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Subject: Re: [PATCH v2 11/17] selftests/x86/ldt_gdt: Prepare for access bit forced
From: Andy Lutomirski <luto@amacapital.net>
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Date: Thu, 14 Dec 2017 14:30:34 -0800
Cc: Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>,
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        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
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To: Linus Torvalds <torvalds@linux-foundation.org>
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> On Dec 14, 2017, at 2:15 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
>> On Thu, Dec 14, 2017 at 2:11 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> 
>> That seems to rather defeat the point of using a VMA, though.
> 
> There never was any point in using a VMA per se.
> 
> The point was always to just map the damn thing in the user page
> tables, wasn't it?
> 
> The vma bit was just an implementation detail.

And all this is why I dislike using a VMA.  My patch puts it at a negative address. We could just as easily put it just above TASK_SIZE_MAX, but I'm a bit nervous about bugs that overrun an access_ok check by a small amount.  IIRC I found one of those in the net code once, and I didn't look very hard.