Re: [PATCH v6 02/10] x86, mpx: add MPX specific mmap interface

[Andy Lutomirski <luto@amacapital.net>]

On Thu, Jun 26, 2014 at 5:19 PM, Dave Hansen <dave.hansen@intel.com> wrote:
> On 06/26/2014 04:15 PM, Andy Lutomirski wrote:
>> So here's my mental image of how I might do this if I were doing it
>> entirely in userspace: I'd create a file or memfd for the bound tables
>> and another for the bound directory.  These files would be *huge*: the
>> bound directory file would be 2GB and the bounds table file would be
>> 2^48 bytes or whatever it is.  (Maybe even bigger?)
>>
>> Then I'd just map pieces of those files wherever they'd need to be,
>> and I'd make the mappings sparse.  I suspect that you don't actually
>> want a vma for each piece of bound table that gets mapped -- the space
>> of vmas could end up incredibly sparse.  So I'd at least map (in the
>> vma sense, not the pte sense) and entire bound table at a time.  And
>> I'd probably just map the bound directory in one big piece.
>>
>> Then I'd populate it in the fault handler.
>>
>> This is almost what the code is doing, I think, modulo the files.
>>
>> This has one killer problem: these mappings need to be private (cowed
>> on fork).  So memfd is no good.
>
> This essentially uses the page cache's radix tree as a parallel data
> structure in order to keep a vaddr->mpx_vma map.  That's not a bad idea,
> but it is a parallel data structure that does not handle copy-on-write
> very well.
>
> I'm pretty sure we need the semantics that anonymous memory provides.
>
>> There's got to be an easyish way to
>> modify the mm code to allow anonymous maps with vm_ops.  Maybe a new
>> mmap_region parameter or something?  Maybe even a special anon_vma,
>> but I don't really understand how those work.
>
> Yeah, we very well might end up having to go down that path.
>
>> Also, egads: what happens when a bound table entry is associated with
>> a MAP_SHARED page?
>
> Bounds table entries are for pointers.  Do we keep pointers inside of
> MAP_SHARED-mapped things? :)

Sure, if it's MAP_SHARED | MAP_ANONYMOUS.  For example:

struct thing {
  struct thing *next;
};

struct thing *storage = mmap(..., MAP_SHARED | MAP_ANONYMOUS, ...);
storage[0].next = &storage[1];
fork();

I'm not suggesting that this needs to *work* in the first incarnation of this :)

--Andy