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Subject: Re: [PHP-DEV] PCRE JIT stack size limit
From: cmbecker69@gmx.de (Christoph Becker)

Adam Harvey wrote:

> On 23 July 2015 at 11:47, Christoph Becker <cmbecker69@gmx.de> wrote:
> <snip great explanation, thanks>
>> 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.
> 
> I'm not a big fan of that, although it's at least in the spirit of
> what configuration settings are meant to be used for.
> 
> What if we added the PCRE_ERROR_JIT_STACKLIMIT error constant to those
> exposed to userland so that it's more easily noticed via
> preg_last_error(), and adding a modifier that can be used to disable
> the JIT on a per-pattern basis (by setting PCRE_NO_START_OPTIMIZE,
> which admittedly disables other stuff too, but at least the regex will
> run)? At least then users could check the error when the regex fails
> and re-run the regex without the JIT if they chose to.

Yes, that would be an option.  I'm not sure if we need to touch
PCRE_NO_START_OPTIMIZE (or if it would even help); simply not calling
pcre_study() respectively not passing the studied extra data (second
argument) to pcre_exec() should be sufficient.

> How likely is the average user to hit this, do you think?

I don't know.  The libpcre man pages say:

| JIT uses far less memory for recursion than the interpretive code,
| and a maximum stack size of 512K to 1M should be more than enough for
| any pattern.

Of course, a hard-coded 512K or even more would be rather much.  The
default stack size (32K) for the regex in the test script of #70110
would be sufficient, if the subject is repeated only about 2000 times,
but that doesn't say much (the preg_match() is rather contrived).

Anyway, that shows that it's not only the pattern that's relevant for
the needed JIT stack space, but also the subject string.

-- 
Christoph M. Becker