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: Thu, 27 Dec 2018 18:56:02 +0100 Message-ID: <87wonuua4t.fsf@mid.deneb.enyo.de> References: <87bm56vqg4.fsf@mid.deneb.enyo.de> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Return-path: In-Reply-To: (Andy Lutomirski's message of "Thu, 27 Dec 2018 10:38:10 -0700") Sender: linux-kernel-owner@vger.kernel.org To: Andy Lutomirski 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, lucho@ionkov.net, hpa@zytor.com, arnd@arndb.de List-Id: linux-api@vger.kernel.org * Andy Lutomirski: >> On Dec 27, 2018, at 10:18 AM, 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). > > I imagine you’re encountering this bug: > > https://lkml.org/lkml/2018/10/18/859 It's definitely in this area. However, the original collision problem with 32-bit hashes is also real, so I can see the desire to use more bits. > Presumably the right fix involves modifying the relevant VFS file > operations to indicate the relevant ABI to the implementations. Not sure. How does NFS solve this problem when access happens from a 32-bit process and the rest (client kernel, transport, server kernel) is 64-bit all the way? > I would guess that 9p is triggering the “not really in the syscall you > think you’re in” issue. I think the issue is more like the networking case for 9p. In this scenario, the server shouldn't have to care whether the client process is in 32-bit mode or 64-bit mode. But maybe the only solution is to pass through some sort of flag, as Peter Maydell has just suggested. From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:36583) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gcZtL-0000aF-2g for qemu-devel@nongnu.org; Thu, 27 Dec 2018 12:56:20 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gcZtG-0003tf-RD for qemu-devel@nongnu.org; Thu, 27 Dec 2018 12:56:18 -0500 Received: from albireo.enyo.de ([5.158.152.32]:54760) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gcZtG-0003pY-JB for qemu-devel@nongnu.org; Thu, 27 Dec 2018 12:56:14 -0500 From: Florian Weimer References: <87bm56vqg4.fsf@mid.deneb.enyo.de> Date: Thu, 27 Dec 2018 18:56:02 +0100 In-Reply-To: (Andy Lutomirski's message of "Thu, 27 Dec 2018 10:38:10 -0700") Message-ID: <87wonuua4t.fsf@mid.deneb.enyo.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable 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: Andy Lutomirski 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, lucho@ionkov.net, hpa@zytor.com, arnd@arndb.de * Andy Lutomirski: >> On Dec 27, 2018, at 10:18 AM, Florian Weimer wrote: >>=20 >> We have a bit of an interesting problem with respect to the d_off >> field in struct dirent. >>=20 >> 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): >>=20 >> getdents(3, [ >> {d_ino=3D1494304, d_off=3D3901177228673045825, d_reclen=3D40, >> d_name=3D"authorized_keys", d_type=3DDT_REG}, >> {d_ino=3D1494277, d_off=3D7491915799041650922, d_reclen=3D24, d_name=3D= ".", >> d_type=3DDT_DIR}, >> {d_ino=3D1314655, d_off=3D9223372036854775807, d_reclen=3D24, >> d_name=3D"..", d_type=3DDT_DIR} >> ], 32768) =3D 88 >>=20 >> 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). > > I imagine you=E2=80=99re encountering this bug: > > https://lkml.org/lkml/2018/10/18/859 It's definitely in this area. However, the original collision problem with 32-bit hashes is also real, so I can see the desire to use more bits. > Presumably the right fix involves modifying the relevant VFS file > operations to indicate the relevant ABI to the implementations. Not sure. How does NFS solve this problem when access happens from a 32-bit process and the rest (client kernel, transport, server kernel) is 64-bit all the way? > I would guess that 9p is triggering the =E2=80=9Cnot really in the syscal= l you > think you=E2=80=99re in=E2=80=9D issue. I think the issue is more like the networking case for 9p. In this scenario, the server shouldn't have to care whether the client process is in 32-bit mode or 64-bit mode. But maybe the only solution is to pass through some sort of flag, as Peter Maydell has just suggested.