Fix for bug #32241: memory corruption due to large index map in 'Range

checked for each record'

The problem was in incorrectly calculated length of the buffer used to
store a hexadecimal representation of an index map in
select_describe(). This could result in buffer overrun and stack
corruption under some circumstances.

Fixed by correcting the calculation.


mysql-test/r/explain.result:
  Added a test case for bug #32241.
mysql-test/t/explain.test:
  Added a test case for bug #32241.
sql/sql_select.cc:
  Corrected the buffer length calculation. Count one hex digit as 4 bits,
  not 8.

mysql-test/r/explain.result

@@ -87,3 +87,23 @@ Warnings:
 Note	1003	select '1' AS `f1`,'1' AS `f2` from `test`.`t1` having 1
 drop view v1;
 drop table t1;
+CREATE TABLE t1(c INT);
+INSERT INTO t1 VALUES (),();
+CREATE TABLE t2 (b INT,
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b));
+INSERT INTO t2 VALUES (),(),();
+EXPLAIN SELECT 1 FROM
+(SELECT 1 FROM t2,t1 WHERE b < c GROUP BY 1 LIMIT 1) AS d2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+X	X	X	X	X	X	X	X	X	const row not found
+X	X	X	X	X	X	X	X	X	
+X	X	X	X	X	X	X	X	X	Range checked for each record (index map: 0xFFFFFFFFFF)
+DROP TABLE t2;
+DROP TABLE t1;

mysql-test/t/explain.test

@@ -66,4 +66,32 @@ explain extended select * from t1 having 1;
 drop view v1;
 drop table t1;
 
+#
+# Bug #32241: memory corruption due to large index map in 'Range checked for 
+#             each record'
+#
+
+CREATE TABLE t1(c INT);
+INSERT INTO t1 VALUES (),();
+
+CREATE TABLE t2 (b INT,
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b),
+KEY(b),KEY(b),KEY(b),KEY(b),KEY(b));
+
+INSERT INTO t2 VALUES (),(),();
+
+# We only need to make sure that there is no buffer overrun and the index map
+# is displayed correctly
+--replace_column 1 X 2 X 3 X 4 X 5 X 6 X 7 X 8 X 9 X
+EXPLAIN SELECT 1 FROM
+  (SELECT 1 FROM t2,t1 WHERE b < c GROUP BY 1 LIMIT 1) AS d2;
+DROP TABLE t2;
+DROP TABLE t1;
+
 # End of 5.0 tests.

sql/sql_select.cc

@@ -15282,7 +15282,8 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
 	{
 	  if (tab->use_quick == 2)
 	  {
-            char buf[MAX_KEY/8+1];
+            /* 4 bits per 1 hex digit + terminating '\0' */
+            char buf[MAX_KEY / 4 + 1];
             extra.append(STRING_WITH_LEN("; Range checked for each "
                                          "record (index map: 0x"));
             extra.append(tab->keys.print(buf));