Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

[Bryan Donlan]

On Fri, Jan 15, 2010 at 7:58 PM, Jim Keniston <jkenisto@xxxxxxxxxx> wrote:

> 4. Emulation removes the need for the XOL area, but requires pretty much
> total knowledge of the instruction set.  It's also a performance win for
> architectures that can't do #3.  I see kvm implemented on 4
> architectures (ia64, powerpc, s390, x86).  Coincidentally, those are the
> architectures to which uprobes (old uprobes, with ubp and xol bundled
> in) has already been ported (though Intel hasn't been maintaining their
> ia64 port).  So it sort of comes down to how objectionable the XOL vma
> (or page) really is.

On x86 at least, wouldn't one option to be to run the instruction to
be emulated in CPL ('ring') 2, from a XOL page above the user-kernel
split, not accessible to userspace at CPL 3? Linux hasn't
traditionally used anything other than CPL 0 and CPL 3 (plus CPL 1 on
Xen), but it would seem to avoid many of the problems here - it's
invisible to normal userspace code and so doesn't pollute userspace
memory maps with kernel-private stuff, but since it's running at a
higher CPL than the kernel, we can still protect kernel memory and
protect against privileged instructions.
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