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Re: d_off field in struct dirent and 32-on-64 emulation
- From: Florian Weimer <fw at deneb dot enyo dot de>
- To: "Dmitry V. Levin" <ldv at altlinux dot org>
- Cc: linux-fsdevel at vger dot kernel dot org, linux-api at vger dot kernel dot org, linux-ext4 at vger dot kernel dot org, linux-kernel at vger dot kernel dot org, v9fs-developer at lists dot sourceforge dot net, libc-alpha at sourceware dot org, qemu-devel at nongnu dot org, ericvh at gmail dot com, rminnich at sandia dot gov, lucho at ionkov dot net, hpa at zytor dot com, arnd at arndb dot de
- Date: Fri, 28 Dec 2018 08:38:17 +0100
- Subject: Re: d_off field in struct dirent and 32-on-64 emulation
- References: <87bm56vqg4.fsf@mid.deneb.enyo.de> <20181228022338.GA27992@altlinux.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.