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Date: Mon, 10 Mar 2025 15:43:07 +0100
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Subject: Re: [PHP-DEV] OPcache should zero out cache slots?
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From: tovilo.ilija@gmail.com (Ilija Tovilo)

Hi Rob

On Mon, Mar 10, 2025 at 9:23=E2=80=AFAM Rob Landers <rob@bottled.codes> wro=
te:
>
> I=E2=80=99ve been trying to chase down a very subtle bug in 8.4 that only=
 happens when OPcache is enabled (I'm trying to create a reproducer to file=
 an actual bug). From what I can tell, OPcache doesn=E2=80=99t zero out cac=
he slots, so occasionally, a cache slot will contain garbage that happens t=
o pass asserts/checks that causes the program to behave incorrectly. Withou=
t OPcache, cache slots are always NULL if not set.
>
> It's quite rare to end up in that situation, though, but I was wondering =
if anyone has any opinions on this?

I don't believe this is accurate. If we're talking about the cache
used within the VM, "cache slots" are a contiguous list of arbitrary
void pointers. The compiler tracks how many cache slots each function
needs, but the actual array of pointers is allocated at runtime, in
init_func_run_time_cache_i(), when the function is executed for the
first time on each request. init_func_run_time_cache_i() zeros the
cache with memset. If it wouldn't, we'd run into issues quickly,
because a non-0 cache is generally interpreted as a primed cache.

Potentially, you might also be referring to the pointer map, which is
a related concept. But this map is also zero'ed for each request (in
zend_activate()).

If you think you might be dealing with uninitialized memory in your
case, MemorySanitizer or Valgrind may help.

Does that help?

Ilija