php_admin_[flag|value] falsely applies config changes

Description

Quick explanation of the problem.

When using strace I noticed stat system calls for the files already in opcache, even though opcache.enable_file_override is set to 1. The system calls are triggered by is_file() being invoked from PHP.

php_admin_flag[opcache.enable_file_override] = On is set in the pool config /etc/php/8.1/fpm/pool.d/www.conf

var_dump(opcache_get_configuration()) when being invoked through an fpm process says:

["opcache.enable_file_override"]=>
  | bool(true)

phpinfo() says the same, enable_file_override is On.

I suspect the actual value of the flag is Off.

How to reproduce the issue.

1. Ensure that opcache is enabled and enable_file_override is disabled (disabled by default). Also, set in the ini file:

opcache.file_update_protection=0
opcache.validate_permission=0
opcache.validate_timestamps=0

2. Apply the fpm pool settings for simplicity of testing:

pm = static
pm.max_children = 1

3. Add three test scripts to the document root of apache server:
   test_opcache.php

<?php

echo "test begin\n";

$file1 = 'include_1.php';
$file2 = 'include_2.php';

if (is_file($file1)) {
    include $file1;
}

if (is_file($file2)) {
    include $file2;
}

echo "test end\n";

plus two more scripts in the same directory:
include_1.php

<?php

echo __FILE__ . "\n";

include_2.php

<?php

echo __FILE__ . "\n";

4. Invoke the test_opcache.php to warm up the opcache via curl request or browser.
5. Attach to the child fpm process using strace -p <pid> -e newfstatat,fstat,stat.
6. Invoke the test_opcache.php again. Expected strace output:

newfstatat(AT_FDCWD, "/", {st_mode=S_IFDIR|0755, st_size=4096, ...}, 0) = 0
newfstatat(AT_FDCWD, "include_1.php", {st_mode=S_IFREG|0644, st_size=29, ...}, 0) = 0
newfstatat(AT_FDCWD, "include_2.php", {st_mode=S_IFREG|0644, st_size=29, ...}, 0) = 0

7. Set php_admin_flag[opcache.enable_file_override] = On in the pool config and restart apache and fpm.
8. Check if the config value is changed via var_dump(opcache_get_configuration()).
9. Repeat steps 4, 5, and 6. The strace output won't change.
10. Set opcache.enable_file_override=1 in the php.ini file and repeat steps 4, 5, and 6. The output will change to

newfstatat(AT_FDCWD, "/", {st_mode=S_IFDIR|0755, st_size=4096, ...}, 0) = 0

Conclusion

The opcache.enable_file_override flag works only when it's set in the ini file.
The worst thing is that setting the flag in the pool config gives a false positive result in opcache_get_configuration() and phpinfo(), which might be very misleading.

PHP Version

PHP 8.1.13

Operating System

Ubuntu 18.04

Start by tracing how php_admin_flag[opcache.enable_file_override] from /etc/php/8.1/fpm/pool.d/www.conf is applied, comparing it with opcache.enable_file_override in php.ini. Use opcache_get_configuration(), phpinfo(), and the strace reproduction to verify that the reported value and file-stat behavior agree when the pool setting is enabled.