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

[Dmitry Vyukov <dvyukov@google.com>]

On Sat, Jul 11, 2020 at 5:52 PM Hristo Venev <hristo@venev.name> wrote:
>
> On Sat, 2020-07-11 at 17:31 +0200, Dmitry Vyukov wrote:
> > 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.
>
> The layout (after the fix) is roughly as follows:
>
> 1. struct io_rings - ~192 bytes, maybe 256
> 2. cqes - (32 << n) bytes
> 3. sq_array - (4 << n) bytes
>
> The bug was that the sq_array was offset by (4 << n) bytes. I think
> issues can only occur when
>
>     PAGE_ALIGN(192 + (32 << n) + (4 << n) + (4 << n))
>     !=
>     PAGE_ALIGN(192 + (32 << n) + (4 << n))
>
> It looks like this never happens. We got lucky.

Interesting. CQ entries are larger and we have at least that many of
them as SQ entries. I guess this + power-of-2-pages can make it never
overflow.