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

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Date:   Thu, 17 Aug 2017 15:50:33 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Rik van Riel <riel@...hat.com>
Cc:     linux-kernel@...r.kernel.org, mhocko@...nel.org,
        mike.kravetz@...cle.com, linux-mm@...ck.org, fweimer@...hat.com,
        colm@...costs.net, keescook@...omium.org, luto@...capital.net,
        wad@...omium.org, mingo@...nel.org, kirill@...temov.name,
        dave.hansen@...el.com, linux-api@...r.kernel.org,
        torvalds@...ux-foundation.org, willy@...radead.org
Subject: Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK

On Tue, 15 Aug 2017 22:18:19 -0400 Rik van Riel <riel@...hat.com> wrote:

> > > --- a/mm/madvise.c
> > > +++ b/mm/madvise.c
> > > @@ -80,6 +80,17 @@ static long madvise_behavior(struct
> > > vm_area_struct *vma,
> > > __		}
> > > __		new_flags &= ~VM_DONTCOPY;
> > > __		break;
> > > +	case MADV_WIPEONFORK:
> > > +		/* MADV_WIPEONFORK is only supported on anonymous
> > > memory. */
> > > +		if (vma->vm_file || vma->vm_flags & VM_SHARED) {
> > > +			error = -EINVAL;
> > > +			goto out;
> > > +		}
> > > +		new_flags |= VM_WIPEONFORK;
> > > +		break;
> > > +	case MADV_KEEPONFORK:
> > > +		new_flags &= ~VM_WIPEONFORK;
> > > +		break;
> > > __	case MADV_DONTDUMP:
> > > __		new_flags |= VM_DONTDUMP;
> > > __		break;
> > 
> > It seems odd to permit MADV_KEEPONFORK against other-than-anon vmas?
> 
> Given that the only way to set VM_WIPEONFORK is through
> MADV_WIPEONFORK, calling MADV_KEEPONFORK on an
> other-than-anon vma would be equivalent to a noop.
> 
> If new_flags == vma->vm_flags, madvise_behavior() will
> immediately exit.

Yes, but calling MADV_WIPEONFORK against an other-than-anon vma is
presumably a userspace bug.  A bug which will probably result in
userspace having WIPEONFORK memory which it didn't want.  The kernel
can trivially tell userspace that it has this bug so why not do so?