Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate instructions

[Peter Zijlstra]

On Tue, Jun 11, 2019 at 11:22:54AM -0400, Steven Rostedt wrote:
> On Tue, 11 Jun 2019 10:03:07 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> 
> > So what happens is that arch_prepare_optimized_kprobe() <-
> > copy_optimized_instructions() copies however much of the instruction
> > stream is required such that we can overwrite the instruction at @addr
> > with a 5 byte jump.
> > 
> > arch_optimize_kprobe() then does the text_poke_bp() that replaces the
> > instruction @addr with int3, copies the rel jump address and overwrites
> > the int3 with jmp.
> > 
> > And I'm thinking the problem is with something like:
> > 
> > @addr: nop nop nop nop nop
> 
> What would work would be to:
> 
> 	add breakpoint to first opcode.
> 
> 	call synchronize_tasks();
> 
> 	/* All tasks now hitting breakpoint and jumping over affected
> 	code */
> 
> 	update the rest of the instructions.
> 
> 	replace breakpoint with jmp.
> 
> One caveat is that the replaced instructions must not be a call
> function. As if the call function calls schedule then it will
> circumvent the synchronize_tasks(). It would be OK if that call is the
> last of the instructions. But I doubt we modify anything more then a
> call size anyway, so this should still work for all current instances.

Right, something like this could work (although I cannot currently find
synchronize_tasks), but it would make the optprobe stuff fairly slow
(iirc this sync_tasks() thing could be pretty horrible).