/proc/self paths resolve to the parent process after pcntl_fork() (realpath cache) #23986

Description

After pcntl_fork(), a path under /proc/self that the parent already used resolves, in the child, to the parent's /proc/<pid>: the realpath cache kept the resolved symlink across the fork. The child reads the parent's process information.

<?php
$pid = fn() => (int) explode(' ', file_get_contents('/proc/self/stat'))[0];
echo "parent: ", $pid() === getmypid() ? "ok" : "wrong", "\n";
if (pcntl_fork() === 0) {
    echo "child: /proc/self/stat says ", $pid(), ", getmypid() ", getmypid(), "\n";
    exit(0);
}
pcntl_wait($status);

Resulted in this output:

parent: ok
child: /proc/self/stat says 3248499, getmypid() 3248500

But I expected this output instead:

parent: ok
child: /proc/self/stat says 3248500, getmypid() 3248500

clearstatcache(true) in the child works around it. The same happens with scandir('/proc/self/task') and the like, on NTS and ZTS builds. /proc/self (and /proc/thread-self) are symlinks whose target depends on the process reading them, so they shouldn't be cached, or the realpath cache could be cleared in the child of pcntl_fork().

PHP Version

PHP 8.5.11 (cli) (NTS); also 8.4.26 (cli) (ZTS)

Operating System

Ubuntu 24.04 (x86_64); Alpine 3.22 (Docker)