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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Tue, 22 Sep 2020 11:01:32 +0200
Message-ID: <CAK8P3a39jN+t2hhLg0oKZnbYATQXmYE2-Z1JkmFyc1EPdg1HXw@mail.gmail.com>
Subject: Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag
To:     Pavel Begunkov <asml.silence@gmail.com>
Cc:     Andy Lutomirski <luto@kernel.org>, Christoph Hellwig <hch@lst.de>,
        Al Viro <viro@zeniv.linux.org.uk>,
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On Tue, Sep 22, 2020 at 9:59 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
> On 22/09/2020 10:23, Arnd Bergmann wrote:
> > On Tue, Sep 22, 2020 at 8:32 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
> >> On 22/09/2020 03:58, Andy Lutomirski wrote:
> >>> On Mon, Sep 21, 2020 at 5:24 PM Pavel Begunkov <asml.silence@gmail.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.

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.

> > 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
already having to implement that fallback, right?

      Arnd