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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 C8521C43387 for ; Fri, 28 Dec 2018 07:46:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A1C262087F for ; Fri, 28 Dec 2018 07:46:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731800AbeL1Hqg (ORCPT ); Fri, 28 Dec 2018 02:46:36 -0500 Received: from albireo.enyo.de ([5.158.152.32]:52790 "EHLO albireo.enyo.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729313AbeL1Hqf (ORCPT ); Fri, 28 Dec 2018 02:46:35 -0500 Received: from [172.17.203.2] (helo=deneb.enyo.de) by albireo.enyo.de with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) id 1gcmqk-00059a-1l; Fri, 28 Dec 2018 07:46:30 +0000 Received: from fw by deneb.enyo.de with local (Exim 4.89) (envelope-from ) id 1gcmin-0001pk-T5; Fri, 28 Dec 2018 08:38:17 +0100 From: Florian Weimer To: "Dmitry V. Levin" Cc: linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, v9fs-developer@lists.sourceforge.net, libc-alpha@sourceware.org, qemu-devel@nongnu.org, ericvh@gmail.com, rminnich@sandia.gov, lucho@ionkov.net, hpa@zytor.com, arnd@arndb.de Subject: Re: d_off field in struct dirent and 32-on-64 emulation References: <87bm56vqg4.fsf@mid.deneb.enyo.de> <20181228022338.GA27992@altlinux.org> Date: Fri, 28 Dec 2018 08:38:17 +0100 In-Reply-To: <20181228022338.GA27992@altlinux.org> (Dmitry V. Levin's message of "Fri, 28 Dec 2018 05:23:39 +0300") Message-ID: <87bm56w17a.fsf@mid.deneb.enyo.de> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Dmitry V. Levin: > On Thu, Dec 27, 2018 at 06:18:19PM +0100, Florian Weimer wrote: >> We have a bit of an interesting problem with respect to the d_off >> field in struct dirent. >> >> When running a 64-bit kernel on certain file systems, notably ext4, >> this field uses the full 63 bits even for small directories (strace -v >> output, wrapped here for readability): >> >> getdents(3, [ >> {d_ino=1494304, d_off=3901177228673045825, d_reclen=40, >> d_name="authorized_keys", d_type=DT_REG}, >> {d_ino=1494277, d_off=7491915799041650922, d_reclen=24, >> d_name=".", d_type=DT_DIR}, >> {d_ino=1314655, d_off=9223372036854775807, d_reclen=24, >> d_name="..", d_type=DT_DIR} >> ], 32768) = 88 >> >> When running in 32-bit compat mode, this value is somehow truncated to >> 31 bits, for both the getdents and the getdents64 (!) system call (at >> least on i386). > > Why getdents64 system call is affected by this truncation, > isn't it a kernel bug that has to be fixed in the kernel instead? It's required because POSIX specifies that telldir and seekdir use long int (and not off_t) as the seek offset. If the kernel does not truncate while keeping a useful value, these functions would turn unusable.