Fix bug in mb_get_substr_slow (sometimes outputs wrong number of characters)

Thanks to Maurício Fauth for finding and reporting this bug.

The bug was introduced in October 2022. It originally only affected
text encodings which do not have a fixed byte width per characters
and for which mbstring does not have an mblen_table. However, I recently
made another change to mbstring, such that mb_substr no longer relies
on the mblen_table even if one is available. Because of this change,
the bug earlier introduced in October 2022 now affected a greater
number of text encodings, including UTF-8.

Author: alexdowad

ext/mbstring/mbstring.c

@@ -2092,9 +2092,10 @@ static zend_string* mb_get_substr_slow(unsigned char *in, size_t in_len, size_t
 		if (from >= out_len) {
 			from -= out_len;
 		} else {
-			enc->from_wchar(wchar_buf + from, MIN(out_len - from, len), &buf, !in_len || out_len >= len);
+			size_t needed_codepoints = MIN(out_len - from, len);
+			enc->from_wchar(wchar_buf + from, needed_codepoints, &buf, !in_len || out_len >= len);
 			from = 0;
-			len -= MIN(out_len, len);
+			len -= needed_codepoints;
 		}
 	}
 

ext/mbstring/tests/mb_substr.phpt

@@ -133,6 +133,11 @@ echo "Regression:\n";
 $str = "\xbd\xbd\xbd\xbd\xbd\xbd\xbd\xbe\xbd\xbd\xbd\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x89\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x00\x00\x00\x00\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8b\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8f\x8b\x8b\x8b\xbd\xbd\xbd\xbd\xbd\xbd\xbd\xbe\x01:O\xaa\xd3";
 echo bin2hex(mb_substr($str, 0, 128, "JIS")), "\n";
 
+/* Alex messed up when reimplementing mb_substr and, in cases where `from` is non-zero and
+ * the number of characters to extract is more than 128, miscalculated where to end the substring
+ * Thanks to Maurício Fauth for finding the issue */
+var_dump(mb_substr('Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vestibulum dapibus feugiat ex non cursus. Pellentesque vestibulum tellus sit lectus.', 19, -1));
+
 ?>
 --EXPECT--
 EUC-JP:
@@ -213,3 +218,4 @@ Testing agreement with mb_strpos on invalid UTF-8 string:
 ?AAA
 Regression:
 1b28493d3d3d3d3d3d3d3e3d3d3d1b28423f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f000000003f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f1b28493d3d3d3d3d3d3d3e1b2842013a4f1b28492a1b2842
+string(121) "it amet, consectetur adipiscing elit. Vestibulum dapibus feugiat ex non cursus. Pellentesque vestibulum tellus sit lectus"