linux-kernel - Re: [PATCH v2 0/2] mm,fork,security: introduce MADV

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Message-ID: <6a04f59b-b72b-c468-ea5c-230764a24402@redhat.com>
Date:   Fri, 11 Aug 2017 17:24:29 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Colm MacCárthaigh <colm@...costs.net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Rik van Riel <riel@...hat.com>,
        Will Drewry <wad@...omium.org>, akpm@...ux-foundation.org,
        dave.hansen@...el.com, kirill@...temov.name,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, luto@...capital.net, mingo@...nel.org
Subject: Re: [PATCH v2 0/2] mm,fork,security: introduce MADV_WIPEONFORK

On 08/11/2017 04:24 PM, Michal Hocko wrote:
> On Fri 11-08-17 16:11:44, Florian Weimer wrote:
>> On 08/11/2017 04:06 PM, Michal Hocko wrote:
>>
>>> I am sorry to look too insisting here (I have still hard time to reconcile
>>> myself with the madvise (ab)use) but if we in fact want minherit like
>>> interface why don't we simply add minherit and make the code which wants
>>> to use that interface easier to port? Is the only reason that hooking
>>> into madvise is less code? If yes is that a sufficient reason to justify
>>> the (ab)use of madvise? If there is a general consensus on that part I
>>> will shut up and won't object anymore. Arguably MADV_DONTFORK would fit
>>> into minherit API better as well.
>>
>> It does, OpenBSD calls it MAP_INHERIT_NONE.
>>
>> Could you implement MAP_INHERIT_COPY and MAP_INHERIT_SHARE as well?  Or
>> is changing from MAP_SHARED to MAP_PRIVATE and back impossible?
> 
> I haven't explored those two very much. Their semantic seems rather
> awkward, especially map_inherit_share one. I guess MAP_INHERIT_COPY
> would be doable. Do we have to support all modes or a missing support
> would disqualify the syscall completely?

I think it would be a bit awkward if we implemented MAP_INHERIT_ZERO and
it would not turn a shared mapping into a private mapping in the child,
or would not work on shared mappings at all, or deviate in any way from
the OpenBSD implementation.

MAP_INHERIT_SHARE for a MAP_PRIVATE mapping which has been modified is a
bit bizarre, and I don't know how OpenBSD implements any of this.  It
could well be that the exact behavior implemented in OpenBSD is a poor
fit for the Linux VM implementation.

Florian