From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from brightrain.aerifal.cx (216-12-86-13.cv.mvl.ntelos.net [216.12.86.13]) by sourceware.org (Postfix) with ESMTPS id 0C95B385DC08 for ; Mon, 20 Apr 2020 01:29:06 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 0C95B385DC08 Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=libc.org Authentication-Results: sourceware.org; spf=none smtp.mailfrom=dalias@libc.org Date: Sun, 19 Apr 2020 21:29:04 -0400 From: Rich Felker To: Nicholas Piggin Cc: Nicholas Piggin via Libc-alpha , Szabolcs Nagy , libc-dev@lists.llvm.org, linuxppc-dev@lists.ozlabs.org, musl@lists.openwall.com Subject: Re: [musl] Powerpc Linux 'scv' system call ABI proposal take 2 Message-ID: <20200420012904.GY11469@brightrain.aerifal.cx> References: <1586931450.ub4c8cq8dj.astroid@bobo.none> <20200415225539.GL11469@brightrain.aerifal.cx> <1586994952.nnxigedbu2.astroid@bobo.none> <20200416095800.GC23945@port70.net> <1587341904.1r83vbudyf.astroid@bobo.none> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1587341904.1r83vbudyf.astroid@bobo.none> User-Agent: Mutt/1.5.21 (2010-09-15) X-Spam-Status: No, score=-5.8 required=5.0 tests=BAYES_00, KAM_DMARC_STATUS, KAM_LAZY_DOMAIN_SECURITY, KAM_NUMSUBJECT, KHOP_HELO_FCRDNS, RDNS_DYNAMIC, SPF_HELO_NONE, SPF_NONE, TXREP autolearn=no autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 20 Apr 2020 01:29:08 -0000 On Mon, Apr 20, 2020 at 10:27:58AM +1000, Nicholas Piggin wrote: > Excerpts from Szabolcs Nagy's message of April 16, 2020 7:58 pm: > > * Nicholas Piggin via Libc-alpha [2020-04-16 10:16:54 +1000]: > >> Well it would have to test HWCAP and patch in or branch to two > >> completely different sequences including register save/restores yes. > >> You could have the same asm and matching clobbers to put the sequence > >> inline and then you could patch the one sc/scv instruction I suppose. > > > > how would that 'patch' work? > > > > there are many reasons why you don't > > want libc to write its .text > > I guess I don't know what I'm talking about when it comes to libraries. > Shame if there is no good way to load-time patch libc. It's orthogonal > to the scv selection though -- if you don't patch you have to > conditional or indirect branch however you implement it. Patched pages cannot be shared. The whole design of PIC and shared libraries is that the code("text")/rodata is immutable and shared and that only a minimal amount of data, packed tightly together (the GOT) has to exist per-instance. Also, allowing patching of executable pages is generally frowned upon these days because W^X is a desirable hardening property. Rich