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=-3.3 required=5.0 tests=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 f24502b0 for ; Sun, 19 Jan 2020 15:54:04 +0000 (UTC) Received: (qmail 28354 invoked by uid 550); 19 Jan 2020 15:54:02 -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 28336 invoked from network); 19 Jan 2020 15:54:02 -0000 To: musl@lists.openwall.com References: <20200119110743.GD2020@voyager> <20200119113134.GJ23985@port70.net> <8299f261-7870-57a6-37cf-d4ce482ad81e@openwall.com> <20200119142401.GG2020@voyager> From: Alexander Cherepanov Message-ID: Date: Sun, 19 Jan 2020 18:53:49 +0300 MIME-Version: 1.0 In-Reply-To: <20200119142401.GG2020@voyager> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Subject: Re: [musl] Minor style patch to exit.c 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. -- Alexander Cherepanov