Subject: Re: [RFC PATCH v2] grep: allow for run time disabling of JIT in PCRE

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Wed, 31 Jul 2019 16:57:54 +0200

On Wed, Jul 31 2019, Johannes Schindelin wrote:

> Hi,
>
> On Mon, 29 Jul 2019, Carlo Marcelo Arenas Belón wrote:
>
>>   $ git grep 'foo bar'
>>   fatal: Couldn't JIT the PCRE2 pattern 'foo bar', got '-48'
>
> My immediate reaction to this error message was: That's not helpful.
> What is `-48` supposed to mean? Why do we even think it sensible to
> throw such an error message at the end user? Can't we do a much better
> job translating that into something that makes actual sense without
> knowing implementation details?
>
> But then, I realized that -48 must be a well-known constant in PCRE2,
> and my reaction transformed into something much more hopeful: why don't
> we detect the situation where the JIT'ed code was not actually
> executable [*1*], and fall back to the non-JIT'ed code path ourselves,
> without troubling the end user (maybe warning, but maybe better not lest
> we annoy the user with something pointless)?
>
> Even after finding out that -48 disappointingly means
> PCRE2_ERROR_NOMEMORY (as opposed to something like
> PCRE2_ERROR_CANNOT_EXECUTE_JIT_CODE), I like the idea of not bothering
> end users and doing the sensible fallback under the hood.
>
> Ciao,
> Dscho
>
> Footnote *1*: Why anybody would think it sensible to build a PCRE2 with
> JIT on an OS that does not allow executing code that was written by the
> same process is beyond me. Or is there a mode in OpenBSD that *does*
> allow JIT'ed code to be executed?

We do detect if JIT isn't supported and fall back. That's what the
pcre2_config(PCRE2_CONFIG_JIT, &p->pcre2_jit_on) code in grep.c
does. This and is the subsequent pcre2_pattern_info() call is how PCRE
documents that you should do this.

What hasn't been supported is all of that saying "yes, I support JIT"
and the feature then fail whaling. I had not encountered that before.

So far that seems like because Carlo just built a completely broken PCRE
v2 package, so I don't know if that's worth supporting on our
side. I.e. this isn't something I think could plausibly happen in the
wild.

That should *not* be confused with me thinking other stuff Carlo's
raised is a non-issue, e.g. running into the JIT stack limit etc. Some
of that's clearly bugs in our/my grep.c code that need fixing.