Re: [PATCH v4 1/2] fs/aio: Restrict kiocb_set_cancel_fn() to I/O submitted via libaio

[Bart Van Assche <bvanassche@acm.org>]

On 3/4/24 11:10, Eric Biggers wrote:
> If I understand correctly, this patch is supposed to fix a memory safety bug
> when kiocb_set_cancel_fn() is called on a kiocb that is owned by io_uring
> instead of legacy AIO.  However, the kiocb still gets accessed as an aio_kiocb
> at the very beginning of the function, so it's still broken:
> 
> 	struct aio_kiocb *req = container_of(iocb, struct aio_kiocb, rw);
> 	struct kioctx *ctx = req->ki_ctx;

Hi Eric,

Thanks for having reported this. I agree that this needs to be fixed.

> I'm also wondering why "ignore" is the right fix.  The USB gadget driver sees
> that it has asynchronous I/O (kiocb::ki_complete != NULL) and then tries to set
> a cancellation function.  What is the expected behavior when the I/O is owned by
> io_uring?  Should it perhaps call into io_uring to set a cancellation function
> with io_uring?  Or is the concept of cancellation functions indeed specific to
> legacy AIO, and nothing should be done with io_uring I/O?

As far as I know no Linux user space interface for submitting I/O 
supports cancellation of read or write requests other than the AIO
io_cancel() system call.

It would make it easier to maintain the kernel if I/O cancellation
support would be removed. However, there is existing user space code
that depends on USB I/O cancellation so I'm not sure how to proceed to 
remove AIO io_cancel() support from the kernel.

Thanks,

Bart.