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Date:   Fri, 28 Dec 2018 05:23:39 +0300
From:   "Dmitry V. Levin" <ldv@altlinux.org>
To:     Florian Weimer <fw@deneb.enyo.de>
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
Message-ID: <20181228022338.GA27992@altlinux.org>
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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.
>=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).

Why getdents64 system call is affected by this truncation,
isn't it a kernel bug that has to be fixed in the kernel instead?


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