Bug #78010 [Ver->Ana]: Segmentation fault during GC

Edit report at https://bugs.php.net/bug.php?id=78010&edit=1 ID: 78010 Updated by: nikic@php.net Reported by: valera dot ymnik at gmail dot com Summary: Segmentation fault during GC -Status: Verified +Status: Analyzed Type: Bug Package: Reproducible crash Operating System: Debian 9 && Ubuntu 18.04 PHP Version: 7.3.5 Block user comment: N Private report: N New Comment: What I believe is happening here is the following: An object gets assigned GC address 2^20 (or a multiple thereof), which is compressed to 0. The object is found to be part of a cycle and will at the end of root collection be marked as black (color 0). Together these two mean that the object has 0 gc_info. During the GC destruction phase, some other object is destroyed first and recursively destroyed our gc_info=0 object. When that happens we attempt to remove it from the root buffer, but because gc_info=0 it looks like the object is not part of the root buffer and this is skipped, eventually resulting in a double free. Previous Comments: ------------------------------------------------------------------------ [2019-07-13 04:28:59] grzegorz129 at gmail dot com After some more testing this bug seems to be introduced in 7.3.0 (or during it's stabilization period). The 7.2.20 runs my short example without a problem while 7.3.0 crashes with a SIGSEGV. While I'm unable to run the whole suite on 3v4l (since it uses substantial amount of resources after all) I also verified all three master branches (normal/jit/opcache) and all of them are crashing in the same way. ------------------------------------------------------------------------ [2019-07-13 00:00:05] grzegorz129 at gmail dot com I simplified the reproducer and added some comments: <?php //gc_disable(); class foo { public function __construct() { $this->x = $this; for ($i = 0; $i < 898; $i++) { //Will not trigger with <898 $obj = [new stdClass, new stdClass]; //This must have at least 2 elements $this->y[] = $obj; } } } for ($i = 0; $i < 2; ++$i) { //This must run >=2 (increasing the number of elements in the array *2 will not do) $x = []; //This must be reset foreach (array_fill(0, 389, 'x') as &$params) { //Will not trigger <389 $x[] = new foo; } } echo "Completed\n"; Valgring output: https://zerobin.net/?ca2ef1bcab5e1b03#qc85nUSpB0Yp7oK4Bvd1hSOV3B9kybe34XmH6wvZE6o= ------------------------------------------------------------------------ [2019-07-12 20:50:21] grzegorz129 at gmail dot com Maybe stating the obvious but putting gc_disable() at the beginning of the script makes the code complete. Also tested on 7.4alpha3 - the same result. ------------------------------------------------------------------------ [2019-07-12 18:50:46] nikic@php.net Related To: Bug #78280 ------------------------------------------------------------------------ [2019-07-12 16:11:17] kolja dot zuelsdorf at deinhandy dot de We have this issue under strong suspicion to break one of our core business processes and wonder why it was not addressed in the last bugfix releases. Is there anything we can do to help mitigate the issue? So far we are running said process on an old (7.1) version. Furthermore, I've confirmed that the same thing happens on 7.3.6 and 7.3.7 too. ------------------------------------------------------------------------ The remainder of the comments for this report are too long. To view the rest of the comments, please view the bug report online at https://bugs.php.net/bug.php?id=78010 -- Edit this bug report at https://bugs.php.net/bug.php?id=78010&edit=1