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

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Date:	Thu, 30 Apr 2015 12:50:04 +0100
From:	Mark Williamson <mwilliamson@...o-software.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Konstantin Khlebnikov <koct9i@...il.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	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 Wed, Apr 29, 2015 at 10:02 PM, 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.

Yes, it sounds like that should be OK for us.  Our usecase is pretty
restricted, so we're a long way off requiring a generic solution.

Our code will always fork() a fresh child in which to monitor memory
changes.  We run the operations we're interested in, use pagemap to
figure out "what changed" (by comparing whether the pagemap_entry_t
values are different from their parent) and then throw away the child
process.

Currently our code does an entry-by-entry compare of pagemap, so
anything that exposes writes as a change to values in there would
allow us to run unmodified.  That would be really nice.  That said, I
think we'd still be OK to modify our own code too if we can find a
solution that would continue to function on older kernel releases,
-stable trees, etc.

Thanks,
Mark
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