Re: [PATCH v4 04/23] arm64: kprobes: Use generic kretprobe trampoline handler

[Masami Hiramatsu <mhiramat@kernel.org>]

On Fri, 28 Aug 2020 15:37:18 +0200
peterz@infradead.org wrote:

> On Fri, Aug 28, 2020 at 02:31:31PM +0100, Mark Rutland wrote:
> > Hi,
> > 
> > On Fri, Aug 28, 2020 at 09:27:22PM +0900, Masami Hiramatsu wrote:
> > > Use the generic kretprobe trampoline handler, and use the
> > > kernel_stack_pointer(regs) for framepointer verification.
> > > 
> > > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> > > ---
> > >  arch/arm64/kernel/probes/kprobes.c |   78 +-----------------------------------
> > >  1 file changed, 3 insertions(+), 75 deletions(-)
> > 
> > > +	return (void *)kretprobe_trampoline_handler(regs, &kretprobe_trampoline,
> > > +					(void *)kernel_stack_pointer(regs));
> > >  }
> > >  
> > >  void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
> > >  				      struct pt_regs *regs)
> > >  {
> > >  	ri->ret_addr = (kprobe_opcode_t *)regs->regs[30];
> > > +	ri->fp = (void *)kernel_stack_pointer(regs);
> > 
> > This is probably a nomenclature problem, but what exactly is
> > kretprobe_instance::fp used for?
> > 
> > I ask because arm64's frame pointer lives in x29 (and is not necessarily
> > the same as the stack pointer at function boundaries), so the naming
> > is potentially misleading and possibly worth a comment or rename.
> 
> IIUC ri->rp is used for matching up the frame between the moment we
> install the return trampoline on the stack and actually hitting that
> trampoline.

Yeah, it is confusing, sorry. On x86, CONFIG_FRAME_POINTER can be disabled,
so we used the address of stack entry where arch_prepare_kretprobe() stores
the trampoline address.
For arm64 which doesn't use a stack entry for call, I decided to use the
stack address when the target function is called.
Indeed, it should be commented.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>