[VarDumper] Fix FFI caster test

src/Symfony/Component/VarDumper/Caster/FFICaster.php

@@ -115,11 +115,21 @@
  private static function castFFIStringValue(CData $data): string|CutStub
  {
  $result = [];
+ $ffi = \FFI::cdef(<<<C
+ size_t zend_get_page_size(void);
+ C);
 
- for ($i = 0; $i < self::MAX_STRING_LENGTH; ++$i) {
+ $pageSize = $ffi->zend_get_page_size();
+
+ // get cdata address
+ $start = $ffi->cast('uintptr_t', $ffi->cast('char*', $data))->cdata;
+ // accessing memory in the same page as $start is safe
+ $max = min(self::MAX_STRING_LENGTH, ($start | ($pageSize - 1)) - $start);
+
+ for ($i = 0; $i < $max; ++$i) {
  $result[$i] = $data[$i];
 
- if ("\0" === $result[$i]) {
+ if ("\0" === $data[$i]) {
  return implode('', $result);
  }
  }

src/Symfony/Component/VarDumper/Tests/Caster/FFICasterTest.php

@@ -12,7 +12,6 @@
 namespace Symfony\Component\VarDumper\Tests\Caster;
 
 use PHPUnit\Framework\TestCase;
-use Symfony\Component\VarDumper\Caster\FFICaster;
 use Symfony\Component\VarDumper\Test\VarDumperTestTrait;
 
 /**
@@ -191,34 +190,21 @@ public function testCastCuttedPointerToChar()
  PHP, $pointer);
  }
 
- /**
- * It is worth noting that such a test can cause SIGSEGV, as it breaks
- * into "foreign" memory. However, this is only theoretical, since
- * memory is allocated within the PHP process and almost always "garbage
- * data" will be read from the PHP process itself.
- *
- * If this test fails for some reason, please report it: We may have to
- * disable the dumping of strings ("char*") feature in VarDumper.
- *
- * @see FFICaster::castFFIStringValue()
- */
  public function testCastNonTrailingCharPointer()
  {
  $actualMessage = 'Hello World!';
  $actualLength = \strlen($actualMessage);
 
- $string = \FFI::cdef()->new('char['.$actualLength.']');
+ $string = \FFI::cdef()->new('char['.($actualLength + 1).']');
  $pointer = \FFI::addr($string[0]);
-
  \FFI::memcpy($pointer, $actualMessage, $actualLength);
 
- // Remove automatically addition of the trailing "\0" and remove trailing "\0"
  $pointer = \FFI::cdef()->cast('char*', \FFI::cdef()->cast('void*', $pointer));
  $pointer[$actualLength] = "\x01";
 
  $this->assertDumpMatchesFormat(<<<PHP
  FFI\CData<char*> size 8 align 8 {
- cdata: "$actualMessage%s"
+ cdata: %A"$actualMessage%s"
  }
  PHP, $pointer);
  }