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Date: Thu, 23 Jul 2015 20:47:26 +0200
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Subject: Re: [PHP-DEV] PCRE JIT stack size limit
From: cmbecker69@gmx.de (Christoph Becker)

Hi Pierre,

Pierre Joye wrote:

> Hi Christoph,
> 
> There are ways to dymacally increase the stack. Apache's modphp can use the
> apache config. Fpm, fcgi or cli can change it on windows (afair it is in
> the doc). It is possible too on Linux with ulimit.
> 
> An alternative (not a big fan) would be to use setrlimit with an ini
> setting.

Ah, I should have explained better that libpcre's JIT support is
implemented as own virtual machine, and libpcre has functions to use a
custom stack which is allocated on the heap.  If these functions are not
used (as it's now in ext/pcre) a fixed 32K on the machine stack are used.[1]

Anyhow, it is not possible to change the stack (size) during a call to
pcre_exec() (unless, maybe, libpcre would be modified), and it is not
possible (to my knowledge) to calculate the required stack size in
advance.  The required stack size depends on pattern and subject.

So basically, we have to call pcre_exec(), and if it fails due to
limited stack size, we either fail as well, or we try again.  In the
latter case we could either use a bigger stack or do without JIT.  In
the former case we should at least give users a setting to choose the
desired stack size, which is quite comparable to pcre.backtrack_limit
and pcre.recursion_limit.

Therefore I tend to prefer a new ini setting (say, pcre.jitstack_limit).
 That would mean, however, to add yet another ini setting, of which
there are already so many.

[1] <http://www.pcre.org/original/doc/html/pcrejit.html#SEC8>

-- 
Christoph M. Becker