Bug #54461: crash with longblob and union or update with subquery

Queries may crash, if
  1) the GREATEST or the LEAST function has a mixed list of
     numeric and LONGBLOB arguments and
  2) the result of such a function goes through an intermediate
     temporary table.

An Item that references a LONGBLOB field has max_length of
UINT_MAX32 == (2^32 - 1).

The current implementation of GREATEST/LEAST returns REAL
result for a mixed list of numeric and string arguments (that
contradicts with the current documentation, this contradiction
was discussed and it was decided to update the documentation).

The max_length of such a function call was calculated as a
maximum of argument max_length values (i.e. UINT_MAX32).

That max_length value of UINT_MAX32 was used as a length for
the intermediate temporary table Field_double to hold
GREATEST/LEAST function result.

The Field_double::val_str() method call on that field
allocates a String value.

Since an allocation of String reserves an additional byte
for a zero-termination, the size of String buffer was
set to (UINT_MAX32 + 1), that caused an integer overflow:
actually, an empty buffer of size 0 was allocated.

An initialization of the "first" byte of that zero-size
buffer with '\0' caused a crash.

The Item_func_min_max::fix_length_and_dec() has been
modified to calculate max_length for the REAL result like
we do it for arithmetical operators.


******
Bug #54461: crash with longblob and union or update with subquery

Queries may crash, if
  1) the GREATEST or the LEAST function has a mixed list of
     numeric and LONGBLOB arguments and
  2) the result of such a function goes through an intermediate
     temporary table.

An Item that references a LONGBLOB field has max_length of
UINT_MAX32 == (2^32 - 1).

The current implementation of GREATEST/LEAST returns REAL
result for a mixed list of numeric and string arguments (that
contradicts with the current documentation, this contradiction
was discussed and it was decided to update the documentation).

The max_length of such a function call was calculated as a
maximum of argument max_length values (i.e. UINT_MAX32).

That max_length value of UINT_MAX32 was used as a length for
the intermediate temporary table Field_double to hold
GREATEST/LEAST function result.

The Field_double::val_str() method call on that field
allocates a String value.

Since an allocation of String reserves an additional byte
for a zero-termination, the size of String buffer was
set to (UINT_MAX32 + 1), that caused an integer overflow:
actually, an empty buffer of size 0 was allocated.

An initialization of the "first" byte of that zero-size
buffer with '\0' caused a crash.

The Item_func_min_max::fix_length_and_dec() has been
modified to calculate max_length for the REAL result like
we do it for arithmetical operators.

3 files changed, 29 insertions, 0 deletions

diff --git a/mysql-test/r/func_misc.result b/mysql-test/r/func_misc.result
index 81dddd0f648..eee56ae7461 100644
--- a/mysql-test/r/func_misc.result
+++ b/mysql-test/r/func_misc.result
@@ -336,4 +336,19 @@ End of 5.0 tests
 select connection_id() > 0;
 connection_id() > 0
 1
+#
+# Bug #54461: crash with longblob and union or update with subquery
+#
+CREATE TABLE t1 (a INT, b LONGBLOB);
+INSERT INTO t1 VALUES (1, '2'), (2, '3'), (3, '2');
+SELECT DISTINCT LEAST(a, (SELECT b FROM t1 LIMIT 1)) FROM t1 UNION SELECT 1;
+LEAST(a, (SELECT b FROM t1 LIMIT 1))
+1
+2
+SELECT DISTINCT GREATEST(a, (SELECT b FROM t1 LIMIT 1)) FROM t1 UNION SELECT 1;
+GREATEST(a, (SELECT b FROM t1 LIMIT 1))
+2
+3
+1
+DROP TABLE t1;
 End of tests
diff --git a/mysql-test/t/func_misc.test b/mysql-test/t/func_misc.test
index 6590b43f2dc..c6b5ffd5a3f 100644
--- a/mysql-test/t/func_misc.test
+++ b/mysql-test/t/func_misc.test
@@ -467,4 +467,16 @@ select NAME_CONST('_id',1234) as id;
 
 select connection_id() > 0;
 
+--echo #
+--echo # Bug #54461: crash with longblob and union or update with subquery
+--echo #
+
+CREATE TABLE t1 (a INT, b LONGBLOB);
+INSERT INTO t1 VALUES (1, '2'), (2, '3'), (3, '2');
+
+SELECT DISTINCT LEAST(a, (SELECT b FROM t1 LIMIT 1)) FROM t1 UNION SELECT 1;
+SELECT DISTINCT GREATEST(a, (SELECT b FROM t1 LIMIT 1)) FROM t1 UNION SELECT 1;
+
+DROP TABLE t1;
+
 --echo End of tests
diff --git a/sql/item_func.cc b/sql/item_func.cc
index 1bec4700bff..1b13297c951 100644
--- a/sql/item_func.cc
+++ b/sql/item_func.cc
@@ -2243,6 +2243,8 @@ void Item_func_min_max::fix_length_and_dec()
     max_length= my_decimal_precision_to_length_no_truncation(max_int_part +
                                                              decimals, decimals,
                                                              unsigned_flag);
+  else if (cmp_type == REAL_RESULT)
+    max_length= float_length(decimals);
   cached_field_type= agg_field_type(args, arg_count);
 }