Re: Protection key in io uring kthread

[Jeff Xu <jeffxu@chromium.org>]

Hi Jens,
Thanks for responding.

On Wed, May 24, 2023 at 8:06 AM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 5/23/23 8:48?PM, Jeff Xu wrote:
> > Hi
> > I have a question on the protection key in io_uring. Today, when a
> > user thread enters the kernel through syscall, PKRU is preserved, and
> > the kernel  will respect the PKEY protection of memory.
> >
> > For example:
> > sys_mprotect_pkey((void *)ptr, size, PROT_READ | PROT_WRITE, pkey);
> > pkey_write_deny(pkey); <-- disable write access to pkey for this thread.
> > ret = read(fd, ptr, 1); <-- this will fail in the kernel.
> >
> > I wonder what is the case for io_uring, since read is now async, will
> > kthread have the user thread's PKUR ?
>
> There is no kthread. What can happen is that some operation may be
> punted to the io-wq workers, but these act exactly like a thread created
> by the original task. IOW, if normal threads retain the protection key,
> so will any io-wq io_uring thread. If they don't, they do not.
>
Does this also apply to when the IORING_SETUP_SQPOLL [1] flag is used
? it mentions a kernel thread is created to perform submission queue
polling.

[1] https://manpages.debian.org/unstable/liburing-dev/io_uring_setup.2.en.html#IORING_SETUP_SQPOLL

> > In theory, it is possible, i.e. from io_uring_enter syscall. But I
> > don't know the implementation details of io_uring, hence asking the
> > expert in this list.
>
> Right, if the IO is done inline, then it won't make a difference if eg
> read(2) is used or IORING_OP_READ (or similar) with io_uring.
>
Can you please clarify what "IO is done inline" means ? i.e. are there
cases that are not inline ?

Thanks!
-Jeff

> --
> Jens Axboe
>