linux-kernel - Re: [PATCH 02/10] x86/cet: Introduce WRUSS instruction

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Date:   Mon, 11 Jun 2018 10:17:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. J. Lu" <hjl.tools@...il.com>,
        "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Oleg Nesterov <oleg@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        mike.kravetz@...cle.com
Subject: Re: [PATCH 02/10] x86/cet: Introduce WRUSS instruction

On Thu, Jun 07, 2018 at 09:40:02AM -0700, Andy Lutomirski wrote:
> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:

> Peterz, isn't there some fancy better way we're supposed to handle the
> error return these days?

> > +       asm volatile("1:.byte 0x66, 0x0f, 0x38, 0xf5, 0x37\n"
> > +                    "xor %[err],%[err]\n"
> > +                    "2:\n"
> > +                    ".section .fixup,\"ax\"\n"
> > +                    "3: mov $-1,%[err]; jmp 2b\n"
> > +                    ".previous\n"
> > +                    _ASM_EXTABLE(1b, 3b)
> > +               : [err] "=a" (err)
> > +               : [val] "S" (val), [addr] "D" (addr)
> > +               : "memory");

So the alternative is something like:

__visible bool ex_handler_wuss(const struct exception_table_entry *fixup,
			       struct pt_regs *regs, int trapnr)
{
	regs->ip = ex_fixup_addr(fixup);
	regs->ax = -1L;

	return true;
}


	int err = 0;

	asm volatile("1: INSN_WUSS\n"
		     "2:\n"
		     _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_wuss)
		     : "=a" (err)
		     : "S" (val), "D" (addr));

But I'm not at all sure that's actually better.