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Re: d_off field in struct dirent and 32-on-64 emulation
- From: Andy Lutomirski <luto at kernel dot org>
- To: Florian Weimer <fw at deneb dot enyo dot de>
- Cc: Linux FS Devel <linux-fsdevel at vger dot kernel dot org>, Linux API <linux-api at vger dot kernel dot org>, linux-ext4 at vger dot kernel dot org, LKML <linux-kernel at vger dot kernel dot org>, V9FS Developers <v9fs-developer at lists dot sourceforge dot net>, libc-alpha <libc-alpha at sourceware dot org>, "qemu-devel at nongnu dot org Developers" <qemu-devel at nongnu dot org>, Eric Van Hensbergen <ericvh at gmail dot com>, Ron Minnich <rminnich at sandia dot gov>, Latchesar Ionkov <lucho at ionkov dot net>, "H. Peter Anvin" <hpa at zytor dot com>, Arnd Bergmann <arnd at arndb dot de>
- Date: Fri, 28 Dec 2018 07:26:00 -0800
- Subject: Re: d_off field in struct dirent and 32-on-64 emulation
- References: <87bm56vqg4.fsf@mid.deneb.enyo.de>
[sending again, slightly edited, due to email client issues]
On Thu, Dec 27, 2018 at 9:25 AM Florian Weimer <fw@deneb.enyo.de> 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).
>
...
>
> However, both qemu-user and the 9p file system can run in such a way
> that the kernel is entered from a 64-bit process, but the actual usage
> is from a 32-bit process:
I imagine that at least some of the problems you're seeing are due to this bug:
https://lkml.org/lkml/2018/10/18/859
Presumably the right fix involves modifying the relevant VFS file
operations to indicate the relevant ABI to the implementations. I
would guess that 9p is triggering the “not really in the syscall you
think you’re in” issue.