Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag

From: Pavel Begunkov
Date: Wed Sep 23 2020 - 04:04:23 EST

On 22/09/2020 12:01, Arnd Bergmann wrote:
> On Tue, Sep 22, 2020 at 9:59 AM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote:
>> On 22/09/2020 10:23, Arnd Bergmann wrote:
>>> On Tue, Sep 22, 2020 at 8:32 AM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote:
>>>> On 22/09/2020 03:58, Andy Lutomirski wrote:
>>>>> On Mon, Sep 21, 2020 at 5:24 PM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote:
>>>>> I may be looking at a different kernel than you, but aren't you
>>>>> preventing creating an io_uring regardless of whether SQPOLL is
>>>>> requested?
>>>> I diffed a not-saved file on a sleepy head, thanks for noticing.
>>>> As you said, there should be an SQPOLL check.
>>>> ...
>>>> if (ctx->compat && (p->flags & IORING_SETUP_SQPOLL))
>>>> goto err;
>>> Wouldn't that mean that now 32-bit containers behave differently
>>> between compat and native execution?
>>> I think if you want to prevent 32-bit applications from using SQPOLL,
>>> it needs to be done the same way on both to be consistent:
>> The intention was to disable only compat not native 32-bit.
> I'm not following why that would be considered a valid option,
> as that clearly breaks existing users that update from a 32-bit
> kernel to a 64-bit one.

Do you mean users who move 32-bit binaries (without recompiling) to a
new x64 kernel? Does the kernel guarantees that to work? I'd personally
care more native-bitness apps.

> Taking away the features from users that are still on 32-bit kernels
> already seems questionable to me, but being inconsistent
> about it seems much worse, in particular when the regression
> is on the upgrade path.

TBH, this won't fix that entirely (e.g. passing non-compat io_uring
to a compat process should yield the same problem). So, let's put
it aside for now until this bikeshedding would be relevant.

>>> Can we expect all existing and future user space to have a sane
>>> fallback when IORING_SETUP_SQPOLL fails?
>> SQPOLL has a few differences with non-SQPOLL modes, but it's easy
>> to convert between them. Anyway, SQPOLL is a privileged special
>> case that's here for performance/latency reasons, I don't think
>> there will be any non-accidental users of it.
> Ok, so the behavior of 32-bit tasks would be the same as running
> the same application as unprivileged 64-bit tasks, with applications

Yes, something like that, but that's not automatic and in some
(hopefully rare) cases there may be pitfalls. That's in short,
I can expand the idea a bit if anyone would be interested.

> already having to implement that fallback, right?

Well, not everyone _have_ to implement such a fallback, e.g.
applications working only whilst privileged may use SQPOLL only.

Pavel Begunkov