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

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Date:   Mon, 07 Aug 2017 10:59:51 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, mike.kravetz@...cle.com,
        linux-mm@...ck.org, fweimer@...hat.com, colm@...costs.net,
        akpm@...ux-foundation.org, keescook@...omium.org,
        luto@...capital.net, wad@...omium.org, mingo@...nel.org,
        kirill@...temov.name, dave.hansen@...el.com,
        linux-api@...r.kernel.org
Subject: Re: [PATCH v2 0/2] mm,fork,security: introduce MADV_WIPEONFORK

On Mon, 2017-08-07 at 15:46 +0200, Michal Hocko wrote:
> On Mon 07-08-17 15:22:57, Michal Hocko wrote:
> > This is an user visible API so make sure you CC linux-api (added)
> > 
> > On Sun 06-08-17 10:04:23, Rik van Riel wrote:
> > > 
> > > A further complication is the proliferation of clone flags,
> > > programs bypassing glibc's functions to call clone directly,
> > > and programs calling unshare, causing the glibc pthread_atfork
> > > hook to not get called.
> > > 
> > > It would be better to have the kernel take care of this
> > > automatically.
> > > 
> > > This is similar to the OpenBSD minherit syscall with
> > > MAP_INHERIT_ZERO:
> > > 
> > >     https://man.openbsd.org/minherit.2
> 
> I would argue that a MAP_$FOO flag would be more appropriate. Or do
> you
> see any cases where such a special mapping would need to change the
> semantic and inherit the content over the fork again?
> 
> I do not like the madvise because it is an advise and as such it can
> be
> ignored/not implemented and that shouldn't have any correctness
> effects
> on the child process.

Too late for that. VM_DONTFORK is already implemented
through MADV_DONTFORK & MADV_DOFORK, in a way that is
very similar to the MADV_WIPEONFORK from these patches.

I wonder if that was done because MAP_* flags are a
bitmap, with a very limited number of values as a result,
while MADV_* constants have an essentially unlimited
numerical namespace available.