diff --git a/mysys/mf_pack.c b/mysys/mf_pack.c
index d14c24f35d7..9e8b6516f63 100644
--- a/mysys/mf_pack.c
+++ b/mysys/mf_pack.c
@@ -271,7 +271,7 @@ 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
@@ -397,7 +397,7 @@ 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)
 {
diff --git a/sql/log.cc b/sql/log.cc
index 079384f18fe..3afc5f3a57d 100644
--- a/sql/log.cc
+++ b/sql/log.cc
@@ -1287,7 +1287,7 @@ 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);
diff --git a/sql/unireg.cc b/sql/unireg.cc
index 4c980552f9e..68e8219f043 100644
--- a/sql/unireg.cc
+++ b/sql/unireg.cc
@@ -166,6 +166,11 @@ bool mysql_create_frm(THD *thd, my_string file_name,
           create_info->comment.str : "", create_info->comment.length);
   forminfo[46]=(uchar) create_info->comment.length;
 #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
   if (my_pwrite(file,(byte*) fileinfo,64,0L,MYF_RW) ||