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

[Jens Axboe <axboe@kernel.dk>]

On 7/11/20 9:31 AM, Dmitry Vyukov wrote:
> 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.

Yeah, actually doing that right now just to verify it.

-- 
Jens Axboe