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

[Dave Hansen]

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? :)

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