Bug #75415 [Com]: shm_put_var() parallel bug.

Edit report at https://bugs.php.net/bug.php?id=75415&edit=1 ID: 75415 Comment by: abecker at mailbox dot org Reported by: abc905 at mail dot ru Summary: shm_put_var() parallel bug. Status: Open Type: Bug Package: Semaphore related Operating System: Linux Debian 4.9.0-4-amd64 PHP Version: 7.1.10 Block user comment: N Private report: N New Comment: The problem is that the shm extension does not use semaphores to protect the shared memory from parellel access. Parallel operations from different processes can thus result in memory corruption, race conditions, and the error "Warning: shm_put_var(): Not enough shared memory left". The problem is still present in PHP 8.0.12. As a workaround, in PHP one must create a semaphore and always encapsulate shm_put_var within sem_acquire and sem_release calls. Here is an example in C how to use semaphore locks to access shared memory: https://www.ibm.com/docs/en/i/7.2?topic=programs-example-using-semaphore-set-shared-memory-functions Previous Comments: ------------------------------------------------------------------------ [2017-10-22 10:04:41] abc905 at mail dot ru Description: ------------ Run test script 1-10 times to see that in spite of the fact shm_put_var returns true, shm_has_var() returns false. Calling sem_get() fixes the bug Test script: --------------- $processes = 300; $shmem = shm_attach(ftok(__FILE__, 'f'), 100000); // If uncomment next line shm_put_var works correctly // $sem = sem_get(ftok(__FILE__, 'a')); for ($i = 1; $i <= $processes; $i++) { if (pcntl_fork() === 0) { usleep(100000); $pid = getmypid(); $put_ok = shm_put_var($shmem, $pid, $i); if ($put_ok && !shm_has_var($shmem, $pid)) { print "shm_put_var() bug\n"; } exit(0); } } while (pcntl_wait($status) !== -1){} shm_remove($shmem); shm_detach($shmem); ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=75415&edit=1