linux-kernel - Re: [PATCH 1/1] get_nr_restart_syscall() should return __NR_ia32_restart_syscall if __USER32

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Date:   Thu, 30 Mar 2017 17:28:48 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jan Kratochvil <jan.kratochvil@...hat.com>,
        Pedro Alves <palves@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] get_nr_restart_syscall() should return
 __NR_ia32_restart_syscall if __USER32_CS

On 03/29, Andy Lutomirski wrote:
>
> On Wed, Mar 29, 2017 at 8:05 AM, Oleg Nesterov <oleg@...hat.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...

> 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.

Oleg.