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Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
From: |
Andy Lutomirski |
Subject: |
Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation |
Date: |
Fri, 28 Dec 2018 07:26:00 -0800 |
[sending again, slightly edited, due to email client issues]
On Thu, Dec 27, 2018 at 9:25 AM Florian Weimer <address@hidden> 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.
- Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation, (continued)
Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation, Dmitry V. Levin, 2018/12/27
Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation,
Andy Lutomirski <=