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From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 30 Mar 2017 11:36:01 -0700
Message-ID: <CALCETrX1L7fQ=UYpzJHAY4M_XQ5Ht5wOSfvKMtac8j=uO7cnzQ@mail.gmail.com>
Subject: Re: [PATCH 1/1] get_nr_restart_syscall() should return
 __NR_ia32_restart_syscall if __USER32_CS
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Denys Vlasenko <dvlasenk@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Ingo Molnar <mingo@redhat.com>,
        Jan Kratochvil <jan.kratochvil@redhat.com>,
        Pedro Alves <palves@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Thu, Mar 30, 2017 at 8:28 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> On 03/29, Andy Lutomirski wrote:
>>
>> On Wed, Mar 29, 2017 at 8:05 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>> > On 03/28, Oleg Nesterov wrote:
>> >>
>> >> On 03/28, Andy Lutomirski wrote:
>> >> >
>> >> > How about we store the syscall arch to be restored in task_struct
>> >> > along with restart_block?
>> >>
>> >> Yes, perhaps we will have to finally do this. Not really nice too.
>> >
>> > OK, how about the hack below?
>> >
>> > I do not want to a new member into task_struct/restart_block, so the
>> > patch below adds a sticky TS_COMPAT bit which logically is a member
>> > of "struct restart_block".
>>
>> Okay, but I'd much rather we just added a helper that's called in the
>> few places that actually write to restart_block.
>
> Oh, yes, I thought about this too. This obviously needs more changes, and
> every arch needs a dummy definition... I was thinking about
>
>         static inline long setup_restart_block(void)
>         {
>                 if (TS_COMPAT)
>                         set TS_COMPAT_XXX;
>                 else
>                         clear TS_COMPAT_XXX;
>
>                 return -ERESTART_RESTARTBLOCK;
>         }
>
> so that we can do
>
>         -       ret = -ERESTART_RESTARTBLOCK;
>         +       ret = setup_restart_block();
>
> but I don't really like this... Do you strongly prefer it over the
> -ERESTART_RESTARTBLOCK check in syscall_return_slowpath? I agree it doesn't
> look nice too but it connects to other TS_ magic we do in arch/x86/entry/,
> perhaps it is not that bad...

How about:

struct restart_block *restart = set_syscall_restart_fn(do_whatever);
restart->other_stuff = blah.

I'd rather avoid adding stuff to the slow path that runs *that* rarely.

>
>> Or we just add the new syscall nr and see what breaks.  The answer
>> could well be nothing at all.
>
> Well, strace knows about __NR_restart_syscall. It won't be really broken,
> but I guess it will report something like "unknown syscall" rather than
> restart_syscall(...).
>
> However, this still looks like a best solution to me, just I have no idea
> how much we can confuse user-space.

Me neither.