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Bug #75629 Object property references are replaced by references created in functions
Submitted: 2017-12-05 09:21 UTC Modified: 2017-12-05 12:00 UTC
Avg. Score:5.0 ± 0.0
Reproduced:0 of 0 (0.0%)
From: Assigned:
Status: Feedback Package: Scripting Engine problem
PHP Version: Irrelevant OS: Linux x64
Private report: No CVE-ID: None
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 [2017-12-05 09:21 UTC]

The PHP engine seems to globally replace references of object properties when new  object property references are created in a by-ref function call.

The problem occurs when defining a by-ref parameter in a function and then calling it with an object property:

function foo(& $param) {
    // keep a non-gc reference to $param here and things will break

The attached test case demonstrates the issue by using `clone` and an array of references. The same example works when the references are kept in another object.

Possible causes here are:

 * clone is misbehaving
 * references are misbehaving overall (YIKES!)
 * references created in functions are cached somewhere

Test script:
Keeping a reference to an object property in an array should not override existing references

class Container
    /** @var \stdClass|null */
    public $referencedProperty;

    public function __clone()
        $this->referencedProperty = clone $this->referencedProperty;

$references = [];

$container = new Container();

$loadFieldByReference = function (& $referencedProperty) use (& $references) {
    $referencedProperty = new \stdClass();
    $references[]       = & $referencedProperty;


$container->referencedProperty->publicProperty = 123;
$clone                                         = clone $container;
$clone->referencedProperty->publicProperty     = 234;

echo $container->referencedProperty->publicProperty, "\n";
echo $clone->referencedProperty->publicProperty, "\n";
echo $container->referencedProperty === $clone->referencedProperty ? "same\n" : "not same\n";

not same


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 [2017-12-05 09:24 UTC]
Causes (original source of the bug)
 [2017-12-05 11:04 UTC]
-Status: Open +Status: Feedback
 [2017-12-05 11:04 UTC]
Try clarify, is the behavior you object to the fact that PHP preserves references while cloning?
 [2017-12-05 12:00 UTC]
@nikic two incorrect things are going on here:

 1. the `referencedProperty` is cloned too here (via `__clone`), so it should be a new instance anyway
 2. the `Container#$referencedProperty` is never a reference itself, but it is passed by reference to the `$loadFieldByReference()` closure and stored in `$references`
 [2017-12-05 15:28 UTC]
As discussed with Nikic, this issue is likely a misunderstanding in how references work. References are bi-directional, and have to be maintained until necessary. 

In the test case above, the reference becomes unnecessary as soon as the closure execution ends, but it is to be kept when pushing it to an array.

This specific line is actually acting on a reference, and writing a clone to a reference, therefore overwriting both objects:

`$this->referencedProperty = clone $this->referencedProperty`

A solution here is to de-reference the value by moving the reference:

$ref = & $this->referencedProperty;
$this->referencedProperty = clone $ref;

I'm still not convinced that this is right, as it basically makes references a complete nightmare to work it (not that it was a pleasure before this).
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Last updated: Sun Nov 19 01:31:42 2017 UTC