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

[Andi Kleen <andi@firstfloor.org>]

> Re: rewriting instructions that use rip-relative addressing.  We do that
> now.  See handle_riprel_insn() in patch #2.  (As far as we can tell, it
> works, but we'd appreciate your review of it.)

Yes, but how do you get within 2GB of it? Add lots of holes
in the address space? 

> The instruction decoder is used only during instruction analysis, while
> registering the probe -- i.e., in kernel space.

Registering the user probe? That means if there's a buffer overflow
in there it would be exploitable.

> > 
> > In general the trend has been also to make traps faster in the CPU, make 
> > sure you're not optimizing for some old CPU here.
> 
> I won't argue with that.  What Avi seems to be proposing buys us a
> speedup, but at the cost of increased complexity -- among other things,
> splitting the instrumentation code between user space (in the "XOL" area
> -- which would then be used for much more than XOL instruction slots)

You can't have a single XOL area, at least not if you want to support
shared libraries on 64bit & rip relative.

> and kernel space.  The splitting would presumably be handled by
> higher-level code -- SystemTap, perf, or whatever.  It's a neat idea,
> but it seems like a v2 kind of feature.

I'm not sure it can even work, unless you severly limited the allowed
instructions.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.