From owner-svn-src-head@freebsd.org Tue Jun 13 12:08:09 2017 Return-Path: Delivered-To: svn-src-head@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 416CFBF2B57; Tue, 13 Jun 2017 12:08:09 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id DAA3F794D5; Tue, 13 Jun 2017 12:08:08 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from tom.home (kib@localhost [127.0.0.1]) by kib.kiev.ua (8.15.2/8.15.2) with ESMTPS id v5DC83g9024642 (version=TLSv1.2 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NO); Tue, 13 Jun 2017 15:08:04 +0300 (EEST) (envelope-from kostikbel@gmail.com) DKIM-Filter: OpenDKIM Filter v2.10.3 kib.kiev.ua v5DC83g9024642 Received: (from kostik@localhost) by tom.home (8.15.2/8.15.2/Submit) id v5DC83NP024641; Tue, 13 Jun 2017 15:08:03 +0300 (EEST) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Tue, 13 Jun 2017 15:08:03 +0300 From: Konstantin Belousov To: Mark Johnston Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r319868 - in head/sys: kern sys Message-ID: <20170613120803.GY2088@kib.kiev.ua> References: <201706122014.v5CKEitK071089@repo.freebsd.org> <20170612202952.GU2088@kib.kiev.ua> <20170612204704.GA73695@wkstn-mjohnston.west.isilon.com> <20170612213300.GV2088@kib.kiev.ua> <20170613002052.GC73695@wkstn-mjohnston.west.isilon.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170613002052.GC73695@wkstn-mjohnston.west.isilon.com> User-Agent: Mutt/1.8.2 (2017-04-18) X-Spam-Status: No, score=-2.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_ADSP_CUSTOM_MED,FREEMAIL_FROM,NML_ADSP_CUSTOM_MED autolearn=no autolearn_force=no version=3.4.1 X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on tom.home X-BeenThere: svn-src-head@freebsd.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: SVN commit messages for the src tree for head/-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 13 Jun 2017 12:08:09 -0000 On Mon, Jun 12, 2017 at 05:20:52PM -0700, Mark Johnston wrote: > On Tue, Jun 13, 2017 at 12:33:00AM +0300, Konstantin Belousov wrote: > > On Mon, Jun 12, 2017 at 01:47:05PM -0700, Mark Johnston wrote: > > > On Mon, Jun 12, 2017 at 11:29:52PM +0300, Konstantin Belousov wrote: > > > > On Mon, Jun 12, 2017 at 08:14:44PM +0000, Mark Johnston wrote: > > > > > +int > > > > > +uuidcmp(const struct uuid *uuid1, const struct uuid *uuid2) > > > > > +{ > > > > > + > > > > > + return (memcmp(uuid1, uuid2, sizeof(struct uuid))); > > > > > +} > > > > > > > > This is unsafe. The function operation depends on the ABI properties > > > > that there is no padding either between members, or at the end of > > > > the structure. Why not use by-member comparision ? > > > > > > I interpreted the CTASSERT at the beginning of kern_uuid.c as a > > > guarantee that no such padding will be present. kern_uuid.c also defines > > > an alternate representation, struct uuid_private, and casts between the > > > two. > > > > I agree, your addition is consistent with the other code in kern_uuid.c, > > which already depends on these features. At least it contradicts > > to what compiler authors try to teach C language users. > > > > BTW, does uuid_private low/mid/hi union split of ll depend on endianess ? > > It seems so. The current usage is ok because the two representations > aren't mixed. That is, ll is only used to access the time fields of the > global uuid_last, and the low/mid/hi fields are never used with > uuid_last. It does look like a bit of a landmine though; I think it > would be worth adding a small hint: > > diff --git a/sys/kern/kern_uuid.c b/sys/kern/kern_uuid.c > index 0953d901a592..78088e1c6f32 100644 > --- a/sys/kern/kern_uuid.c > +++ b/sys/kern/kern_uuid.c > @@ -58,7 +58,7 @@ CTASSERT(sizeof(struct uuid) == 16); > /* We use an alternative, more convenient representation in the generator. */ > struct uuid_private { > union { > - uint64_t ll; /* internal. */ > + uint64_t ll; /* internal, for uuid_last only */ > struct { > uint32_t low; > uint16_t mid; Indeed helpful, IMO.