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=-3.8 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 38040C43464 for ; Sat, 19 Sep 2020 13:50:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ED678208B8 for ; Sat, 19 Sep 2020 13:50:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726468AbgISNuq convert rfc822-to-8bit (ORCPT ); Sat, 19 Sep 2020 09:50:46 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:49513 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726159AbgISNuq (ORCPT ); Sat, 19 Sep 2020 09:50:46 -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-217-sBKr2MWsPJyQWXL4zt-j_w-1; Sat, 19 Sep 2020 14:50:42 +0100 X-MC-Unique: sBKr2MWsPJyQWXL4zt-j_w-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; Sat, 19 Sep 2020 14:50:41 +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; Sat, 19 Sep 2020 14:50:41 +0100 From: David Laight To: 'Samuel Holland' , Benjamin Herrenschmidt , Paul Mackerras , "Michael Ellerman" , "linuxppc-dev@lists.ozlabs.org" CC: "linux-kernel@vger.kernel.org" Subject: RE: [PATCH] powerpc: Select HAVE_FUTEX_CMPXCHG Thread-Topic: [PATCH] powerpc: Select HAVE_FUTEX_CMPXCHG Thread-Index: AQHWjjP8ICmOrEPCtEqSzubXo06uGalv9oPA Date: Sat, 19 Sep 2020 13:50:41 +0000 Message-ID: <7c3e299f9c134eab9ba1e6d07a77f7b6@AcuMS.aculab.com> References: <20200919032009.8346-1-samuel@sholland.org> In-Reply-To: <20200919032009.8346-1-samuel@sholland.org> Accept-Language: en-GB, 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 Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Samuel Holland > Sent: 19 September 2020 04:20 > > On powerpc, access_ok() succeeds for the NULL pointer. This breaks the > dynamic check in futex_detect_cmpxchg(), which expects -EFAULT. As a > result, robust futex operations are not functional on powerpc. access_ok(NULL, sane_count) will succeed on all (maybe most) architectures. All access_ok() does is check that kernel addresses aren't referenced. (access_ok(kernel_adress, 0) is also likely to succeed.) It is the access to user-address 0 that is expected to fault. If this isn't faulting something else is wrong. Historically (at least pre-elf, if not before) user programs were linked to address zero - so the page was mapped. (Linux may be too new to actually require it.) Not sure what 'wine' requires for win-32 execuatbles. ISTR there are also some 'crazy' ARM? cpu that read the interrupt vectors from address 0 in user-space. So assuming: static void __init futex_detect_cmpxchg(void) { #ifndef CONFIG_HAVE_FUTEX_CMPXCHG u32 curval; /* * This will fail and we want it. Some arch implementations do * runtime detection of the futex_atomic_cmpxchg_inatomic() * functionality. We want to know that before we call in any * of the complex code paths. Also we want to prevent * registration of robust lists in that case. NULL is * guaranteed to fault and we get -EFAULT on functional * implementation, the non-functional ones will return * -ENOSYS. */ if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT) futex_cmpxchg_enabled = 1; #endif } will fail -EFAULT because user address 0 is invalid seems hopeful. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)