imagesettile() and imagesetbrush() keep a dangling pointer once the tile is freed

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Descrizione

Description

imagesettile() passes the libgd pointer of the tile to gdImageSetTile() without keeping a reference to the tile object. Once the tile GdImage is freed, the image keeps a dangling pointer, and the next fill with IMG_COLOR_TILED reads freed memory. imagesetbrush() has the same shape, with IMG_COLOR_BRUSHED.

The following code:

<?php
$canvas = imagecreatetruecolor(100, 100);
(function () use ($canvas) {
    $tile = imagecreatetruecolor(10, 10);
    imagesettile($canvas, $tile);
})();
// The tile is freed: its memory is reused
$strings = [];
for ($i = 0; $i < 10000; ++$i) {
    $strings[] = str_repeat("\xff", 7000 + $i % 64);
}
imagefilledrectangle($canvas, 0, 0, 99, 99, IMG_COLOR_TILED);
echo "survived\n";

Resulted in this output:

Segmentation fault

But I expected this output instead:

survived

It crashes every time on PHP 8.5.12 (macOS, system libgd 2.3.3) and on PHP 8.4.27 (the php:8.4-cli Docker image, bundled GD). The same script with imagesetbrush() and imageline(..., IMG_COLOR_BRUSHED) also crashes on 8.5.12.

In ext/gd/gd.c on master, PHP_FUNCTION(imagesettile) and PHP_FUNCTION(imagesetbrush) only call gdImageSetTile(im, tile) and gdImageSetBrush(im, tile), and php_gd_image_object has no field that would hold the tile or the brush. A fix could keep a reference to the tile and brush objects in php_gd_image_object, released when they are replaced and when the image is freed (and reported by get_gc).

I found it while drawing SVG patterns with GD in a userland library: keeping the tile in a variable until the drawing ends works around it.

PHP Version
PHP 8.5.12 (cli) (ZTS)
PHP 8.4.27 (cli) (php:8.4-cli Docker image)
Operating System

macOS 27; Debian (Docker)

Lingua principale
C
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Merge medio
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PR unite (30g)
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