Fix bug #71572

Discussed on the internals list as part of
http://markmail.org/message/fa625yc2eegk2dio.

Author: flaupretre

NEWS

@@ -5,6 +5,8 @@ PHP                                                                        NEWS
 - Core:
   . Fixed bug #62210 (Exceptions can leak temporary variables). (Dmitry, Bob)
   . Fixed bug #69989 (Generators don't participate in cycle GC). (Nikita)
+  . Fixed buf #71572 (String offset assignment from an empty string inserts
+    null byte). (Francois)
   . Implemented the RFC `Support Class Constant Visibility`. (Sean DuBois,
     Reeze Xia, Dmitry)
   . Added void return type. (Andrea)

Zend/tests/bug71572.phpt

@@ -0,0 +1,27 @@
+--TEST--
+Bug #71572: String offset assignment from an empty string inserts null byte
+--FILE--
+<?php
+
+$str = "abc";
+var_dump($str{0} = "");
+var_dump($str{1} = "");
+var_dump($str{3} = "");
+var_dump($str{10} = "");
+var_dump($str);
+?>
+==DONE==
+--EXPECTF--
+Warning: Cannot assign an empty string to a string offset in %s on line %d
+NULL
+
+Warning: Cannot assign an empty string to a string offset in %s on line %d
+NULL
+
+Warning: Cannot assign an empty string to a string offset in %s on line %d
+NULL
+
+Warning: Cannot assign an empty string to a string offset in %s on line %d
+NULL
+string(3) "abc"
+==DONE==

Zend/zend_execute.c

@@ -1169,8 +1169,11 @@ static zend_never_inline void zend_binary_assign_op_obj_dim(zval *object, zval *
 static void zend_assign_to_string_offset(zval *str, zend_long offset, zval *value, zval *result)
 {
 	zend_string *old_str;
+	zend_uchar c;
+	size_t string_len;
 
 	if (offset < 0) {
+		/* Error on negative offset */
 		zend_error(E_WARNING, "Illegal string offset:  " ZEND_LONG_FMT, offset);
 		zend_string_release(Z_STR_P(str));
 		if (result) {
@@ -1179,8 +1182,31 @@ static void zend_assign_to_string_offset(zval *str, zend_long offset, zval *valu
 		return;
 	}
 
+	if (Z_TYPE_P(value) != IS_STRING) {
+		/* Convert to string, just the time to pick the 1st byte */
+		zend_string *tmp = zval_get_string(value);
+
+		string_len = ZSTR_LEN(tmp);
+		c = (zend_uchar)ZSTR_VAL(tmp)[0];
+		zend_string_release(tmp);
+	} else {
+		string_len = Z_STRLEN_P(value);
+		c = (zend_uchar)Z_STRVAL_P(value)[0];
+	}
+
+	if (string_len == 0) {
+		/* Error on empty input string */
+		zend_error(E_WARNING, "Cannot assign an empty string to a string offset");
+		zend_string_release(Z_STR_P(str));
+		if (result) {
+			ZVAL_NULL(result);
+		}
+		return;
+	}
+
 	old_str = Z_STR_P(str);
 	if ((size_t)offset >= Z_STRLEN_P(str)) {
+		/* Extend string if needed */
 		zend_long old_len = Z_STRLEN_P(str);
 		Z_STR_P(str) = zend_string_extend(Z_STR_P(str), offset + 1, 0);
 		Z_TYPE_INFO_P(str) = IS_STRING_EX;
@@ -1191,23 +1217,11 @@ static void zend_assign_to_string_offset(zval *str, zend_long offset, zval *valu
 		Z_TYPE_INFO_P(str) = IS_STRING_EX;
 	}
 
-	if (Z_TYPE_P(value) != IS_STRING) {
-		zend_string *tmp = zval_get_string(value);
-
-		Z_STRVAL_P(str)[offset] = ZSTR_VAL(tmp)[0];
-		zend_string_release(tmp);
-	} else {
-		Z_STRVAL_P(str)[offset] = Z_STRVAL_P(value)[0];
-	}
-	/*
-	 * the value of an assignment to a string offset is undefined
-	T(result->u.var).var = &T->str_offset.str;
-	*/
+	Z_STRVAL_P(str)[offset] = c;
 
 	zend_string_release(old_str);
 	if (result) {
-		zend_uchar c = (zend_uchar)Z_STRVAL_P(str)[offset];
-
+		/* Return the new character */
 		if (CG(one_char_string)[c]) {
 			ZVAL_INTERNED_STR(result, CG(one_char_string)[c]);
 		} else {