Bug#31752: check strmake() bounds

post-fixes: prevent semi-related overflow, additional comments

mysys/mf_pack.c

@@ -272,7 +272,7 @@ void symdirget(char *dir)
 
   SYNOPSIS
     unpack_dirname()
-    to			Store result here.  May be = from
+    to			result-buffer, FN_REFLEN characters. may be == from
     from		'Packed' directory name (may contain ~)
 
  IMPLEMENTATION
@@ -398,7 +398,7 @@ uint unpack_filename(my_string to, const char *from)
 
 	/* Convert filename (unix standard) to system standard */
 	/* Used before system command's like open(), create() .. */
-	/* Returns length of to */
+	/* Returns used length of to; total length should be FN_REFLEN */
 
 uint system_filename(my_string to, const char *from)
 {

sql/log.cc

@@ -963,7 +963,7 @@ err:
 void MYSQL_LOG::make_log_name(char* buf, const char* log_ident)
 {
   uint dir_len = dirname_length(log_file_name); 
-  if (dir_len > FN_REFLEN)
+  if (dir_len >= FN_REFLEN)
     dir_len=FN_REFLEN-1;
   strnmov(buf, log_file_name, dir_len);
   strmake(buf+dir_len, log_ident, FN_REFLEN - dir_len -1);

sql/sql_show.cc

@@ -136,7 +136,8 @@ int mysqld_show_tables(THD *thd,const char *db,const char *wild)
 {
   Item_string *field=new Item_string("",0,thd->charset());
   List<Item> field_list;
-  char path[FN_REFLEN],*end;
+  char path[FN_REFLEN],*end;                    // for unpack_dirname()
+
   List<char> files;
   char *file_name;
   Protocol *protocol= thd->protocol;
@@ -457,7 +458,7 @@ int mysqld_extend_show_tables(THD *thd,const char *db,const char *wild)
   Item *item;
   List<char> files;
   List<Item> field_list;
-  char path[FN_REFLEN];
+  char path[FN_REFLEN];                         // for unpack_dirname()
   char *file_name;
   TABLE *table;
   Protocol *protocol= thd->protocol;

sql/unireg.cc

@@ -141,6 +141,11 @@ bool mysql_create_frm(THD *thd, my_string file_name,
 	  60);
   forminfo[46]=(uchar) strlen((char*)forminfo+47);	// Length of comment
 #ifdef EXTRA_DEBUG
+  /*
+    EXTRA_DEBUG causes strmake() to initialize its buffer behind the
+    payload with a magic value to detect wrong buffer-sizes. We
+    explicitly zero that segment again.
+  */
   memset((char*) forminfo+47 + forminfo[46], 0, 61 - forminfo[46]);
 #endif