Subject: bug#24751: 26.0.50; Regex stack overflow not detected properly (gets "Variable binding depth exceeds max-specpdl-size")

From: Eli Zaretskii <[email-masked]>
Date: Tue, 15 Nov 2016 18:12:09 +0200

> From: [email-masked]
> Cc: [email-masked]
> Date: Mon, 14 Nov 2016 22:08:18 -0500
> 
> Actually, we should avoid increasing this limit if the stack wasn't
> increased, right?  Here's what I came up with, I think it doesn't cover
> Cygwin/Windows though.
> 
> diff --git c/src/emacs.c i/src/emacs.c
> index b74df21..d4655c8 100644
> --- c/src/emacs.c
> +++ i/src/emacs.c
> @@ -831,8 +831,8 @@ main (int argc, char **argv)
>  	 re_max_failures, then add 33% to cover the size of the
>  	 smaller stacks that regex.c successively allocates and
>  	 discards on its way to the maximum.  */
> -      int ratio = 20 * sizeof (char *);
> -      ratio += ratio / 3;
> +      int min_ratio = 20 * sizeof (char *);
> +      int ratio = min_ratio + min_ratio / 3;
>  
>        /* Extra space to cover what we're likely to use for other reasons.  */
>        int extra = 200000;
> @@ -869,6 +869,7 @@ main (int argc, char **argv)
>  
>        /* Don't let regex.c overflow the stack.  */
>        re_max_failures = lim < extra ? 0 : min (lim - extra, SIZE_MAX) / ratio;
> +      emacs_re_safe_alloca = re_max_failures * min_ratio;
>      }
>  #endif /* HAVE_SETRLIMIT and RLIMIT_STACK and not CYGWIN */

Right, but I have 2 comments:

  . we shouldn't set re_max_failures to zero if the amount of stack is
    less than 'extra', since in that case we will allocate the failure
    stack off the heap;
  . emacs_re_safe_alloca should have its minimum value MAX_ALLOCA, not
    zero, because SAFE_ALLOCA can still be used in regex.c, even
    though the failure stack will be malloc'ed.

Thanks.