PCRE: Check whether start offset is on char boundary

We need not just the whole string to be UTF-8, but the start
position to be on a character boundary as well. Check this by
looking for a continuation byte.

Author: nikic

ext/pcre/php_pcre.c

@@ -1130,6 +1130,22 @@ static void php_do_pcre_match(INTERNAL_FUNCTION_PARAMETERS, int global) /* {{{ *
 }
 /* }}} */
 
+static zend_always_inline zend_bool is_known_valid_utf8(
+		zend_string *subject_str, PCRE2_SIZE start_offset) {
+	if (!(GC_FLAGS(subject_str) & IS_STR_VALID_UTF8)) {
+		/* We don't know whether the string is valid UTF-8 or not. */
+		return 0;
+	}
+
+	if (start_offset == ZSTR_LEN(subject_str)) {
+		/* Degenerate case: Offset points to end of string. */
+		return 1;
+	}
+
+	/* Check that the offset does not point to an UTF-8 continuation byte. */
+	return (ZSTR_VAL(subject_str)[start_offset] & 0xc0) != 0x80;
+}
+
 /* {{{ php_pcre_match_impl() */
 PHPAPI void php_pcre_match_impl(pcre_cache_entry *pce, zend_string *subject_str, zval *return_value,
 	zval *subpats, int global, int use_flags, zend_long flags, zend_off_t start_offset)
@@ -1247,7 +1263,7 @@ PHPAPI void php_pcre_match_impl(pcre_cache_entry *pce, zend_string *subject_str,
 		}
 	}
 
-	options = (pce->compile_options & PCRE2_UTF) && !(GC_FLAGS(subject_str) & IS_STR_VALID_UTF8)
+	options = (pce->compile_options & PCRE2_UTF) && !is_known_valid_utf8(subject_str, start_offset2)
 		? 0 : PCRE2_NO_UTF_CHECK;
 
 	/* Execute the regular expression. */

ext/pcre/tests/bug79241.phpt

@@ -0,0 +1,22 @@
+--TEST--
+Bug #79241: Segmentation fault on preg_match()
+--FILE--
+<?php
+
+// if "’" string is used directly without json_decode,
+// the issue does not reproduce
+$text = json_decode('"’"');
+
+$pattern = '/\b/u';
+
+// it has to be exact two calls to preg_match(),
+// with the second call offsetting after the tick symbol
+var_dump(preg_match($pattern, $text, $matches, 0, 0));
+var_dump(preg_match($pattern, $text, $matches, 0, 1));
+var_dump(preg_last_error() == PREG_BAD_UTF8_OFFSET_ERROR);
+
+?>
+--EXPECT--
+int(0)
+bool(false)
+bool(true)