Messages in this thread | | | Subject | Re: [PATCH v4 2/2] arm64: kprobe: Enable OPTPROBE for arm64 | From | "liuqi (BA)" <> | Date | Tue, 30 Nov 2021 14:48:06 +0800 |
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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
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