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Subject: Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag
From:   Pavel Begunkov <asml.silence@gmail.com>
To:     Andy Lutomirski <luto@amacapital.net>,
        Arnd Bergmann <arnd@arndb.de>
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>,
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Message-ID: <486c92d0-0f2e-bd61-1ab8-302524af5e08@gmail.com>
Date:   Mon, 21 Sep 2020 19:13:18 +0300
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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.

-- 
Pavel Begunkov