Re: [PATCH 2/4] x86/kprobes: Fix frame pointer annotations

[Masami Hiramatsu]

On Fri, 10 May 2019 14:31:31 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Fri, May 10, 2019 at 01:58:31PM +0900, Masami Hiramatsu wrote:
> > On Thu, 9 May 2019 19:14:16 +0200
> > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > > Ideally also the optimized kprobe trampoline, but I've not managed to
> > > fully comprehend that one.
> > 
> > As you pointed in other reply, save/restore can be a macro, but
> > each trampoline code is slightly different. Optprobe template has
> > below parts
> > 
> > (jumped from probed address)
> > [store regs]
> > [setup function arguments (pt_regs and probed address)]
> > [handler call]
> > [restore regs]
> > [execute copied instruction]
> 
>  instruction_s_ ?

Yes.

> 
> The JMP to this trampoline is likely 5 bytes and could have clobbered
> multiple instructions, we'd then have to place them all here, and
> 
> > [jump back to probed address]
> 
> jump to after whatever instructions were clobbered by the JMP.

Right!

> > Note that there is a limitation that if it is optiomized probe, user
> > handler can not change regs->ip. (we can not use "ret" after executed
> > a copied instruction, which must run on same stack)
> 
> Changing regs->ip in this case is going to be massively dodgy indeed :-)
> But so would changing much else; changing stack layout would also be
> somewhat tricky.

Yes, so the stack must be same after [restore regs].

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>