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

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Date:   Fri, 18 Aug 2017 20:02:41 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
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 Fri, 2017-08-18 at 11:15 -0700, Andrew Morton wrote:
> On Fri, 18 Aug 2017 12:28:29 -0400 Rik van Riel <riel@...hat.com>
> wrote:
> 
> > On Thu, 2017-08-17 at 15:50 -0700, Andrew Morton wrote:
> > > 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?
> > 
> > Uh, what?
> > 
> 
> Braino.  I meant MADV_KEEPONFORK.  Calling MADV_KEEPONFORK against an
> other-than-anon vma is a presumptive userspace bug and the kernel
> should report that.

All MADV_KEEPONFORK does is clear the flag set by
MADV_WIPEONFORK. Since there is no way to set the
WIPEONFORK flag on other-than-anon VMAs, that means
MADV_KEEPONFORK is always a noop for those VMAs.

You remind me that I should send in a man page
addition, though, when this code gets sent to
Linus.