Re: [PATCH 0/5] Volatile Ranges (v12) & LSF-MM discussion fodder

From: Jan Kara
Date: Wed Apr 02 2014 - 18:44:57 EST

On Wed 02-04-14 13:13:34, John Stultz wrote:
> On 04/02/2014 12:47 PM, Johannes Weiner wrote:
> > On Wed, Apr 02, 2014 at 12:01:00PM -0700, John Stultz wrote:
> >> On Wed, Apr 2, 2014 at 10:58 AM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> >>> On Wed, Apr 02, 2014 at 10:40:16AM -0700, John Stultz wrote:
> >>>> That point beside, I think the other problem with the page-cleaning
> >>>> volatility approach is that there are other awkward side effects. For
> >>>> example: Say an application marks a range as volatile. One page in the
> >>>> range is then purged. The application, due to a bug or otherwise,
> >>>> reads the volatile range. This causes the page to be zero-filled in,
> >>>> and the application silently uses the corrupted data (which isn't
> >>>> great). More problematic though, is that by faulting the page in,
> >>>> they've in effect lost the purge state for that page. When the
> >>>> application then goes to mark the range as non-volatile, all pages are
> >>>> present, so we'd return that no pages were purged. From an
> >>>> application perspective this is pretty ugly.
> >>>>
> >>>> Johannes: Any thoughts on this potential issue with your proposal? Am
> >>>> I missing something else?
> >>> No, this is accurate. However, I don't really see how this is
> >>> different than any other use-after-free bug. If you access malloc
> >>> memory after free(), you might receive a SIGSEGV, you might see random
> >>> data, you might corrupt somebody else's data. This certainly isn't
> >>> nice, but it's not exactly new behavior, is it?
> >> The part that troubles me is that I see the purged state as kernel
> >> data being corrupted by userland in this case. The kernel will tell
> >> userspace that no pages were purged, even though they were. Only
> >> because userspace made an errant read of a page, and got garbage data
> >> back.
> > That sounds overly dramatic to me. First of all, this data still
> > reflects accurately the actions of userspace in this situation. And
> > secondly, the kernel does not rely on this data to be meaningful from
> > a userspace perspective to function correctly.
> <insert dramatic-chipmunk video w/ text overlay "errant read corrupted
> volatile page purge state!!!!1">
> Maybe you're right, but I feel this is the sort of thing application
> developers would be surprised and annoyed by.
> > It's really nothing but a use-after-free bug that has consequences for
> > no-one but the faulty application. The thing that IS new is that even
> > a read is enough to corrupt your data in this case.
> >
> > MADV_REVIVE could return 0 if all pages in the specified range were
> > present, -Esomething if otherwise. That would be semantically sound
> > even if userspace messes up.
> So its semantically more of just a combined mincore+dirty operation..
> and nothing more?
> What are other folks thinking about this? Although I don't particularly
> like it, I probably could go along with Johannes' approach, forgoing
> SIGBUS for zero-fill and adapting the semantics that are in my mind a
> bit stranger. This would allow for ashmem-like style behavior w/ the
> additional write-clears-volatile-state and read-clears-purged-state
> constraints (which I don't think would be problematic for Android, but
> am not totally sure).
> But I do worry that these semantics are easier for kernel-mm-developers
> to grasp, but are much much harder for application developers to
> understand.
Yeah, I have to admit that although the simplicity of the implementation
looks compelling, the interface from a userspace POV looks weird.

Jan Kara <jack@xxxxxxx>
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