Messages in this thread | | | Date | Wed, 1 Dec 2021 10:50:01 +0900 | From | Masami Hiramatsu <> | Subject | Re: [PATCH v4 2/2] arm64: kprobe: Enable OPTPROBE for arm64 |
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On Tue, 30 Nov 2021 14:48:06 +0800 "liuqi (BA)" <liuqi115@huawei.com> wrote:
> > > 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 :(
Hmm, OK, we need to introduce trampoline buffer allocation pool for modules for such arch. But that should be another story. I think you should start from the core-kernel. At this moment, if the probe address is in the module, please return -ERANGE from arch_prepare_optimized_kprobe(). Module support must be done in the next step (series), since that will involve the kprobes generic code change.
Thank you,
> > Thanks, > Qi
-- Masami Hiramatsu <mhiramat@kernel.org>
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