From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A3305C433E0 for ; Mon, 3 Aug 2020 08:27:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7B55E206E2 for ; Mon, 3 Aug 2020 08:27:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726118AbgHCI1a convert rfc822-to-8bit (ORCPT ); Mon, 3 Aug 2020 04:27:30 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:24997 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725867AbgHCI13 (ORCPT ); Mon, 3 Aug 2020 04:27:29 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-238-5RDLBwWzPy-44Oq5Oxj5QQ-1; Mon, 03 Aug 2020 09:27:26 +0100 X-MC-Unique: 5RDLBwWzPy-44Oq5Oxj5QQ-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Mon, 3 Aug 2020 09:27:25 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Mon, 3 Aug 2020 09:27:25 +0100 From: David Laight To: 'Mark Rutland' , "Madhavan T. Venkataraman" CC: Andy Lutomirski , Kernel Hardening , Linux API , linux-arm-kernel , Linux FS Devel , linux-integrity , LKML , "LSM List" , Oleg Nesterov , X86 ML Subject: RE: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor Thread-Topic: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor Thread-Index: AQHWZ2jYT+e4gDrzGEmP/30MMvDTCqkmD9qg Date: Mon, 3 Aug 2020 08:27:25 +0000 Message-ID: References: <20200728131050.24443-1-madvenka@linux.microsoft.com> <6540b4b7-3f70-adbf-c922-43886599713a@linux.microsoft.com> <46a1adef-65f0-bd5e-0b17-54856fb7e7ee@linux.microsoft.com> <20200731183146.GD67415@C02TD0UTHF1T.local> In-Reply-To: <20200731183146.GD67415@C02TD0UTHF1T.local> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Mark Rutland > Sent: 31 July 2020 19:32 ... > > It requires PC-relative data references. I have not worked on all architectures. > > So, I need to study this. But do all ISAs support PC-relative data references? > > Not all do, but pretty much any recent ISA will as it's a practical > necessity for fast position-independent code. i386 has neither PC-relative addressing nor moves from %pc. The cpu architecture knows that the sequence: call 1f 1: pop %reg is used to get the %pc value so is treated specially so that it doesn't 'trash' the return stack. So PIC code isn't too bad, but you have to use the correct sequence. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)