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, URIBL_BLOCKED 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 5B04AC433E0 for ; Mon, 4 Jan 2021 22:39:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 287E022525 for ; Mon, 4 Jan 2021 22:39:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726923AbhADWji convert rfc822-to-8bit (ORCPT ); Mon, 4 Jan 2021 17:39:38 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:42417 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726525AbhADWji (ORCPT ); Mon, 4 Jan 2021 17:39:38 -0500 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-281-CM56XzXDPwKrUvpunGmZUQ-1; Mon, 04 Jan 2021 22:34:48 +0000 X-MC-Unique: CM56XzXDPwKrUvpunGmZUQ-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, 4 Jan 2021 22:34:48 +0000 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, 4 Jan 2021 22:34:48 +0000 From: David Laight To: "'Eric W. Biederman'" , Al Viro CC: Christoph Hellwig , "linux-kernel@vger.kernel.org" , X86 ML Subject: RE: in_compat_syscall() on x86 Thread-Topic: in_compat_syscall() on x86 Thread-Index: AQHW4tnp52+Fp/HYSH2MXxqk00t/OqoYCInQ Date: Mon, 4 Jan 2021 22:34:48 +0000 Message-ID: References: <20210104165827.GJ3579531@ZenIV.linux.org.uk> <87sg7gfnaa.fsf@x220.int.ebiederm.org> In-Reply-To: <87sg7gfnaa.fsf@x220.int.ebiederm.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-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Eric W. Biederman > Sent: 04 January 2021 20:41 > > Al Viro writes: > > > On Mon, Jan 04, 2021 at 12:16:56PM +0000, David Laight wrote: > >> On x86 in_compat_syscall() is defined as: > >> in_ia32_syscall() || in_x32_syscall() > >> > >> Now in_ia32_syscall() is a simple check of the TS_COMPAT flag. > >> However in_x32_syscall() is a horrid beast that has to indirect > >> through to the original %eax value (ie the syscall number) and > >> check for a bit there. > >> > >> So on a kernel with x32 support (probably most distro kernels) > >> the in_compat_syscall() check is rather more expensive than > >> one might expect. > > I suggest you check the distro kernels. I suspect they don't compile in > support for x32. As far as I can tell x32 is an undead beast of a > subarchitecture that just enough people use that it can't be removed, > but few enough people use it likely has a few lurking scary bugs. It is defined in the Ubuntu kernel configs I've got lurking: Both 3.8.0-19_generic (Ubuntu 13.04) and 5.4.0-56_generic (probably 20.04). Which is probably why it is in my test builds (I've just cut out a lot of modules). > >> It would be muck better if both checks could be done together. > >> I think this would require the syscall entry code to set a > >> value in both the 64bit and x32 entry paths. > >> (Can a process make both 64bit and x32 system calls?) > > > > Yes, it bloody well can. > > > > And I see no benefit in pushing that logics into syscall entry, > > since anything that calls in_compat_syscall() more than once > > per syscall execution is doing the wrong thing. Moreover, > > in quite a few cases we don't call the sucker at all, and for > > all of those pushing that crap into syscall entry logics is > > pure loss. > > The x32 system calls have their own system call table and it would be > trivial to set a flag like TS_COMPAT when looking up a system call from > that table. I expect such a change would be purely in the noise. Certainly a write of 0/1/2 into a dirtied cache line of 'current' could easily cost absolutely nothing. Especially if current has already been read. I also wondered about resetting it to zero when an x32 system call exits (rather than entry to a 64bit one). For ia32 the flag is set (with |=) on every syscall entry. Even though I'm pretty sure it can only change during exec. > > What's the point, really? > > Before we came up with the current games with __copy_siginfo_to_user > and x32_copy_siginfo_to_user I was wondering if we should make such > a change. The delivery of compat signal frames and core dumps which > do not go through the system call entry path could almost benefit from > a flag that could be set/tested when on those paths. For signal delivery it should (probably) depend on the system call that setup the signal handler. Although I'm sure I remember one kernel where some of it was done in libc (with a single entrypoint for all hadlers). > The fact that only SIGCHLD (which can not trigger a coredump) is > different saves the coredump code from needing such a test. > > The fact that the signal frame code is simple enough it can directly > call x32_copy_siginfo_to_user or __copy_siginfo_to_user saves us there. > > So I don't think we have any cases where we actually need a flag that > is independent of the system call but we have come very close. If a program can do both 64bit and x32 system calls you probably need to generate a 64bit core dump if it has ever made a 64bit system call?? > For people who want to optimize I suggest tracking down the handful of > users of x32 and see if x32 can be made to just go away. Unlikely since Ubuntu seem to have enabled it for years. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)