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 10:23:52 +0200

On 07/29/2014 09:52 AM, Andreas Schwab wrote:
> Jan Chaloupka <[email-masked]> writes:
>
>> Changelog:
>> 	line wrapping to 80 characters
>>
>> 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.
> So the correct solution is to initialize kbd_macro_ptr together with
> kbd_macro_buffer.  Otherwise the same situation can still happen any
> time garbage collection is called.
Yes, for garbage collector. However, Vlast_kbd_macro will continue being 
Qnil.
The patch is still valid (just without comment about random value of
kbd_macro_ptr and garbage collection):

In function Fstart_kbd_macro (macros.c), Vlast_kbd_macro of 
current_kboard is
Qnil for the first invocation. If NILP (append) is false
CHECK_VECTOR_OR_STRING fails (invocation
of wrong_type_argument resulting in emacs_abort). However, it has to pass.

> Andreas.