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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Sun, 20 Sep 2020 15:55:47 +0200
Message-ID: <CAK8P3a3QApj3isPu3TkLahArsfb5jaABb7DJ7EKMxey1T1YPbA@mail.gmail.com>
Subject: Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jens Axboe <axboe@kernel.dk>,
        David Howells <dhowells@redhat.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@vger.kernel.org>,
        Parisc List <linux-parisc@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
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        linux-block <linux-block@vger.kernel.org>,
        linux-scsi <linux-scsi@vger.kernel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
        linux-aio <linux-aio@kvack.org>, io-uring@vger.kernel.org,
        linux-arch <linux-arch@vger.kernel.org>,
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On Sun, Sep 20, 2020 at 12:09 AM Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Fri, Sep 18, 2020 at 05:16:15PM +0200, Christoph Hellwig wrote:
> > On Fri, Sep 18, 2020 at 02:58:22PM +0100, Al Viro wrote:
> > > Said that, why not provide a variant that would take an explicit
> > > "is it compat" argument and use it there?  And have the normal
> > > one pass in_compat_syscall() to that...
> >
> > That would help to not introduce a regression with this series yes.
> > But it wouldn't fix existing bugs when io_uring is used to access
> > read or write methods that use in_compat_syscall().  One example that
> > I recently ran into is drivers/scsi/sg.c.
>
> So screw such read/write methods - don't use them with io_uring.
> That, BTW, is one of the reasons I'm sceptical about burying the
> decisions deep into the callchain - we don't _want_ different
> data layouts on read/write depending upon the 32bit vs. 64bit
> caller, let alone the pointer-chasing garbage that is /dev/sg.

Would it be too late to limit what kind of file descriptors we allow
io_uring to read/write on?

If io_uring can get changed to return -EINVAL on trying to
read/write something other than S_IFREG file descriptors,
that particular problem space gets a lot simpler, but this
is of course only possible if nobody actually relies on it yet.

      Arnd