Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK

[Rik van Riel <riel@redhat.com>]

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@redhat.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?

Calling MADV_WIPEONFORK on an other-than-anon vma results
in NOT getting VM_WIPEONFORK semantics on that VMA.

The code you are commenting on is the bit that CLEARS
the VM_WIPEONFORK code, not the bit where it gets set.