3469 dbuf_read_impl shows too much enthusiasm
Reviewed by: Bryan Cantrill <bryan@joyent.com>

*** 20,29 ****
--- 20,30 ----
   */
  /*
   * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
   * Copyright (c) 2012 by Delphix. All rights reserved.
+  * Copyright (c) 2013, Joyent, Inc. All rights reserved.
   */
  
  #include <sys/zfs_context.h>
  #include <sys/dmu.h>
  #include <sys/dmu_impl.h>
*** 530,541 ****
                  ASSERT3U(bonuslen, <=, db->db.db_size);
                  db->db.db_data = zio_buf_alloc(DN_MAX_BONUSLEN);
                  arc_space_consume(DN_MAX_BONUSLEN, ARC_SPACE_OTHER);
                  if (bonuslen < DN_MAX_BONUSLEN)
                          bzero(db->db.db_data, DN_MAX_BONUSLEN);
!                 if (bonuslen)
!                         bcopy(DN_BONUS(dn->dn_phys), db->db.db_data, bonuslen);
                  DB_DNODE_EXIT(db);
                  dbuf_update_data(db);
                  db->db_state = DB_CACHED;
                  mutex_exit(&db->db_mtx);
                  return;
--- 531,553 ----
                  ASSERT3U(bonuslen, <=, db->db.db_size);
                  db->db.db_data = zio_buf_alloc(DN_MAX_BONUSLEN);
                  arc_space_consume(DN_MAX_BONUSLEN, ARC_SPACE_OTHER);
                  if (bonuslen < DN_MAX_BONUSLEN)
                          bzero(db->db.db_data, DN_MAX_BONUSLEN);
! 
!                 if (bonuslen) {
!                         /*
!                          * Absent byzantine on-disk corruption, we fully expect
!                          * our bonuslen to be no more than DN_MAX_BONUSLEN --
!                          * but we nonetheless explicitly clamp it on the bcopy()
!                          * to prevent any on-disk corruption from becoming
!                          * rampant in-kernel corruption.
!                          */
!                         bcopy(DN_BONUS(dn->dn_phys), db->db.db_data,
!                             MIN(bonuslen, DN_MAX_BONUSLEN));
!                 }
! 
                  DB_DNODE_EXIT(db);
                  dbuf_update_data(db);
                  db->db_state = DB_CACHED;
                  mutex_exit(&db->db_mtx);
                  return;