From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751564AbcFWPWd (ORCPT ); Thu, 23 Jun 2016 11:22:33 -0400 Received: from mail-pa0-f68.google.com ([209.85.220.68]:33418 "EHLO mail-pa0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750866AbcFWPWb (ORCPT ); Thu, 23 Jun 2016 11:22:31 -0400 Subject: Re: [PATCH v6 3/6] crypto: AF_ALG -- add asymmetric cipher interface To: Stephan Mueller , Mat Martineau References: <20160515041645.15888.94903.stgit@tstruk-mobl1> <3250613.UAo0YkFYZb@tauon.atsec.com> Cc: Tadeusz Struk , dhowells@redhat.com, herbert@gondor.apana.org.au, linux-api@vger.kernel.org, marcel@holtmann.org, linux-kernel@vger.kernel.org, keyrings@vger.kernel.org, linux-crypto@vger.kernel.org, dwmw2@infradead.org, davem@davemloft.net From: Denis Kenzior Message-ID: <576BFEB3.5080009@gmail.com> Date: Thu, 23 Jun 2016 10:22:27 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.0 MIME-Version: 1.0 In-Reply-To: <3250613.UAo0YkFYZb@tauon.atsec.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Stephan, >> >> This brings me to another proposal for read buffer sizing: AF_ALG akcipher >> can guarantee that partial reads (where the read buffer is shorter than >> the output of the crypto op) will work using the same semantics as >> SOCK_DGRAM/SOCK_SEQPACKET. With those sockets, as much data as will fit is >> copied in to the read buffer and the remainder is discarded. >> >> I realize there's a performance and memory tradeoff, since the crypto >> algorithm needs a sufficiently large output buffer that would have to be >> created by AF_ALG akcipher. The user could manage that tradeoff by >> providing a larger buffer (typically key_size?) if it wants to avoid >> allocating and copying intermediate buffers inside the kernel. > > How shall the user know that something got truncated or that the kernel > created memory? > To the former point, recall the signature of recv: ssize_t recv(int sockfd, void *buf, size_t len, int flags); Traditionally, userspace apps can know that the buffer provided to recv was too small in two ways: The return value from recv / recvmsg was >= len. In the case of recvmsg, the MSG_TRUNC flag is set. To quote man recv: "All three calls return the length of the message on successful comple‐ tion. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the message is received from." and "MSG_TRUNC (since Linux 2.2) For raw (AF_PACKET), Internet datagram (since Linux 2.4.27/2.6.8), netlink (since Linux 2.6.22), and UNIX datagram (since Linux 3.4) sockets: return the real length of the packet or datagram, even when it was longer than the passed buffer. " Regards, -Denis