From mboxrd@z Thu Jan 1 00:00:00 1970 From: Florian Weimer Subject: Re: d_off field in struct dirent and 32-on-64 emulation Date: Fri, 28 Dec 2018 08:38:17 +0100 Message-ID: <87bm56w17a.fsf@mid.deneb.enyo.de> References: <87bm56vqg4.fsf@mid.deneb.enyo.de> <20181228022338.GA27992@altlinux.org> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: <20181228022338.GA27992@altlinux.org> (Dmitry V. Levin's message of "Fri, 28 Dec 2018 05:23:39 +0300") Sender: linux-kernel-owner@vger.kernel.org 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 List-Id: linux-api@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. From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:60405) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gcmsd-0006zr-Lt for qemu-devel@nongnu.org; Fri, 28 Dec 2018 02:48:30 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gcmqx-00076s-G8 for qemu-devel@nongnu.org; Fri, 28 Dec 2018 02:46:46 -0500 Received: from albireo.enyo.de ([5.158.152.32]:59072) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gcmqx-00070z-8E for qemu-devel@nongnu.org; Fri, 28 Dec 2018 02:46:43 -0500 From: Florian Weimer 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 Subject: Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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 * 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.