Re: [PATCH v4 2/2] arm64: kprobe: Enable OPTPROBE for arm64

[liuqi (BA)]

On 2021/11/29 22:35, Masami Hiramatsu wrote:
> > > > > > Hi all,
> > > > > >
> > > > > > I meet a problem when I use SYM_CODE_START(optprobe_template) to replace
> > > > > > optprobe_template_entry.
> > > > > >
> > > > > > If SYM_CODE_START is used, all optprobe will share one trampoline space.
> > > > > > Under this circumstances, if user register two optprobes, trampoline
> > > > > > will be overwritten by the newer one, and this will cause kernel panic
> > > > > > when the old optprobe is trigger.
> > > > > Hm, this is curious, because the template should be copied to the
> > > > > trampoline buffer for each optprobe and be modified.
> > > > >
> > > > > > Using optprobe_template_entry will not have this problem, as each
> > > > > > optprobe has its own trampoline space (alloced in get_opinsn_slot()).
> > > > > Yes, it is designed to do so.
> > > > >
> > > > > Thank you,
> > > > Hi Masami,
> > > >
> > > > Thanks for your reply. But I also met a problem when using
> > > > get_opinsn_slot() to alloc trampoline buffer.
> > > >
> > > > As module_alloc(like x86) is used to alloc buffer, trampoline is in
> > > > module space, so if origin insn is in kernel space, the range between
> > > > origin insn and trampoline is out of 128M.
> > > >
> > > > As module PLT cannot used here, I have no idea to achieve long jump in
> > > > this situation. Do you have any good idea?
> > Hi Masami,
> >
> > Thanks so much for your reply.
> >
> > > One possible solution is to use pre-allocated trampoline space in
> > > the text area, as same as ppc64 does.
> > > (See arch/powerpc/kernel/optprobes_head.S, it embeds a space at "optinsn_slot")
> > I find something interesting in arch/powerpc/kernel/optprobes.c, it use
> > "optinsn_slot" as a public buffer, and use a static "insn_page_in_use"
> > to make sure there is only one optprobe in kernel.
> >
> > If we use this solution , users could only register one optprobe each
> > time. This will also be a limitation for users, what's your opinion
> > about this?
> No, that is just a memory area for pooling trampoline buffer. So optprobe
> can allocate the buffer from that area. Please see kernel/kprobes.c:344.
> optprobe allocates "insn_slot" from kprobe_optinsn_slots, which uses
> alloc_optinsn_page() to allocate the pool of slots.
>
> Thank you,

Hi,

Thanks for your reply, I test alloc_optinsn_page() and it does work well 
to alloc the pool of slots.

But when I tried to use module PLT, something seems wrong here.
Arm64 Module PLT in mod->arch.ftrace_trampolines is set in 
module_finalize, after that, mod->arch.ftrace_trampolines seems to be a 
read-only memory. But in arch_optimize_kprobes() we need to modify the 
destination of PLT (as each optprobe has its own trampoline buffer), if 
so, we cannot get rid of the 128M branch limit :(

Thanks,
Qi