Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation

[Andy Lutomirski]

> On Dec 27, 2018, at 10:18 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).

I imagine you’re encountering 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.