linux-kernel - Re: io_uring vs in_compat

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Message-ID: <CALCETrWMQpKe7jqw2t39yn4HgGhGTSEFGK6MPR4wPs=tBBhjbg@mail.gmail.com>
Date:   Tue, 21 Jul 2020 10:25:24 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Andy Lutomirski <luto@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, io-uring@...r.kernel.org
Subject: Re: io_uring vs in_compat_syscall()

On Tue, Jul 21, 2020 at 7:34 AM Christoph Hellwig <hch@....de> wrote:
>
> On Tue, Jul 21, 2020 at 07:31:02AM -0700, Andy Lutomirski wrote:
> > > What do you mean with "properly wired up".  Do you really want to spread
> > > ->compat_foo methods everywhere, including read and write?  I found
> > > in_compat_syscall() a lot small and easier to maintain than all the
> > > separate compat cruft.
> >
> > I was imagining using a flag.  Some of the net code uses
> > MSG_CMSG_COMPAT for this purpose.
>
> Killing that nightmarish monster is what actually got me into looking
> io_uring and starting this thread.

I agree that MSG_CMSG_COMPAT is nasty, but I think the concept is
sound -- rather than tracking whether we're compat by using a
different function or a per-thread variable, actually explicitly
tracking the mode seems sensible.

If we're going to play in_compat_syscall() games, let's please make
io_uring_enter() return -EINVAL if in_compat_syscall() != ctx->compat.

--Andy