linux-kernel - Re: [RFC, PATCHv1 16/28] x86/asm: remove __VIRTUAL_MASK

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Message-ID: <20161208192254.GB30380@node.shutemov.name>
Date:   Thu, 8 Dec 2016 22:22:54 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC, PATCHv1 16/28] x86/asm: remove __VIRTUAL_MASK_SHIFT==47
 assert

On Thu, Dec 08, 2016 at 10:39:57AM -0800, Andy Lutomirski wrote:
> On Thu, Dec 8, 2016 at 8:21 AM, Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> wrote:
> > We don't need it anymore. 17be0aec74fb ("x86/asm/entry/64: Implement
> > better check for canonical addresses") made canonical address check
> > generic wrt. address width.
> 
> This code existed in part to remind us that this needs very careful
> adjustment when the paging size becomes dynamic.  If you want to
> remove it, please add test cases to tools/testing/selftests/x86 that
> verify:
> 
> a. Either mmap(2^47-4096, ..., MAP_FIXED, ...) fails or that, if it
> succeeds and you put a syscall instruction at the very end, that
> invoking the syscall instruction there works.  The easiest way to do
> this may be to have the selftest literally have a page of text that
> has 4094 0xcc bytes and a syscall and to map that page or perhaps move
> it into place with mremap.  That will avoid annoying W^X userspace
> stuff from messing up the test.  You'll need to handle the signal when
> you fall off the end of the world after the syscall.
> 
> b. Ditto for the new highest possible userspace page.
> 
> c. Ditto for one page earlier to make sure that your test actually works.
> 
> d. For each possible maximum address, call raise(SIGUSR1) and, in the
> signal handler, change RIP to point to the first noncanonical address
> and RCX to match RIP.  Return and catch the resulting exception.  This
> may be easy to integrate into the sigreturn tests, and I can help with
> that.

Thanks, for hints.

I'll come back to you with testcases to verify that they are you wanted
to see.

-- 
 Kirill A. Shutemov