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

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Message-ID: <YqyPmhKLzvc7iXxT@worktop.programming.kicks-ass.net>
Date:   Fri, 17 Jun 2022 16:28:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "H.J. Lu" <hjl.tools@...il.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "andreyknvl@...il.com" <andreyknvl@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "ryabinin.a.a@...il.com" <ryabinin.a.a@...il.com>,
        "glider@...gle.com" <glider@...gle.com>
Subject: Re: [PATCHv3 5/8] x86/uaccess: Provide untagged_addr() and remove
 tags before address check

On Fri, Jun 17, 2022 at 07:22:57AM -0700, H.J. Lu wrote:
> On Fri, Jun 17, 2022 at 4:36 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Thu, Jun 16, 2022 at 07:44:40PM +0300, Kirill A. Shutemov wrote:
> > > Any objection against this variant (was posted in the thread):
> > >
> > >               #define untagged_addr(mm, addr) ({                                      \
> > >                       u64 __addr = (__force u64)(addr);                               \
> > >                       s64 sign = (s64)__addr >> 63;                                   \
> > >                       __addr &= (mm)->context.untag_mask | sign;                      \
> > >                       (__force __typeof__(addr))__addr;                               \
> > >               })
> > >
> > > ?
> >
> > Yeah, I suppose that should work fine.
> 
> Won't the sign bit be put at the wrong place?

sign is either 0 or ~0, by or-ing ~0 into the mask, the masking becomes
no-op.