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

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Date:   Fri, 11 Aug 2017 17:31:52 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Florian Weimer <fweimer@...hat.com>
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 Fri 11-08-17 17:24:29, Florian Weimer wrote:
> 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.

If we go with minherit API then I think we should adhere with the BSD
semantic and alloc MAP_INHERIT_ZERO for shared mappings as well

> 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.

yeah, it would be MAP_INHERIT_SHARE that I would consider problematic
and rather go with ENOSUPP or even EINVAL.
-- 
Michal Hocko
SUSE Labs