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[2021-05-31 18:34 UTC] mark at klb dot jp
Description:
------------
When using filters to output bzip2 compressed data, one may be tempted to call fflush() before stream_filter_remove() to ensure all data has been written.
This used to work fine, however since php8, this causes the generated stream to be corrupt.
Test script:
---------------
<?php
if ($_SERVER['argv'][1]??'' == 'output') {
$stream = fopen('php://output', 'wb+');
$filter = stream_filter_append($stream, 'bzip2.compress', STREAM_FILTER_WRITE, ['blocks' => 9, 'work' => 0]);
fwrite($stream, random_bytes(8192));
fflush($stream);
stream_filter_remove($filter); // should call dtor
exit;
}
$script = $_SERVER['argv'][0];
$res = shell_exec('php '.escapeshellarg($script).' output'); // should output bz encoded data
$data = bzdecompress($res);
var_dump(strlen($data));
Expected result:
----------------
int(8192)
Actual result:
--------------
int(0)
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Last updated: Thu Nov 21 15:01:30 2024 UTC |
A couple notes: I am not sure this is a bz2 issue, this could come from filters too. Outputting to php://output using a filter and fflush()ing before removing the filter are conditions for this bug to happen. The bz2 stream generated is cut partially, using bzcat will for example output: php bug.php output | bzcat -tvv (stdin): [1: huff+mtf file ends unexpectedlyI can confirm that the script outputs int(0), but I get the same result with PHP-7.4 either (regardless of explicitly calling fflush()). Interestingly, using a str_repeat("*", 8192) instead of random_bytes(8192) gives the expected result. If I use the zlib.deflate and gzinflate(), I get an explicit: Warning: gzinflate(): data error So apparently a general stream (compression) filter issue.Somehow this issue does not happen for me with php 7.4, and we started to have that issue as we upgraded from 7.4 to 8.0.6, so I assumed it was a php8 issue. With: $ php -v PHP 7.4.14 (cli) (built: Mar 6 2021 04:12:14) ( NTS ) Copyright (c) The PHP Group Zend Engine v3.4.0, Copyright (c) Zend Technologies with Zend OPcache v7.4.14, Copyright (c), by Zend Technologies (php 7.4.14 from Linux Gentoo, so might contain some patches) $ php bug.php int(8192) This seems to be related to large output of compressed data, so outputting something that compresses well (str_repeat, etc) will not cause this issue. random_bytes() generate a string with a lot of entropy, which will be difficult to compress and will result in larger compressed block(s).