Re: [PATCH 3/3] grep: plug leak of pcre chartables in PCRE2

[Carlo Arenas <carenas@gmail.com>]

On Mon, Jul 29, 2019 at 1:35 PM René Scharfe <l.s.r@web.de> wrote:
>
> Am 28.07.19 um 03:41 schrieb Carlo Arenas:
> > On Sat, Jul 27, 2019 at 4:48 PM Ævar Arnfjörð Bjarmason
> > <avarab@gmail.com> wrote:
> >>> +     free((void *)p->pcre_tables);
> >>
> >> Is the cast really needed? I'm rusty on the rules, removing it from the
> >> pcre_free() you might have copied this from produces a warning for me,
> >> but not for free() itself. This is on GCC 8.3.0. How about for you &
> >> what compiler(s)?
> >
> > both will trigger warnings for the same reason
> > (-Wincompatible-pointer-types-discards-qualifiers)
> > with Apple LLVM version 10.0.1 (clang-1001.0.46.4)
> >
> > gcc-9 in macOS triggers 2 "warnings"; one for discarding the const
> > qualifier (-Wdiscarded-qualifiers)
> > and another for mismatching parameter to free():
> >
> > note: expected 'void *' but argument is of type 'const uint8_t *' {aka
> > 'const unsigned char *'}
>
> Right: pcre_maketables() returns a const pointer, and you have to cast
> away this const'ness at some point if you want to free(3) that memory.
> Returning a non-const pointer would be more fitting, but I guess the
> idea is that users of the library are not supposed to change the
> contents of the table.

note that this pointer was generated by pcre2_maketables() instead
which is actually "const uint8_t *", but yes these tables are meant to
be inmutable and that is why they are "cost".

there is even a compile time generated table for the C locale that is
used internally and obviously shouldn't be free().

> But wouldn't it be more correct to use pcre_free()?  As long as we keep
> pcre_malloc() and pcre_free() at their default values it doesn't matter
> in practice, but using free(3) directly is a layering violation, no?

yes, but that is the only option PCRE2 gives when not using a global
context which is what the comment in the commit refers to.

FWIW pcre_free() doesn't exist anymore in PCRE2.

> Perhaps just UNLEAK that thing?  There is only a single way to build it
> and we can reuse it throughout the lifetime of the program, so there is
> no real need to clean it up before the OS does.

That would be a better fit if it would be created once in cmd_grep and
then shared with all worker threads (which I thought would be nice to
do in the future anyway), but this change was trying to be conservative
and just to the minimum to close the leak.

Carlo