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

[Peter Zijlstra]

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_ ?

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.

> 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.