linux-kernel - Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED

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Message-ID: <5b56c88e477d879e5a0e3c15627cb05901a812f4.camel@intel.com>
Date:   Fri, 10 Jun 2022 18:08:10 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "andreyknvl@...il.com" <andreyknvl@...il.com>,
        "kcc@...gle.com" <kcc@...gle.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>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "glider@...gle.com" <glider@...gle.com>
Subject: Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and
 ARCH_ENABLE_TAGGED_ADDR

On Fri, 2022-06-10 at 21:06 +0300, Kirill A. Shutemov wrote:
> On Fri, Jun 10, 2022 at 04:16:01PM +0000, Edgecombe, Rick P wrote:
> > On Fri, 2022-06-10 at 17:35 +0300, Kirill A. Shutemov wrote:
> > > +static int prctl_enable_tagged_addr(unsigned long nr_bits)
> > > +{
> > > +       struct mm_struct *mm = current->mm;
> > > +
> > > +       /* Already enabled? */
> > > +       if (mm->context.lam_cr3_mask)
> > > +               return -EBUSY;
> > > +
> > > +       /* LAM has to be enabled before spawning threads */
> > > +       if (get_nr_threads(current) > 1)
> > > +               return -EBUSY;
> > 
> > Does this work for vfork()? I guess the idea is that locking is not
> > needed below because there is only one thread with the MM, but with
> > vfork() another task could operate on the MM, call fork(), etc. I'm
> > not
> > sure...
> 
> I'm not sure I follow. vfork() blocks parent process until child exit
> or
> execve(). I don't see how it is a problem.

Oh yea, you're right.