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Date:   Sun, 20 Sep 2020 07:52:38 +1000 (AEST)
From:   Finn Thain <fthain@telegraphics.com.au>
To:     Arnd Bergmann <arnd@arndb.de>
cc:     Peter Maydell <peter.maydell@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        Al Viro <viro@zeniv.linux.org.uk>,
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Subject: Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag
In-Reply-To: <CAK8P3a2Mi+1yttyGk4k7HxRVrMtmFqJewouVhynqUL0PJycmog@mail.gmail.com>
Message-ID: <alpine.LNX.2.23.453.2009200745310.6@nippy.intranet>
References: <20200918124533.3487701-1-hch@lst.de> <20200918124533.3487701-2-hch@lst.de> <20200918134012.GY3421308@ZenIV.linux.org.uk> <20200918134406.GA17064@lst.de> <20200918135822.GZ3421308@ZenIV.linux.org.uk> <20200918151615.GA23432@lst.de>
 <CALCETrW=BzodXeTAjSvpCoUQoL+MKaKPEeSTRWnB=-C9jMotbQ@mail.gmail.com> <CAK8P3a2Mi+1yttyGk4k7HxRVrMtmFqJewouVhynqUL0PJycmog@mail.gmail.com>
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On Sat, 19 Sep 2020, Arnd Bergmann 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?
> 

Quite possibly. The ext4 vs. compat getdents bug is still unresolved. 
Please see,
https://lore.kernel.org/lkml/CAFEAcA9W+JK7_TrtTnL1P2ES1knNPJX9wcUvhfLwxLq9augq1w@mail.gmail.com/

>        Arnd
>