MDEV-39450: Memory corruption: overlapping memory ranges in Field_longstr::compress on UPDATE of compressed column

mysql-test/main/column_compression.result

@@ -3035,3 +3035,19 @@ select * from (select * from t) as t natural join (select * from t) as t1;
 a	b
 drop tables t, t4;
 # End of 10.5 tests
+#
+# MDEV-39450: Memory corruption: overlapping memory ranges in `Field_longstr::compress` on UPDATE of compressed column
+#
+CREATE TABLE t (c VARCHAR(255) COMPRESSED);
+REPLACE INTO t VALUES ('abcdefghijklm');
+UPDATE t SET c=RIGHT(c,10);
+DROP TABLE t;
+CREATE TABLE t (c VARCHAR(255) COMPRESSED);
+REPLACE INTO t VALUES ('abcdefghijklm');
+UPDATE t SET c=SUBSTRING(c,3);
+DROP TABLE t;
+CREATE TABLE t (c VARCHAR(255) COMPRESSED);
+REPLACE INTO t VALUES ('abcdefghijklm');
+UPDATE t SET c=MID(c,2,4);
+DROP TABLE t;
+# End of 10.6 tests

mysql-test/main/column_compression.test

@@ -573,3 +573,24 @@ select * from (select * from t) as t natural join (select * from t) as t1;
 drop tables t, t4;
 
 --echo # End of 10.5 tests
+
+--echo #
+--echo # MDEV-39450: Memory corruption: overlapping memory ranges in `Field_longstr::compress` on UPDATE of compressed column
+--echo #
+
+CREATE TABLE t (c VARCHAR(255) COMPRESSED);
+REPLACE INTO t VALUES ('abcdefghijklm');
+UPDATE t SET c=RIGHT(c,10);
+DROP TABLE t;
+
+CREATE TABLE t (c VARCHAR(255) COMPRESSED);
+REPLACE INTO t VALUES ('abcdefghijklm');
+UPDATE t SET c=SUBSTRING(c,3);
+DROP TABLE t;
+
+CREATE TABLE t (c VARCHAR(255) COMPRESSED);
+REPLACE INTO t VALUES ('abcdefghijklm');
+UPDATE t SET c=MID(c,2,4);
+DROP TABLE t;
+
+--echo # End of 10.6 tests

sql/field.cc

@@ -8543,7 +8543,7 @@ int Field_longstr::compress(char *to, uint to_length,
     /* Store uncompressed */
     to[0]= 0;
     if (buf_length < to_length)
-      memcpy(to + 1, buf, buf_length);
+      memmove(to + 1, buf, buf_length);
     else
     {
       /* Storing string at blob capacity, e.g. 255 bytes string to TINYBLOB. */