linux-kernel - Re: [PATCHv3 5/8] x86/uaccess: Provide untagged_addr() and remove tags before address check

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Message-ID: <Yqr/qBeIbLZYzgXi@hirez.programming.kicks-ass.net>
Date:   Thu, 16 Jun 2022 12:02:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv3 5/8] x86/uaccess: Provide untagged_addr() and remove
 tags before address check

On Fri, Jun 10, 2022 at 05:35:24PM +0300, Kirill A. Shutemov wrote:
> +#ifdef CONFIG_X86_64
> +/*
> + * Mask out tag bits from the address.
> + *
> + * Magic with the 'sign' allows to untag userspace pointer without any branches
> + * while leaving kernel addresses intact.
> + */
> +#define untagged_addr(mm, addr)	({					\
> +	u64 __addr = (__force u64)(addr);				\
> +	s64 sign = (s64)__addr >> 63;					\
> +	__addr ^= sign;							\
> +	__addr &= (mm)->context.untag_mask;				\
> +	__addr ^= sign;							\
> +	(__force __typeof__(addr))__addr;				\
> +})

Can't we make that mask a constant and *always* unmask U57 irrespective
of LAM being on?