PHP <=7.3 Opcache is broken on Bookworm

[mauzi]

Hi,

- Debian 11 (Bullseye) amd64, up-to-date @ today, PHP-FPM:
7.1.33-54+0~20230814.83+debian11~1.gbp8adb1a - everything works flawlessly
7.2.34-40+0~20230814.86+debian11~1.gbpe8e12b - everything works flawlessly
7.3.33-12+0~20230814.109+debian11~1.gbpa3309d - everything works flawlessly
7.4.33-7+0~20230814.85+debian11~1.gbp091944 - everything works flawlessly

- Debian 12 (Bookworm) amd64, up-to-date @ today, PHP-FPM:
7.1.33-54+0~20230815.84+debian12~1.gbp8adb1a - some Wordpress, Joomla, and other sites fail (100% CPU usage for several minutes, then timeout)
7.2.34-40+0~20230815.87+debian12~1.gbpe8e12b - some Wordpress, Joomla, and other sites fail (100% CPU usage several for minutes, then timeout)
7.3.33-12+0~20230815.111+debian12~1.gbpa3309d - some Wordpress, Joomla, and other sites fail (100% CPU usage several for minutes, then timeout)
7.4.33-7+0~20230815.86+debian12~1.gbp091944 - everything works flawlessly

Do you have any idea?
Thanks!

[oerdnj]

Try capturing the stack with eu-stack. Alternatively, you might want to do some perf record on the php process to sample what's happening.

[mauzi]

It has something to do with libgd3.
The problem occurs in functions using the gd extension.
Investigating further...

[mauzi]

Please forget my post about libgd3.
It's the opcache.
I have disabled the opcache, and now everything work good. Re-enabling opcache 100% reproduces the problem.

What can be the difference between php7.3-fpm on Debian 11 vs. Debian 12? Are the build options the same?

[oerdnj]

Different compiler. The opcache is a bit fragile, so this doesn't surprise me at all.

[mauzi]

Look at this code:

$x = 0;
$y = hexdec("0200");

for ($i = 0; $i < 100; $i++) {
    if ($x >= $y) {
        $i += $y % 4;
        $x = 0;
    }

    $x++;
}

This code always fail on my Debian 12 (amd64) box with PHP 7.3-FPM + opcache enabled.

But:
var_dump($y) is int(512), but if I type cast it, then the code does not fail:

$y = (int)hexdec("0200");

The if() expression should never eval to TRUE, but if I change $i += $width % 4; to $i += 5; then the code does not fail.

[oerdnj]

This one could be worked around with opcache.optimization_level=0x7FFEBF5F