Re: io_uring vs in_compat_syscall()

On Jul 20, 2020, at 9:37 AM, Jens Axboe [off-list ref] wrote:

On 7/20/20 12:10 AM, Christoph Hellwig wrote:

quoted

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.