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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 15 Jan 2021 16:07:46 -0800
Message-ID: <CALCETrUtyVaGSE9fcFAkhrGCpkyYcYnZb6tj8227o2EH5hgOfg@mail.gmail.com>
Subject: Re: [PATCH] Adds a new ioctl32 syscall for backwards compatibility layers
To:     Arnd Bergmann <arnd@kernel.org>, Christoph Hellwig <hch@lst.de>
Cc:     Ryan Houdek <sonicadvance1@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Arnd Bergmann <arnd@arndb.de>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Minchan Kim <minchan@kernel.org>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Sargun Dhillon <sargun@sargun.me>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        "Amanieu d'Antras" <amanieu@gmail.com>,
        Willem de Bruijn <willemb@google.com>,
        YueHaibing <yuehaibing@huawei.com>,
        Xiaoming Ni <nixiaoming@huawei.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Joe Perches <joe@perches.com>, Jan Kara <jack@suse.cz>,
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On Fri, Jan 15, 2021 at 1:03 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Fri, Jan 15, 2021 at 3:06 AM Ryan Houdek <sonicadvance1@gmail.com> wrote:
> > On Wed, Jan 6, 2021 at 12:49 AM Arnd Bergmann <arnd@kernel.org> wrote:
> >> On Wed, Jan 6, 2021 at 7:48 AM <sonicadvance1@gmail.com> wrote:
> >> > From: Ryan Houdek <Sonicadvance1@gmail.com>
> >> ...
> >>
> >> For x86, this has another complication, as some ioctls also need to
> >> check whether they are in an ia32 task (with packed u64 and 32-bit
> >> __kernel_old_time_t) or an x32 task (with aligned u64 and 64-bit
> >> __kernel_old_time_t). If the new syscall gets wired up on x86 as well,
> >> you'd need to decide which of the two behaviors you want.
> >
> >
> > I can have a follow-up patch that makes this do ni-syscall on x86_64 since
> > we can go through the int 0x80 handler, or x32 handler path and choose
> > whichever one there.
>
> I'd say for consistency
>

We need to make it crystal clear on x86 what this ioctl does.  We have
a silly selection of options:

 - ioctl32() via SYSCALL, x32 bit clear -- presumably does an i386 ioctl?
 - ioctl32() via SYSCALL, x32 bit set -- this needs to do something
clearly documented.
 - ioctl32() via int80 -- presumably you're not wiring this up

In any case, the compat alloc thing should just go away.  It's a hack
and serves no real purpose.

Finally, I'm not convinced that this patch works correctly.  We have
in_compat_syscall(), and code that uses it may well be reachable from
ioctl.  I personally would like to see in_compat_syscall() go away,
but some other people (Hi, Christoph!) disagree, and usage seems to be
increasing, not decreasing.