MDEV-18195 ASAN use-after-poison in my_strcasecmp_utf8 / Item::eq upon prepared statement with ORDER BY NAME_CONST

ASAN noticed a freed memory access during EXECUTE in this script:
  PREPARE stmt FROM "SELECT 'x' ORDER BY NAME_CONST( 'f', 'foo' )";
  EXECUTE stmt;

In case of a PREPARE statement, all Items, including Item_name_const,
are created on Prepared_statement::main_mem_root.
Item_name_const::fix_fields() did not take this into account
and could allocate the value of Item::name on a wrong memory root,
in this code:

  if (is_autogenerated_name)
  {
    set_name(thd, item_name->c_ptr(), (uint) item_name->length(),
             system_charset_info);
  }

When fix_fields() is called in the reported SQL script, THD's arena already
points to THD::main_mem_root rather than to Prepared_statement::main_mem_root,
so Item::name was allocated on THD::main_mem_root.
Then, at the end of the dispatch_command() for the PREPARE statement,
THD::main_mem_root got cleared. So during EXECUTE, Item::name
pointed to an already freed memory.

This patch changes the code to set the implicit name for Item_name_const
at the constructor time rather than at fix_fields time. This guarantees
that Item_name_const and its Item::name always reside on the same memory root.

Note, this change makes the code for Item_name_const symmetric with other
constant-alike items that set their default implicit names at the constructor
call time rather than at fix_fields() time:
- Item_string
- Item_int
- Item_real
- Item_decimal
- Item_null
- Item_param

1 files changed, 6 insertions, 12 deletions

diff --git a/sql/item.cc b/sql/item.cc
index 1a2c4eba5fb..d8f27bdb30e 100644
--- a/sql/item.cc
+++ b/sql/item.cc
@@ -1672,10 +1672,14 @@ bool Item_name_const::is_null()
 Item_name_const::Item_name_const(THD *thd, Item *name_arg, Item *val):
   Item(thd), value_item(val), name_item(name_arg)
 {
+  StringBuffer<128> name_buffer;
+  String *name_str;
   Item::maybe_null= TRUE;
   valid_args= true;
-  if (!name_item->basic_const_item())
+  if (!name_item->basic_const_item() ||
+      !(name_str= name_item->val_str(&name_buffer))) // Can't have a NULL name
     goto err;
+  set_name(name_str->ptr(), name_str->length(), name_str->charset());
 
   if (value_item->basic_const_item())
     return; // ok
@@ -1737,24 +1741,14 @@ Item::Type Item_name_const::type() const
 
 bool Item_name_const::fix_fields(THD *thd, Item **ref)
 {
-  char buf[128];
-  String *item_name;
-  String s(buf, sizeof(buf), &my_charset_bin);
-  s.length(0);
-
   if (value_item->fix_fields(thd, &value_item) ||
       name_item->fix_fields(thd, &name_item) ||
       !value_item->const_item() ||
-      !name_item->const_item() ||
-      !(item_name= name_item->val_str(&s))) // Can't have a NULL name 
+      !name_item->const_item())
   {
     my_error(ER_RESERVED_SYNTAX, MYF(0), "NAME_CONST");
     return TRUE;
   }
-  if (is_autogenerated_name)
-  {
-    set_name(item_name->ptr(), (uint) item_name->length(), system_charset_info);
-  }
   if (value_item->collation.derivation == DERIVATION_NUMERIC)
     collation.set_numeric();
   else