* Andy Lutomirski:
>> On Dec 27, 2018, at 10:18 AM, Florian Weimer <fw@xxxxxxxxxxxxx> 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
It's definitely in this area. However, the original collision problem
with 32-bit hashes is also real, so I can see the desire to use more
bits.
> Presumably the right fix involves modifying the relevant VFS file
> operations to indicate the relevant ABI to the implementations.
Not sure. How does NFS solve this problem when access happens from a
32-bit process and the rest (client kernel, transport, server kernel)
is 64-bit all the way?
> I would guess that 9p is triggering the “not really in the syscall you
> think you’re in” issue.
I think the issue is more like the networking case for 9p. In this
scenario, the server shouldn't have to care whether the client process
is in 32-bit mode or 64-bit mode. But maybe the only solution is to
pass through some sort of flag, as Peter Maydell has just suggested.
