Return-Path: Received: from mail.kernel.org ([198.145.29.99]:51306 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726543AbeL2Qtf (ORCPT ); Sat, 29 Dec 2018 11:49:35 -0500 Received: from mail-wm1-f50.google.com (mail-wm1-f50.google.com [209.85.128.50]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id D38D621907 for ; Sat, 29 Dec 2018 16:49:33 +0000 (UTC) Received: by mail-wm1-f50.google.com with SMTP id f81so21764612wmd.4 for ; Sat, 29 Dec 2018 08:49:33 -0800 (PST) MIME-Version: 1.0 References: <87bm56vqg4.fsf@mid.deneb.enyo.de> <9C6A7D45-CF53-4C61-B5DD-12CA0D419972@dilger.ca> <1EF1B31A-83D8-4642-BEBF-F56E45485223@dilger.ca> <20181229015453.GA6310@bombadil.infradead.org> In-Reply-To: <20181229015453.GA6310@bombadil.infradead.org> From: Andy Lutomirski Date: Sat, 29 Dec 2018 08:49:19 -0800 Message-ID: Subject: Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation To: Matthew Wilcox Cc: Peter Maydell , Andreas Dilger , Florian Weimer , linux-fsdevel , Linux API , Ext4 Developers List , Latchesar Ionkov , libc-alpha , Arnd Bergmann , Eric Van Hensbergen , "H. Peter Anvin" , lkml - Kernel Mailing List , QEMU Developers , Ron Minnich , V9FS Developers Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Sender: linux-ext4-owner@vger.kernel.org List-ID: > On Dec 28, 2018, at 6:54 PM, Matthew Wilcox wrote: > >> On Sat, Dec 29, 2018 at 12:12:27AM +0000, Peter Maydell wrote: >> On Fri, 28 Dec 2018 at 23:16, Andreas Dilger wrot >>> On Dec 28, 2018, at 4:18 AM, Peter Maydell w= rote: >>>> The problem is that there is no 32-bit API in some cases >>>> (unless I have misunderstood the kernel code) -- not all >>>> host architectures implement compat syscalls or allow them >>>> to be called from 64-bit processes or implement all the older >>>> syscall variants that had smaller offets. If there was a guaranteed >>>> "this syscall always exists and always gives me 32-bit offsets" >>>> we could use it. >>> >>> The "32bitapi" mount option would use 32-bit hash for seekdir >>> and telldir, regardless of what kernel API was used. That would >>> just set the FMODE_32BITHASH flag in the file->f_mode for all files. >> >> A mount option wouldn't be much use to QEMU -- we can't tell >> our users how to mount their filesystems, which they're >> often doing lots of other things with besides running QEMU. >> (Otherwise we could just tell them "don't use ext4", which >> would also solve the problem :-)) We need something we can >> use at the individual-syscall level. > > Could you use a prctl to set whether you were running in 32 or 64 bit > mode? Or do you change which kind of task you're emulating too often > to make this a good idea? How would this work? We already have the separate COMPAT_DEFINE_SYSCALL entries *and* in_compat_syscall(). Now we=E2=80=99d h= ave a third degree of freedom. Either the arches people care about should add reasonable ways to issue 32-bit syscalls from 64-bit mode or there should be an explicit way to ask for the 32-bit directory offsets.