On Fri, 10 May 2019 14:31:31 +0200 Peter Zijlstra peterz@infradead.org 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@infradead.org 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,