From mboxrd@z Thu Jan  1 00:00:00 1970
From: Arnd Bergmann <arnd@arndb.de>
Date: Tue, 22 Sep 2020 07:23:37 +0000
Subject: Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag
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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>, Andrew Morton <akpm@linux-foundation.org>, Jens Axboe <axboe@kernel.dk>, David Howells <dhowells@redhat.com>, linux-arm-kernel <linux-arm-kernel@lists.infradead.org>, X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>, "open list:MIPS" <linux-mips@vger.kernel.org>, Parisc List <linux-parisc@vger.kernel.org>, linuxppc-dev <linuxppc-dev@lists.ozlabs.org>, linux-s390 <linux-s390@vger.kernel.org>, sparclinux <sparclinux@vger.kernel.org>, linux-block <linux-block@vger.kernel.org>, Linux SCSI List <linux-scsi@vger.kernel.org>, Linux FS Devel <linux-fsdevel@vger.kernel.org>, linux-aio <linux-aio@kvack.org>, io-uring@vger.kernel.org, linux-arch <linux-arch@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>, Network Development <netdev@vger.kernel.org>, keyrings@vger.kernel.org, LSM List <linux-security-module@vger.kernel.org>

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:

   if ((!IS_ENABLED(CONFIG_64BIT) || ctx->compat) &&
        (p->flags & IORING_SETUP_SQPOLL))
            goto err;

I don't really see how taking away SQPOLL from 32-bit tasks is
any better than just preventing access to the known-broken files
as Al suggested, or adding the hack to make it work as in
Christoph's original patch.

Can we expect all existing and future user space to have a sane
fallback when IORING_SETUP_SQPOLL fails?

      Arnd