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From: Jens Axboe <axboe@kernel.dk>
To: Hristo Venev <hristo@venev.name>, Dmitry Vyukov <dvyukov@google.com>
Cc: Necip Fazil Yildiran <necip@google.com>, io-uring@vger.kernel.org
Subject: Re: [PATCH] io_uring: fix sq array offset calculation
Date: Sat, 11 Jul 2020 09:55:20 -0600	[thread overview]
Message-ID: <4620d87d-1862-ed28-9f42-7b98bd49179b@kernel.dk> (raw)
In-Reply-To: <7f128319f405358aa448a869a3a634a6cbc1469f.camel@venev.name>

On 7/11/20 9:52 AM, Hristo Venev 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.

A bit of luck, but if that wasn't the case, then I'm sure we would
have found it when the original patch was tested. But thanks for
double checking!

-- 
Jens Axboe


  reply	other threads:[~2020-07-11 15:55 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-11  9:31 [PATCH] io_uring: fix sq array offset calculation Dmitry Vyukov
2020-07-11  9:37 ` Hristo Venev
2020-07-11 15:15 ` Jens Axboe
2020-07-11 15:31   ` Dmitry Vyukov
2020-07-11 15:36     ` Jens Axboe
2020-07-11 15:47       ` Jens Axboe
2020-07-11 15:52     ` Hristo Venev
2020-07-11 15:55       ` Jens Axboe [this message]
2020-07-11 15:56       ` Hristo Venev
2020-07-11 16:16       ` Dmitry Vyukov
2020-07-17 13:48         ` Dmitry Vyukov
2020-07-17 14:05           ` Jens Axboe
2020-07-17 14:08             ` Jens Axboe
2020-07-17 14:08             ` Dmitry Vyukov

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