netdev - Re: [PATCH 1/9] kernel: add a PF_FORCE

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Date:   Wed, 23 Sep 2020 11:01:34 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andy Lutomirski <luto@...nel.org>, Christoph Hellwig <hch@....de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        David Howells <dhowells@...hat.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        Parisc List <linux-parisc@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        sparclinux <sparclinux@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Linux SCSI List <linux-scsi@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        linux-aio <linux-aio@...ck.org>, io-uring@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Network Development <netdev@...r.kernel.org>,
        keyrings@...r.kernel.org,
        LSM List <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag

On 22/09/2020 12:01, Arnd Bergmann wrote:
> On Tue, Sep 22, 2020 at 9:59 AM Pavel Begunkov <asml.silence@...il.com> wrote:
>> On 22/09/2020 10:23, Arnd Bergmann wrote:
>>> On Tue, Sep 22, 2020 at 8:32 AM Pavel Begunkov <asml.silence@...il.com> wrote:
>>>> On 22/09/2020 03:58, Andy Lutomirski wrote:
>>>>> On Mon, Sep 21, 2020 at 5:24 PM Pavel Begunkov <asml.silence@...il.com> 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