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bug#18140: [PATCH] macros.c: CHECK_VECTOR_OR_STRING invokes wrong_type_a


From: Jan Chaloupka
Subject: bug#18140: [PATCH] macros.c: CHECK_VECTOR_OR_STRING invokes wrong_type_argument for Qnil instead of return 0
Date: Tue, 29 Jul 2014 10:23:52 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0


On 07/29/2014 09:52 AM, Andreas Schwab wrote:
Jan Chaloupka <address@hidden> 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.







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