Re: [RFC PATCH v4 24/27] mm/mmap: Create a guard area between VMAs

[Jann Horn <jannh@google.com>]

On Wed, Oct 3, 2018 at 6:32 PM Eugene Syromiatnikov <esyr@redhat.com> wrote:
> On Wed, Oct 03, 2018 at 09:00:04AM -0700, Yu-cheng Yu wrote:
> > On Tue, 2018-10-02 at 22:36 -0700, Andy Lutomirski wrote:
> > > On Tue, Oct 2, 2018 at 9:55 PM Eugene Syromiatnikov <esyr@redhat.com> wrote:
> > > >
> > > > On Fri, Sep 21, 2018 at 08:03:48AM -0700, Yu-cheng Yu wrote:
> > > > > Create a guard area between VMAs, to detect memory corruption.
> > > >
> > > > Do I understand correctly that with this patch a user space program
> > > > no longer be able to place two mappings back to back? If it is so,
> > > > it will likely break a lot of things; for example, it's a common ring
> > > > buffer implementations technique, to map buffer memory twice back
> > > > to back in order to avoid special handling of items wrapping its end.
> > >
> > > I haven't checked what the patch actually does, but it shouldn't have
> > > any affect on MAP_FIXED or the new no-replace MAP_FIXED variant.
> > >
> > > --Andy
> >
> > I did some mmap tests with/without MAP_FIXED, and it works as intended.
> > In addition to the ring buffer, are there other test cases?
>
> Right, after some more code reading I figured out that it indeed
> shouldn't affect MAP_FIXED, thank you for confirmation.
>
> I'm not sure, however, whether such a change that provides no ability
> to configure or affect it will go well with all the supported
> architectures.

Is there a concrete reason why you think an architecture might not
like this? As far as I can tell, the virtual address space overhead
should be insignificant even for 32-bit systems.