Bug #54476: crash when group_concat and 'with rollup' in

                     prepared statements

Using GROUP_CONCAT() together with the WITH ROLLUP modifier
could crash the server.

The reason was a combination of several facts:

1. The Item_func_group_concat class stores pointers to ORDER
objects representing the columns in the ORDER BY clause of
GROUP_CONCAT().

2. find_order_in_list() called from
Item_func_group_concat::setup() modifies the ORDER objects so
that their 'item' member points to the arguments list
allocated in the Item_func_group_concat constructor.

3. In some cases (e.g. in JOIN::rollup_make_fields) a copy of
the original Item_func_group_concat object could be created by
using the Item_func_group_concat::Item_func_group_concat(THD
*thd, Item_func_group_concat *item) copy constructor. The
latter essentially creates a shallow copy of the source
object. Memory for the arguments array is allocated on
thd->mem_root, but the pointers for arguments and ORDER are
copied verbatim.

What happens in the test case is that when executing the query
for the first time, after a copy of the original
Item_func_group_concat object has been created by
JOIN::rollup_make_fields(), find_order_in_list() is called for
this new object. It then resolves ORDER BY by modifying the
ORDER objects so that they point to elements of the arguments
array which is local to the cloned object. When thd->mem_root
is freed upon completing the execution, pointers in the ORDER
objects become invalid. Those ORDER objects, however, are also
shared with the original Item_func_group_concat object which is
preserved between executions of a prepared statement. So the
first call to find_order_in_list() for the original object on
the second execution tries to dereference an invalid pointer.

The solution is to create copies of the ORDER objects when
copying Item_func_group_concat to not leave any stale pointers
in other instances with different lifecycles.



mysql-test/r/func_gconcat.result:
  Test case for bug #54476.
mysql-test/t/func_gconcat.test:
  Test case for bug #54476.
sql/item_sum.cc:
  Copy the ORDER objects pointed to by the elements of the 
  'order' array in the copy constructor of 
  Item_func_group_concat.
sql/table.h:
  Removed the unused 'item_copy' member of the ORDER class.

1 files changed, 15 insertions, 1 deletions

diff --git a/mysql-test/t/func_gconcat.test b/mysql-test/t/func_gconcat.test
index e832ea316eb..926c1f92855 100644
--- a/mysql-test/t/func_gconcat.test
+++ b/mysql-test/t/func_gconcat.test
@@ -708,6 +708,7 @@ SELECT 1 FROM
 
 DROP TABLE t1;
 
+--echo End of 5.0 tests
 
 --echo #
 --echo # Bug #52397: another crash with explain extended and group_concat
@@ -719,5 +720,18 @@ EXPLAIN EXTENDED SELECT 1 FROM
     t1 t2, t1 GROUP BY t1.a) AS d;
 DROP TABLE t1;
 
+--echo #
+--echo # Bug #54476: crash when group_concat and 'with rollup' in prepared statements
+--echo # 
 
---echo End of 5.0 tests
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1), (2);
+
+PREPARE stmt FROM "SELECT GROUP_CONCAT(t1.a ORDER BY t1.a) FROM t1 JOIN t1 t2 GROUP BY t1.a WITH ROLLUP";
+EXECUTE stmt;
+EXECUTE stmt;
+
+DEALLOCATE PREPARE stmt;
+DROP TABLE t1;
+
+--echo End of 5.1 tests