Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag

Andy Lutomirski
2020-09-21T23:51:18Z

On Mon, Sep 21, 2020 at 9:15 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> On 21/09/2020 19:10, Pavel Begunkov wrote:
> > On 20/09/2020 01:22, Andy Lutomirski wrote:
> >>
> >>> On Sep 19, 2020, at 2:16 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> >>>
> >>> ﻿On Sat, Sep 19, 2020 at 6:21 PM Andy Lutomirski <luto@kernel.org> wrote:
> >>>>> On Fri, Sep 18, 2020 at 8:16 AM Christoph Hellwig <hch@lst.de> 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.
> >>>
> >>> Ah, so reading /dev/input/event* would suffer from the same issue,
> >>> and that one would in fact be broken by your patch in the hypothetical
> >>> case that someone tried to use io_uring to read /dev/input/event on x32...
> >>>
> >>> For reference, I checked the socket timestamp handling that has a
> >>> number of corner cases with time32/time64 formats in compat mode,
> >>> but none of those appear to be affected by the problem.
> >>>
> >>>> Aside from the potentially nasty use of per-task variables, one thing
> >>>> I don't like about PF_FORCE_COMPAT is that it's one-way. If we're
> >>>> going to have a generic mechanism for this, shouldn't we allow a full
> >>>> override of the syscall arch instead of just allowing forcing compat
> >>>> so that a compat syscall can do a non-compat operation?
> >>>
> >>> The only reason it's needed here is that the caller is in a kernel
> >>> thread rather than a system call. Are there any possible scenarios
> >>> where one would actually need the opposite?
> >>>
> >>
> >> I can certainly imagine needing to force x32 mode from a kernel thread.
> >>
> >> As for the other direction: what exactly are the desired bitness/arch semantics of io_uring? Is the operation bitness chosen by the io_uring creation or by the io_uring_enter() bitness?
> >
> > It's rather the second one. Even though AFAIR it wasn't discussed
> > specifically, that how it works now (_partially_).
>
> Double checked -- I'm wrong, that's the former one. Most of it is based
> on a flag that was set an creation.
>

Could we get away with making io_uring_enter() return -EINVAL (or
maybe -ENOTTY?) if you try to do it with bitness that doesn't match
the io_uring? And disable SQPOLL in compat mode?

--Andy