Fix #79096: FFI Struct Segfault

We must not assume that the size of a function's return value is at
most `sizeof(ffi_arg)`, but rather have to use the size which already
has been determined for the return type if it is larger than
`sizeof(ffi_arg)`.

To be able to have a regression test, we export the required test
function from the zend-test extension, and make sure that the test
can be run on different platforms regardless of whether zend-tests was
built statically or dynamically.

Author: cmb69

NEWS

@@ -6,6 +6,9 @@ PHP                                                                        NEWS
   . Fixed bug #79078 (Hypothetical use-after-free in curl_multi_add_handle()).
     (cmb)
 
+- FFI:
+  . Fixed bug #79096 (FFI Struct Segfault). (cmb)
+
 - MySQLnd:
   . Fixed bug #79084 (mysqlnd may fetch wrong column indexes with MYSQLI_BOTH).
     (cmb)

ext/ffi/ffi.c

@@ -2613,10 +2613,11 @@ static ZEND_FUNCTION(ffi_trampoline) /* {{{ */
 	ffi_type **arg_types = NULL;
 	void **arg_values = NULL;
 	uint32_t n, arg_count;
-	ffi_arg ret;
+	void *ret;
 	zend_ffi_type *arg_type;
 	ALLOCA_FLAG(arg_types_use_heap = 0)
 	ALLOCA_FLAG(arg_values_use_heap = 0)
+	ALLOCA_FLAG(ret_use_heap = 0)
 
 	ZEND_ASSERT(type->kind == ZEND_FFI_TYPE_FUNC);
 	arg_count = type->func.args ? zend_hash_num_elements(type->func.args) : 0;
@@ -2704,7 +2705,8 @@ static ZEND_FUNCTION(ffi_trampoline) /* {{{ */
 		}
 	}
 
-	ffi_call(&cif, addr, &ret, arg_values);
+	ret = do_alloca(MAX(ret_type->size, sizeof(ffi_arg)), ret_use_heap);
+	ffi_call(&cif, addr, ret, arg_values);
 
 	for (n = 0; n < arg_count; n++) {
 		if (arg_types[n]->type == FFI_TYPE_STRUCT) {
@@ -2720,7 +2722,8 @@ static ZEND_FUNCTION(ffi_trampoline) /* {{{ */
 		free_alloca(arg_values, arg_values_use_heap);
 	}
 
-	zend_ffi_cdata_to_zval(NULL, (void*)&ret, ZEND_FFI_TYPE(type->func.ret_type), BP_VAR_R, return_value, 0, 1);
+	zend_ffi_cdata_to_zval(NULL, ret, ZEND_FFI_TYPE(type->func.ret_type), BP_VAR_R, return_value, 0, 1);
+	free_alloca(ret, ret_use_heap);
 
 exit:
 	zend_string_release(EX(func)->common.function_name);

ext/ffi/tests/bug79096.phpt

@@ -0,0 +1,39 @@
+--TEST--
+Bug #79096 (FFI Struct Segfault)
+--SKIPIF--
+<?php
+if (!extension_loaded('ffi')) die('skip ffi extension not available');
+if (!extension_loaded('zend-test')) die('skip zend-test extension not available');
+?>
+--FILE--
+<?php
+$header = <<<HEADER
+struct bug79096 {
+	uint64_t a;
+	uint64_t b;
+};
+
+struct bug79096 bug79096(void);
+HEADER;
+
+if (PHP_OS_FAMILY !== 'Windows') {
+    $ffi = FFI::cdef($header);
+} else {
+    try {
+        $ffi = FFI::cdef($header, 'php_zend_test.dll');
+    } catch (FFI\Exception $ex) {
+        $dll = $dll = 'php7' . (PHP_ZTS ? 'ts' : '') . (PHP_DEBUG ? '_debug' : '') . '.dll';
+        $ffi = FFI::cdef($header, $dll);
+    }
+}
+
+$struct = $ffi->bug79096();
+var_dump($struct);
+?>
+--EXPECTF--
+object(FFI\CData:struct bug79096)#%d (2) {
+  ["a"]=>
+  int(1)
+  ["b"]=>
+  int(1)
+}

ext/zend_test/php_test.h

@@ -32,4 +32,11 @@ extern zend_module_entry zend_test_module_entry;
 ZEND_TSRMLS_CACHE_EXTERN()
 #endif
 
+struct bug79096 {
+	uint64_t a;
+	uint64_t b;
+};
+
+ZEND_API struct bug79096 bug79096(void);
+
 #endif

ext/zend_test/test.c

@@ -319,3 +319,12 @@ ZEND_TSRMLS_CACHE_DEFINE()
 #endif
 ZEND_GET_MODULE(zend_test)
 #endif
+
+struct bug79096 bug79096(void)
+{
+  struct bug79096 b;
+
+  b.a = 1;
+  b.b = 1;
+  return b;
+}