From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F37B8C4363D for ; Wed, 23 Sep 2020 14:51:18 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id A1A5521D43 for ; Wed, 23 Sep 2020 14:51:18 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A1A5521D43 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=zeniv.linux.org.uk Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 387246B0072; Wed, 23 Sep 2020 10:51:18 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 3110E6B0073; Wed, 23 Sep 2020 10:51:18 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 1D7D76B0074; Wed, 23 Sep 2020 10:51:18 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0075.hostedemail.com [216.40.44.75]) by kanga.kvack.org (Postfix) with ESMTP id 048916B0073 for ; Wed, 23 Sep 2020 10:51:18 -0400 (EDT) Received: from smtpin19.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 97FBB185FD673 for ; Wed, 23 Sep 2020 14:51:17 +0000 (UTC) X-FDA: 77294614194.19.roof04_59117e727157 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin19.hostedemail.com (Postfix) with ESMTP id AA3A13D7B74; Wed, 23 Sep 2020 14:49:27 +0000 (UTC) X-HE-Tag: roof04_59117e727157 X-Filterd-Recvd-Size: 3859 Received: from ZenIV.linux.org.uk (zeniv.linux.org.uk [195.92.253.2]) by imf23.hostedemail.com (Postfix) with ESMTP; Wed, 23 Sep 2020 14:49:26 +0000 (UTC) Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kL657-004bBq-D5; Wed, 23 Sep 2020 14:49:17 +0000 Date: Wed, 23 Sep 2020 15:49:17 +0100 From: Al Viro To: David Laight Cc: Christoph Hellwig , Andrew Morton , Jens Axboe , Arnd Bergmann , David Howells , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "linux-mips@vger.kernel.org" , "linux-parisc@vger.kernel.org" , "linuxppc-dev@lists.ozlabs.org" , "linux-s390@vger.kernel.org" , "sparclinux@vger.kernel.org" , "linux-block@vger.kernel.org" , "linux-scsi@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-aio@kvack.org" , "io-uring@vger.kernel.org" , "linux-arch@vger.kernel.org" , "linux-mm@kvack.org" , "netdev@vger.kernel.org" , "keyrings@vger.kernel.org" , "linux-security-module@vger.kernel.org" , Linus Torvalds Subject: Re: [PATCH 3/9] iov_iter: refactor rw_copy_check_uvector and import_iovec Message-ID: <20200923144917.GM3421308@ZenIV.linux.org.uk> References: <20200923060547.16903-1-hch@lst.de> <20200923060547.16903-4-hch@lst.de> <20200923141654.GJ3421308@ZenIV.linux.org.uk> <200cf2b9ce5e408f8838948fda7ce9a0@AcuMS.aculab.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200cf2b9ce5e408f8838948fda7ce9a0@AcuMS.aculab.com> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Sep 23, 2020 at 02:38:24PM +0000, David Laight wrote: > From: Al Viro > > Sent: 23 September 2020 15:17 > > > > On Wed, Sep 23, 2020 at 08:05:41AM +0200, Christoph Hellwig wrote: > > > > > +struct iovec *iovec_from_user(const struct iovec __user *uvec, > > > + unsigned long nr_segs, unsigned long fast_segs, > > > > Hmm... For fast_segs unsigned long had always been ridiculous > > (4G struct iovec on caller stack frame?), but that got me wondering about > > nr_segs and I wish I'd thought of that when introducing import_iovec(). > > > > The thing is, import_iovec() takes unsigned int there. Which is fine > > (hell, the maximal value that can be accepted in 1024), except that > > we do pass unsigned long syscall argument to it in some places. > > It will make diddly-squit difference. > The parameters end up in registers on most calling conventions. > Plausibly you get an extra 'REX' byte on x86 for the 64bit value. > What you want to avoid is explicit sign/zero extension and value > masking after arithmetic. Don't tell me what I want; your telepathic abilities are consistently sucky. I am *NOT* talking about microoptimization here. I have described the behaviour change of syscall caused by commit 5 years ago. Which is generally considered a problem. Then I asked whether that behaviour change would fall under the "if nobody noticed, it's not a userland ABI breakage" exception. Could you show me the point where I have expressed concerns about the quality of amd64 code generated for that thing, before or after the change in question?