len = CHECK_VECTOR_OR_STRING (KVAR (current_kboard, Vlast_kbd_macro));
+      /* If Vlast_kbd_macro is Qnil, skip the check and set len to 0.
+       * Flength returns 0 for Qnil, CHECK_VECTOR_OR_STRING has to do the same.
+       * Otherwise CHECK_VECTOR_OR_STRING fails and results in garbage collecting,
+       * which results in (keyboard.c, mark_kboards(void))
+       *
+       * for (p = kb->kbd_macro_buffer; p < kb->kbd_macro_ptr; p++)
+       * 	  mark_object (*p);
+       *
+       * Here, kb->kbd_macro_ptr is not initialized and can contain address
+       * 0x5353535353535353, which results in SIGSEGV trying to access the address.
+       *
+       * https://bugzilla.redhat.com/show_bug.cgi?id=1104012
+       */
+      if (!NILP (KVAR (current_kboard, Vlast_kbd_macro) ))
+        len = CHECK_VECTOR_OR_STRING (KVAR (current_kboard, Vlast_kbd_macro));
+      else
+        len = 0;
  
        /* Copy last-kbd-macro into the buffer, in case the Lisp code
  	 has put another macro there.  */