Fix overflow in memory limit checks

Due to overflows in the memory limit checks, we were missing cases
where the allocation size was close to the address space size, and
caused an OOM condition rather than a memory limit error.

Author: nikic

Zend/zend_alloc.c

@@ -980,7 +980,7 @@ static void *zend_mm_alloc_pages(zend_mm_heap *heap, uint32_t pages_count ZEND_F
 				heap->cached_chunks = chunk->next;
 			} else {
 #if ZEND_MM_LIMIT
-				if (UNEXPECTED(heap->real_size + ZEND_MM_CHUNK_SIZE > heap->limit)) {
+				if (UNEXPECTED(ZEND_MM_CHUNK_SIZE > heap->limit - heap->real_size)) {
 					if (zend_mm_gc(heap)) {
 						goto get_chunk;
 					} else if (heap->overflow == 0) {
@@ -1484,8 +1484,8 @@ static void *zend_mm_realloc_heap(zend_mm_heap *heap, void *ptr, size_t size, si
 				}
 			} else /* if (new_size > old_size) */ {
 #if ZEND_MM_LIMIT
-				if (UNEXPECTED(heap->real_size + (new_size - old_size) > heap->limit)) {
-					if (zend_mm_gc(heap) && heap->real_size + (new_size - old_size) <= heap->limit) {
+				if (UNEXPECTED(new_size - old_size > heap->limit - heap->real_size)) {
+					if (zend_mm_gc(heap) && new_size - old_size <= heap->limit - heap->real_size) {
 						/* pass */
 					} else if (heap->overflow == 0) {
 #if ZEND_DEBUG
@@ -1730,8 +1730,8 @@ static void *zend_mm_alloc_huge(zend_mm_heap *heap, size_t size ZEND_FILE_LINE_D
 	void *ptr;
 
 #if ZEND_MM_LIMIT
-	if (UNEXPECTED(heap->real_size + new_size > heap->limit)) {
-		if (zend_mm_gc(heap) && heap->real_size + new_size <= heap->limit) {
+	if (UNEXPECTED(new_size > heap->limit - heap->real_size)) {
+		if (zend_mm_gc(heap) && new_size <= heap->limit - heap->real_size) {
 			/* pass */
 		} else if (heap->overflow == 0) {
 #if ZEND_DEBUG

ext/standard/tests/strings/wordwrap_memory_limit.phpt

@@ -0,0 +1,18 @@
+--TEST--
+No overflow should occur during the memory_limit check for wordwrap()
+--SKIPIF--
+<?php
+if (getenv("USE_ZEND_ALLOC") === "0") die("skip Zend MM disabled");
+?>
+--INI--
+memory_limit=128M
+--FILE--
+<?php
+
+$str = str_repeat('x', 65534);
+$str2 = str_repeat('x', 65535);
+wordwrap($str, 1, $str2);
+
+?>
+--EXPECTF--
+Fatal error: Allowed memory size of 134217728 bytes exhausted%s(tried to allocate %d bytes) in %s on line %d