Messages in this thread | | | Subject | Re: [PATCH v4 2/2] arm64: kprobe: Enable OPTPROBE for arm64 | From | "liuqi (BA)" <> | Date | Wed, 1 Dec 2021 10:55:20 +0800 |
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On 2021/12/1 9:50, Masami Hiramatsu wrote: > 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 :( > Hi Masami,
> Hmm, OK, we need to introduce trampoline buffer allocation pool for modules > for such arch.
Yes, but will this expand the size of Image?
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. >
got it, I'll send a new patchset which support core-kernel optprobe soon. thanks a lot for your reply.
Qi > Thank you, > >> >> Thanks, >> Qi > >
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