`JIT_G(enabled)` not set correctly on other threads

[dktapps]

Description

Disclaimer: I encountered this issue using a threading extension. Well aware this may not be the best source of truth on issues like this, especially due to the difficulty of reproducing.

With the following INI:

opcache.jit=tracing
opcache.jit_buffer_size=0

the following happens:

• INI is loaded on the main thread and sets JIT_G(enabled) = 1 and JIT_G(on) = 1
• accel_post_startup() is called during process init, and overrides thread-locals here to force-disable JIT:
  

  php-src/ext/opcache/ZendAccelerator.c

  Lines 3279 to 3280 in 6dec6a6

  	JIT_G(enabled) = false;
  	JIT_G(on) = false;

• dasm_buf is therefore null because zend_jit_startup() wasn't called due to jit_buffer_size=0:

  php-src/ext/opcache/ZendAccelerator.c

  Lines 3277 to 3286 in 6dec6a6

  	if (JIT_G(enabled)) {
  	if (JIT_G(buffer_size) == 0) {
  	JIT_G(enabled) = false;
  	JIT_G(on) = false;
  	} else if (!ZSMMG(reserved)) {
  	zend_accel_error_noreturn(ACCEL_LOG_FATAL, "Could not enable JIT: could not use reserved buffer!");
  	} else {
  	zend_jit_startup(ZSMMG(reserved), jit_size, reattached);
  	}
  	}

  &

  php-src/ext/opcache/jit/zend_jit.c

  Line 3675 in 6dec6a6

  dasm_buf = buf;

• when a second thread is started, INI is loaded again, setting JIT_G(enabled) = 1 and JIT_G(on) = 1, but this time accel_post_startup() does not override these settings
• finally, the following calls fail on Windows with errors like VirtualProtect() failed [87] The parameter is incorrect during zend_accel_persist_script() because dasm_buf is NULL - this is because JIT_G(on) = 1 although it was supposed to be forcibly disabled:

  php-src/ext/opcache/zend_persist.c

  Lines 1384 to 1386 in 6dec6a6

  	if (JIT_G(on) && for_shm) {
  	zend_jit_unprotect();
  	}

  &

  php-src/ext/opcache/zend_persist.c

  Lines 1419 to 1424 in 6dec6a6

  	if (JIT_G(on) && for_shm) {
  	if (JIT_G(opt_level) >= ZEND_JIT_LEVEL_OPT_SCRIPT) {
  	zend_jit_script(&script->script);
  	}
  	zend_jit_protect();
  	}

Proposed solution

• JIT_G(enabled) needs to become a true global
• zend_jit_config() needs to respect the existing value of JIT_G(enabled) (or its replacement true global) instead of overwriting it like it currently does
• JIT_G(enabled) (or its replacement true global) should be set to true in accel_post_startup() after zend_jit_startup() returns a success code

PHP Version

8.1, 8.2, 8.3, 8.4, master

Operating System

Windows

[cmb69]

See also krakjoe/parallel#266 (comment). It seems to me the real problem is that you can enable JIT (opcache.jit) without enabling it (opcache.jit_buffer_size=0).

[dktapps]

Yeah, that looks like the same issue. Unfortunately these are the default values for opcache.jit and opcache.jit_buffer_size so you can run into this just by loading opcache and not even touching JIT.

[cmb69]

Unfortunately these are the default values for opcache.jit and opcache.jit_buffer_size

Note that as of PHP 8.4.0, the defaults are opcache.jit=disable and opcache.jit_buffer_size=64M (see https://wiki.php.net/rfc/jit_config_defaults), though.