Re: io_uring vs in_compat_syscall()

[Andy Lutomirski <luto@amacapital.net>]

> On Jul 20, 2020, at 10:02 AM, Jens Axboe <axboe@kernel.dk> wrote:
> 
> On 7/20/20 10:58 AM, Andy Lutomirski wrote:
>> 
>>>> On Jul 20, 2020, at 9:37 AM, Jens Axboe <axboe@kernel.dk> wrote:
>>> 
>>> On 7/20/20 12:10 AM, Christoph Hellwig wrote:
>>>> Hi Jens,
>>>> 
>>>> I just found a (so far theoretical) issue with the io_uring submission
>>>> offloading to workqueues or threads.  We have lots of places using
>>>> in_compat_syscall() to check if a syscall needs compat treatmenet.
>>>> While the biggest users is iocttl(), we also have a fair amount of
>>>> places using in_compat_task() in read and write methods, and these
>>>> will not do the wrong thing when used with io_uring under certain
>>>> conditions.  I'm not sure how to best fix this, except for making sure
>>>> in_compat_syscall() returns true one way or another for these cases.
>>> 
>>> We can probably propagate this information in the io_kiocb via a flag,
>>> and have the io-wq worker set TS_COMPAT if that's the case.
>>> 
>> 
>> Is TS_COMPAT actually a cross-arch concept for which this is safe?
>> Having a real arch helper for “set the current syscall arch for the
>> current kernel thread” seems more sensible to me. 
> 
> Sure, I'd consider that implementation detail for the actual patch(es)
> for this issue.

There’s a corner case, though: doesn’t io_uring submission frequently do the work synchronously in the context of the calling thread?  If so, can a thread do a 64-bit submit with 32-bit work or vice versa?

Sometimes I think that in_compat_syscall() should have a mode in which calling it warns (e.g. not actually in a syscall when doing things in io_uring).  And the relevant operations should be properly wired up to avoid global state like this.

> 
> -- 
> Jens Axboe
>