From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-3.0 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H3, RCVD_IN_MSPIKE_WL autolearn=ham autolearn_force=no version=3.4.2 Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by inbox.vuxu.org (OpenSMTPD) with SMTP id b8b0f6aa for ; Sun, 19 Jan 2020 16:22:47 +0000 (UTC) Received: (qmail 7436 invoked by uid 550); 19 Jan 2020 16:22:45 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 7415 invoked from network); 19 Jan 2020 16:22:44 -0000 Date: Sun, 19 Jan 2020 11:22:33 -0500 From: Rich Felker To: musl@lists.openwall.com Message-ID: <20200119162233.GD30412@brightrain.aerifal.cx> References: <20200119110743.GD2020@voyager> <20200119113134.GJ23985@port70.net> <8299f261-7870-57a6-37cf-d4ce482ad81e@openwall.com> <20200119142401.GG2020@voyager> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: Rich Felker Subject: Re: [musl] Minor style patch to exit.c On Sun, Jan 19, 2020 at 06:53:49PM +0300, Alexander Cherepanov wrote: > On 19/01/2020 17.24, Markus Wichmann wrote: > >On Sun, Jan 19, 2020 at 04:33:47PM +0300, Alexander Cherepanov wrote: > >>Couldn't _start defined as an array? Then separate values could be accessed > >>simply as elements of this array. And casts to integers could be limited to > >>calculating the number of elements, the terminating value or something. > > > >That reminds me of something I read in the C standard: Two pointers must > >compare equal if, among other possibilities, one is a pointer to > >one-past its underlying array, and the other is a pointer to the start > >of its array, and the arrays happen to lie behind one another in address > >space. > > One[1] of the gcc bug reports I mentioned is exactly about this > issue. DR 260[2] allows to take the provenance of the pointers into > account when comparing them and gcc really does this. > > [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61502 > [2] http://open-std.org/jtc1/sc22/wg14/www/docs/dr_260.htm > > As a side note, I thinks this is the wildest gcc bug report, it > contains really mind-blowing comments (like comment 3). I don't mean > it in a bad way at all and if you want to turn your understanding of > C language inside-out you can try to read it. OTOH I think it's all > wrong after all and I have some hope for it to be settled after my > recent comments there. But I don't hold my breath. > > >Therefore, if _start and _end were arrays, even the GCC devs must agree > >that there might be an integer i such that _start + i == _end. For the C > >language, _start and _end would be arrays that happen to lie adjacent in > >address space. > > > >And if we have guarantees from the outside attesting to that, then > >_end - _start is no longer an undefined expression, right? > > Even if we know that _start + k == _end it doesn't mean that we > allowed to subtract them. Consider a function that takes a pointer p, an array a, and a length l, and does: for (i=0; i