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

[Andy Lutomirski <luto@kernel.org>]

> On Dec 28, 2018, at 6:54 PM, Matthew Wilcox <willy@infradead.org> wrote:
>
>> On Sat, Dec 29, 2018 at 12:12:27AM +0000, Peter Maydell wrote:
>> On Fri, 28 Dec 2018 at 23:16, Andreas Dilger <adilger@dilger.ca> wrot
>>> On Dec 28, 2018, at 4:18 AM, Peter Maydell <peter.maydell@linaro.org> wrote:
>>>> The problem is that there is no 32-bit API in some cases
>>>> (unless I have misunderstood the kernel code) -- not all
>>>> host architectures implement compat syscalls or allow them
>>>> to be called from 64-bit processes or implement all the older
>>>> syscall variants that had smaller offets. If there was a guaranteed
>>>> "this syscall always exists and always gives me 32-bit offsets"
>>>> we could use it.
>>>
>>> The "32bitapi" mount option would use 32-bit hash for seekdir
>>> and telldir, regardless of what kernel API was used.  That would
>>> just set the FMODE_32BITHASH flag in the file->f_mode for all files.
>>
>> A mount option wouldn't be much use to QEMU -- we can't tell
>> our users how to mount their filesystems, which they're
>> often doing lots of other things with besides running QEMU.
>> (Otherwise we could just tell them "don't use ext4", which
>> would also solve the problem :-)) We need something we can
>> use at the individual-syscall level.
>
> Could you use a prctl to set whether you were running in 32 or 64 bit
> mode?  Or do you change which kind of task you're emulating too often
> to make this a good idea?


How would this work?  We already have the separate
COMPAT_DEFINE_SYSCALL entries *and* in_compat_syscall(). Now we’d have
a third degree of freedom.

Either the arches people care about should add reasonable ways to
issue 32-bit syscalls from 64-bit mode or there should be an explicit
way to ask for the 32-bit directory offsets.