From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by sourceware.org (Postfix) with ESMTPS id E09C63858D28 for ; Wed, 15 Dec 2021 22:40:13 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org E09C63858D28 Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-161-8B5qTGYrPPS5t9Ggnl5o9A-1; Wed, 15 Dec 2021 17:40:12 -0500 X-MC-Unique: 8B5qTGYrPPS5t9Ggnl5o9A-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 3235981CCB4; Wed, 15 Dec 2021 22:40:11 +0000 (UTC) Received: from oldenburg.str.redhat.com (unknown [10.2.17.223]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 599025F91F; Wed, 15 Dec 2021 22:40:10 +0000 (UTC) From: Florian Weimer To: Mark Wielaard Cc: elfutils-devel@sourceware.org Subject: Re: [PATCH] libelf: Use offsetof to get field of unaligned References: <20211215220544.625735-1-mark@klomp.org> Date: Wed, 15 Dec 2021 23:40:08 +0100 In-Reply-To: <20211215220544.625735-1-mark@klomp.org> (Mark Wielaard's message of "Wed, 15 Dec 2021 23:05:44 +0100") Message-ID: <87wnk5o5kn.fsf@oldenburg.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (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=-6.5 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, RCVD_IN_DNSWL_LOW, RCVD_IN_MSPIKE_H3, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_NONE, TXREP autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: elfutils-devel@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Elfutils-devel mailing list List-Unsubscribe: , List-Archive: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 15 Dec 2021 22:40:15 -0000 * Mark Wielaard: > This seems a wrong warning since we aren't accessing the field member > of the struct, but are taking the address of it. But we can do the > same by adding the field offsetof to the base address. Which doesn't > trigger a runtime error. I think the warning is correct. I believe it is motivated by the GCC optimizers using this to infer alignment of the original pointer. It won't make a difference for this expression, but it can cause crashes elsewhere with strict-alignment targets. Thanks, Florian