linux-kernel - Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap causes application-level breakage

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Message-ID: <CALYGNiNoVyH2YgtoXOxO-6TgVfaFraXdJEyFeHkfAJuQ+E0W+A@mail.gmail.com>
Date:	Thu, 30 Apr 2015 16:32:57 +0300
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mark Williamson <mwilliamson@...o-software.com>,
	Mark Seaborn <mseaborn@...omium.org>,
	kernel list <linux-kernel@...r.kernel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linux API <linux-api@...r.kernel.org>,
	Finn Grimwood <fgrimwood@...o-software.com>,
	Daniel James <djames@...o-software.com>
Subject: Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap
 causes application-level breakage

On Thu, Apr 30, 2015 at 4:22 PM, Kirill A. Shutemov
<kirill@...temov.name> wrote:
> On Thu, Apr 30, 2015 at 04:11:30PM +0300, Konstantin Khlebnikov wrote:
>> On Thu, Apr 30, 2015 at 2:43 PM, Konstantin Khlebnikov <koct9i@...il.com> wrote:
>> > On Thu, Apr 30, 2015 at 12:02 AM, Linus Torvalds
>> > <torvalds@...ux-foundation.org> wrote:
>> >> On Wed, Apr 29, 2015 at 1:44 PM, Konstantin Khlebnikov <koct9i@...il.com> wrote:
>> >>>
>> >>> This's no longer true. After recent fixes for "anon_vma endless growing" new vma
>> >>> might reuse old anon_vma from grandparent vma.
>> >>
>> >> Oh well. I guess that was too simple.
>> >>
>> >> If Mark is ok with the rule that "it's not reliably if you have two
>> >> nested forks" (ie it only works if you exec for every fork you do), it
>> >> should still work, right? It sounds like Mark doesn't necessarily need
>> >> to handle the *generic* case.
>> >
>> > What about exposing shared/exclusive bit in pagemap == 1 if
>> > page_mapcount() > 1, otherwise 0 (or vise versa).
>> >
>> > Seems like this should work for detecting CoWed pages in child mm.
>>
>> Something like this (see patch in attachment)
>
> THP is not covered.

Ok. Thanks.

>
> Any comments on kcmp() idea?

Should work too. Supporing full equal-less-greater semantics seems
safe -- it's obfuscation is strong enough for that.

>
> --
>  Kirill A. Shutemov
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