From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-1.mimecast.com (us-smtp-2.mimecast.com [205.139.110.61]) by sourceware.org (Postfix) with ESMTP id 497543870888 for ; Mon, 18 May 2020 12:36:08 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 497543870888 Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-38-m6aw_dwwPlutsFfjBBhgUw-1; Mon, 18 May 2020 08:36:06 -0400 X-MC-Unique: m6aw_dwwPlutsFfjBBhgUw-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 30A3DEC1A0; Mon, 18 May 2020 12:36:05 +0000 (UTC) Received: from oldenburg2.str.redhat.com (ovpn-112-142.ams2.redhat.com [10.36.112.142]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7C5B25D9DD; Mon, 18 May 2020 12:36:04 +0000 (UTC) From: Florian Weimer To: "H.J. Lu" Cc: "H.J. Lu via Libc-alpha" Subject: Re: V2 [PATCH] x86: Move CET control to _dl_x86_feature_control [BZ #25887] References: <20200428215243.236312-1-hjl.tools@gmail.com> <20200428215243.236312-2-hjl.tools@gmail.com> <871rnjoknd.fsf@mid.deneb.enyo.de> <87mu65g16k.fsf@oldenburg2.str.redhat.com> Date: Mon, 18 May 2020 14:36:03 +0200 In-Reply-To: (H. J. Lu's message of "Mon, 18 May 2020 05:26:36 -0700") Message-ID: <87367xe7yk.fsf@oldenburg2.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain X-Spam-Status: No, score=-8.8 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H2, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham 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, 18 May 2020 12:36:09 -0000 * H. J. Lu: >> It's still not clear to me why CET control variables have to be in >> _rtld_global. Regular global variables would likely lead to clearer >> code, I think. _rtld_global is needed for read-write data that is >> shared with libc/libdl, and this does not seem to apply to CET control >> settings. > > We don't have a pure local struct for rtld. Yes, but we could use global variables. > _rtld_local is a hidden alias of _rtld_global: > > extern struct rtld_global _rtld_local > __attribute__ ((alias ("_rtld_global"), visibility ("hidden"))); > > struct rtld_global is used to improve IP-relative access. Are you sure? I think it's mainly for sharing data with libc and libdl through a single symbol. Data that's not shared should go into a regular global variable, I think. > It may not be important for x86-64. But it is still useful for i386 > which doesn't have IP-relative addressing. I don't think we use an alternative way to access _rtld_global or _rtld_global_ro on any target. Access always happens through a (hidden) variable. The patch looks okay to me now. Thanks! Florian