diff --git a/mysys/mf_pack.c b/mysys/mf_pack.c
index 049aa59a578..485b2c5ec3d 100644
--- a/mysys/mf_pack.c
+++ b/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)
 {
diff --git a/sql/log.cc b/sql/log.cc
index b91ec2b3dee..4e0964fe9d6 100644
--- a/sql/log.cc
+++ b/sql/log.cc
@@ -963,10 +963,10 @@ 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);
+  strmake(buf+dir_len, log_ident, FN_REFLEN - dir_len -1);
 }
 
 
diff --git a/sql/repl_failsafe.cc b/sql/repl_failsafe.cc
index 4c8703226a6..4ea90346638 100644
--- a/sql/repl_failsafe.cc
+++ b/sql/repl_failsafe.cc
@@ -926,7 +926,7 @@ int load_master_data(THD* thd)
 			     0, (SLAVE_IO | SLAVE_SQL)))
           send_error(thd, ER_MASTER_INFO);
 	strmake(active_mi->master_log_name, row[0],
-		sizeof(active_mi->master_log_name));
+		sizeof(active_mi->master_log_name) -1);
 	active_mi->master_log_pos= my_strtoll10(row[1], (char**) 0, &error);
         /* at least in recent versions, the condition below should be false */
 	if (active_mi->master_log_pos < BIN_LOG_HEADER_SIZE)
diff --git a/sql/sql_show.cc b/sql/sql_show.cc
index bf0e254d3e4..741d30e6a99 100644
--- a/sql/sql_show.cc
+++ b/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_LEN],*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_LEN];
+  char path[FN_REFLEN];                         // for unpack_dirname()
   char *file_name;
   TABLE *table;
   Protocol *protocol= thd->protocol;
diff --git a/sql/unireg.cc b/sql/unireg.cc
index e5ee0222f20..dcb49bc1766 100644
--- a/sql/unireg.cc
+++ b/sql/unireg.cc
@@ -140,6 +140,14 @@ bool mysql_create_frm(THD *thd, my_string file_name,
   strmake((char*) forminfo+47,create_info->comment ? create_info->comment : "",
 	  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
 
   if (my_pwrite(file,(byte*) fileinfo,64,0L,MYF_RW) ||
       my_pwrite(file,(byte*) keybuff,key_info_length,
diff --git a/strings/strmake.c b/strings/strmake.c
index d2252f648f6..47d8a04e361 100644
--- a/strings/strmake.c
+++ b/strings/strmake.c
@@ -28,23 +28,25 @@
 #include <my_global.h>
 #include "m_string.h"
 
-#ifdef BAD_STRING_COMPILER
-
-char *strmake(char *dst,const char *src,uint length)
-{
-  reg1 char *res;
-
-  if ((res=memccpy(dst,src,0,length)))
-    return res-1;
-  dst[length]=0;
-  return dst+length;
-}
-
-#define strmake strmake_overlapp	/* Use orginal for overlapping str */
-#endif
-
 char *strmake(register char *dst, register const char *src, uint length)
 {
+#ifdef EXTRA_DEBUG
+  /*
+    'length' is the maximum length of the string; the buffer needs
+    to be one character larger to accomodate the terminating '\0'.
+    This is easy to get wrong, so we make sure we write to the
+    entire length of the buffer to identify incorrect buffer-sizes.
+    We only initialise the "unused" part of the buffer here, a) for
+    efficiency, and b) because dst==src is allowed, so initialising
+    the entire buffer would overwrite the source-string. Also, we
+    write a character rather than '\0' as this makes spotting these
+    problems in the results easier.
+  */
+  uint n= strlen(src) + 1;
+  if (n <= length)
+    memset(dst + n, (int) 'Z', length - n + 1);
+#endif
+
   while (length--)
     if (! (*dst++ = *src++))
       return dst-1;