MDEV-11752 Unsafe strmov - function definition in include/m_string.h

assert that strmov() cannot be used on overlapping strings.
(because strpcpy cannot)

dbug/dbug.c

@@ -1883,7 +1883,7 @@ static void DBUGOpenFile(CODE_STATE *cs,
       cs->stack->name[len]=0;
     }
     else
-    strmov(cs->stack->name,name);
+      strmov(cs->stack->name,name);
     name=cs->stack->name;
     if (strcmp(name, "-") == 0)
     {

include/m_string.h

@@ -73,11 +73,13 @@ extern void *(*my_str_malloc)(size_t);
 extern void *(*my_str_realloc)(void *, size_t);
 extern void (*my_str_free)(void *);
 
+#ifdef DBUG_OFF
 #if defined(HAVE_STPCPY) && MY_GNUC_PREREQ(3, 4) && !defined(__INTEL_COMPILER)
 #define strmov(A,B) __builtin_stpcpy((A),(B))
 #elif defined(HAVE_STPCPY)
 #define strmov(A,B) stpcpy((A),(B))
 #endif
+#endif
 
 /* Declared in int2str() */
 extern const char _dig_vec_upper[];

strings/strmov.c

@@ -40,6 +40,7 @@
 
 char *strmov(register char *dst, register const char *src)
 {
+  DBUG_ASSERT(src + strlen(src) < dst || dst + strlen(src) < src);
   while ((*dst++ = *src++)) ;
   return dst-1;
 }