SHM_PROTECT/SHM_UNPROTECT race in opcache under ZTS with multiple threads and protect_memory=1

Description

When multiple PHP threads run concurrently in the same process (ZTS build), opcache's SHM_PROTECT()/SHM_UNPROTECT() calls race with each other because mprotect() is process-global, but there is no coordination (refcounting) between threads.

I'm not 100% sure this is a bug vs. a known limitation. But the behavior is surprising and I wanted to report it for discussion.

What happens

SHM_PROTECT() calls mprotect(shm, PROT_READ) and SHM_UNPROTECT() calls mprotect(shm, PROT_READ|PROT_WRITE). These are process-wide — they affect all threads.

In ZEND_RINIT_FUNCTION(zend_accelerator) (ZendAccelerator.c, lines 2714–2778), every php_request_startup() executes an SHM_UNPROTECT() / SHM_PROTECT() pair. In a multi-threaded ZTS process, each thread calls php_request_startup() independently.

When tracing JIT is enabled and compiling a hot trace, it also holds SHM unprotected (zend_jit_trace.c, line 7512). If a second thread's RINIT calls SHM_PROTECT() while the first thread is still writing to SHM inside JIT compilation, the first thread gets SIGSEGV (write to read-only page).

Timeline:

Main thread                              Worker thread
───────────                              ─────────────
zend_jit_compile_root_trace:
  zend_shared_alloc_lock()
  SHM_UNPROTECT()  ← PROT_READ|WRITE
  writing to zend_jit_traces[N]...       php_request_startup():
  ...                                      RINIT(accel):
  ...                                        SHM_UNPROTECT() ← no-op
  ...                                        ...checks restart_pending...
  ...                                        SHM_PROTECT() ← mprotect(PROT_READ)
  ...                                        done, returns
  t->code_start = start  → SIGSEGV
  (page is now read-only)

The zend_shared_alloc_lock() serializes JIT compilation between threads, but it does NOT prevent RINIT's SHM_PROTECT() from running concurrently — RINIT doesn't acquire that lock for its SHM_UNPROTECT/PROTECT pair.

How to reproduce

Any ZTS setup with multiple threads running PHP code concurrently + opcache.protect_memory=1 + opcache.jit=tracing.

Minimal reproduction with TrueAsync threads (but should be reproducible with any ZTS threading extension — parallel, pmmpthread, etc.):

<?php
use Async\ThreadPool;
use function Async\spawn;
use function Async\await;

spawn(function() {
    $pool = new ThreadPool(2);
    $future = $pool->submit(fn() => 42);
    echo await($future) . "\n";
    $pool->close();
    echo "Done\n";
});

Run with:

php -d opcache.enable_cli=1 \
    -d opcache.jit=tracing \
    -d opcache.jit_buffer_size=64M \
    -d opcache.protect_memory=1 \
    -d opcache.jit_hot_loop=1 \
    -d opcache.jit_hot_func=1 \
    test.php

Result: correct output 42\nDone followed by SIGSEGV in zend_jit_trace_add_code (zend_jit_trace.c:227).

The crash does NOT happen with:

• opcache.protect_memory=0 (default) — mprotect is never called, no race
• opcache.jit=function — functions are compiled before threads start, no runtime compilation during concurrent execution

Notes

• The krakjoe/parallel extension has the same architecture (calls php_request_startup() per thread) and avoids this by only testing with opcache.jit=function or opcache.jit=disable in CI. Their ASAN+JIT job explicitly uses -d opcache.jit=function.
• run-tests.php hardcodes opcache.protect_memory=1 (line 300), so any multi-threaded test suite using run-tests.php with tracing JIT will hit this.
• The underlying issue is that mprotect() is process-global but SHM_UNPROTECT/PROTECT pairs are not coordinated across threads — there is no refcount to prevent one thread's PROTECT from overriding another thread's active UNPROTECT window.

PHP Version

PHP 8.6.0-dev (cli) (ZTS DEBUG)
Zend Engine v4.6.0-dev
    with Zend OPcache v8.6.0-dev

Operating System

Ubuntu 24.04 (Linux 6.17)

Start with the SHM_PROTECT/SHM_UNPROTECT pair in ZendAccelerator.c lines 2714–2778, then inspect the JIT window in zend_jit_trace.c around line 7512 and the crash site at line 227. Reproduce with a ZTS build using the provided TrueAsync script and tracing-JIT options. Done means the concurrent test no longer reaches SIGSEGV, with suitable regression coverage identified.