From: David Miller Subject: Re: [PATCH net-next 2/2] xfrm: Fix unaligned access in xfrm_notify_sa() for DELSA Date: Wed, 21 Oct 2015 06:22:13 -0700 (PDT) Message-ID: <20151021.062213.273447452472704627.davem@davemloft.net> References: <20151021065704.GM7701@secunet.com> <20151021105442.GM6948@oracle.com> <20151021123628.GP6948@oracle.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: steffen.klassert@secunet.com, linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, herbert@gondor.apana.org.au, dhowells@redhat.com, zohar@linux.vnet.ibm.com, David.Woodhouse@intel.com To: sowmini.varadhan@oracle.com Return-path: Received: from shards.monkeyblade.net ([149.20.54.216]:43817 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753329AbbJUNFy (ORCPT ); Wed, 21 Oct 2015 09:05:54 -0400 In-Reply-To: <20151021123628.GP6948@oracle.com> Sender: linux-crypto-owner@vger.kernel.org List-ID: From: Sowmini Varadhan Date: Wed, 21 Oct 2015 08:36:28 -0400 > + memcpy((u8 *)p, &tmp, sizeof(tmp)); memcpy() _never_ works for avoiding unaligned accessed. I repeat, no matter what you do, no matter what kinds of casts or fancy typing you use, memcpy() _never_ works for this purpose. The compiler knows that the pointer you are using is supposed to be at least 8 byte aligned, it can look through the cast and that's completely legitimate for it to do. So it can legitimately inline emit loads and stores to implement the memcpy() and those will still get the unaligned accesses. There is one and only one portable way to access unaligned data, and that is with the get_unaligned() and put_unaligned() helpers. Userland must do something similar to deal with this situation as well.