Re: vvar, gup && coredump

[Oleg Nesterov <oleg@redhat.com>]

On 03/12, Andy Lutomirski wrote:
>
> On Thu, Mar 12, 2015 at 7:34 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > What is not clear: do we really want gup() to fail? Or it is not trivial
> > to turn __vvar_page into the "normal" page? (to simplify the discussion,
> > lets ignore hpet mapping for now).
>
> We could presumably fiddle with the vma to allow get_user_pages to
> work on at least the first vvar page.  There are some decently large
> caveats, though:
>
>  - We don't want to COW it.  If someone pokes at that page with
> ptrace, for example, and it gets COWed, everything will stop working
> because the offending process will no longer see updates.  That way
> lies infinite loops.

Of course, but this looks simple... is_cow_mapping() == F so FOLL_FORCE
won't work anyway?

>  - The implementation could be odd.  The vma is either VM_MIXEDMAP or
> VM_PFNMAP, and I don't see any practical way to change that.
>
>  - The HPET and perhaps pvclock stuff.  The HPET probably doesn't have
> a struct page at all, so you can't possibly get_user_pages it.

Yes, this is true. OK, lets not dump it. I'll probably send a patch which
changes vma_dump_size() to check VM_DONTDUMP first...

But this leads to another question: why do we want to expose this
"vvar" vma at all?

For the moment, forget about compat 32-bit applications running under
64-bit kernel.

Can't we simply add FIX_VVAR_PAGE into fixed_addresses{}, map it into
init_mm via set_fixmap(FIX_VVAR_PAGE, __PAGE_USER) and change __vdso.*
functions to use fix_to_virt() address?

I don't really understand the low-level details, I'd like to understand
if this can work or not. And if it can work, why this is undesirable.

As for 32-bit applications. Yes, this can't work because 32-bit simply
can't access this "high" memory. But you know, it would be very nice to
have the fixmap-like "global" area in init_mm which is also visible to
compat applications. If we had it, uprobes could work without xol vma's.

Oleg.