Re: [PATCH] io_uring: fix sq array offset calculation

[Dmitry Vyukov <dvyukov@google.com>]

On Sat, Jul 11, 2020 at 5:16 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 7/11/20 3:31 AM, Dmitry Vyukov wrote:
> > rings_size() sets sq_offset to the total size of the rings
> > (the returned value which is used for memory allocation).
> > This is wrong: sq array should be located within the rings,
> > not after them. Set sq_offset to where it should be.
> >
> > Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
> > Cc: io-uring@vger.kernel.org
> > Cc: Hristo Venev <hristo@venev.name>
> > Fixes: 75b28affdd6a ("io_uring: allocate the two rings together")
> >
> > ---
> > This looks so wrong and yet io_uring works.
> > So I am either missing something very obvious here,
> > or io_uring worked only due to lucky side-effects
> > of rounding size to power-of-2 number of pages
> > (which gave it enough slack at the end),
> > maybe reading/writing some unrelated memory
> > with some sizes.
> > If I am wrong, please poke my nose into what I am not seeing.
> > Otherwise, we probably need to CC stable as well.
>
> Well that's a noodle scratcher, it's definitely been working fine,
> and I've never seen any out-of-bounds on any of the testing I do.
> I regularly run anything with KASAN enabled too.

Looking at the code more, I am not sure how it may not corrupt memory.
There definitely should be some combinations where accessing
sq_entries*sizeof(u32) more memory won't be OK.
May be worth adding a test that allocates all possible sizes for sq/cq
and fills both rings.