Re: OPcache should zero out cache slots?
Hi Rob
On Mon, Mar 10, 2025 at 9:23 AM Rob Landers <[email protected]> wrote:
>
> I’ve 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’t zero out cache slots, so occasionally, a cache slot will contain garbage that
> happens to pass asserts/checks that causes the program to behave incorrectly. Without 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
Thread (3 messages)