Imitate legacy behavior when converting non-encodings using mbstring

Fuzzing revealed that something was missed here when making the new
encoding conversion code match the behavior of the old code. In the
next major release of PHP, support for these non-encodings will be
dropped, but in the meantime, it is better to match the legacy
behavior.

Author: alexdowad

ext/mbstring/libmbfl/mbfl/mbfl_convert.c

@@ -353,6 +353,12 @@ zend_string* mb_fast_convert(unsigned char *in, size_t in_len, const mbfl_encodi
 	uint32_t wchar_buf[128];
 	unsigned int state = 0;
 
+	if (to == &mbfl_encoding_base64 || to == &mbfl_encoding_qprint || to == &mbfl_encoding_7bit) {
+		from = &mbfl_encoding_8bit;
+	} else if (from == &mbfl_encoding_base64 || from == &mbfl_encoding_qprint || from == &mbfl_encoding_uuencode || from == &mbfl_encoding_7bit) {
+		to = &mbfl_encoding_8bit;
+	}
+
 	mb_convert_buf buf;
 	mb_convert_buf_init(&buf, in_len, replacement_char, error_mode);
 

ext/mbstring/tests/other_encodings.phpt

@@ -15,6 +15,7 @@ var_dump(mb_convert_encoding("ABC", "7bit", "ASCII"));
 var_dump(mb_convert_encoding("\x80", "7bit", "ASCII"));
 var_dump(mb_convert_encoding("ABC", "8bit", "7bit"));
 var_dump(mb_check_encoding(chr(255), '7bit'));
+var_dump(mb_convert_encoding("\xAC\xAC", '7bit', 'UHC'));
 echo "7bit done\n";
 
 // "8bit"
@@ -27,6 +28,7 @@ string(3) "ABC"
 string(1) "%"
 string(3) "ABC"
 bool(false)
+string(2) "%%"
 7bit done
 string(1) "%"
 8bit done