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From: Masami Hiramatsu <mhiramat@kernel.org>
To: "liuqi (BA)" <liuqi115@huawei.com>
Cc: Linuxarm <linuxarm@huawei.com>,
	Mark Rutland <mark.rutland@arm.com>, <catalin.marinas@arm.com>,
	<will@kernel.org>, <naveen.n.rao@linux.ibm.com>,
	<anil.s.keshavamurthy@intel.com>, <davem@davemloft.net>,
	<linux-arm-kernel@lists.infradead.org>,
	<song.bao.hua@hisilicon.com>, <prime.zeng@hisilicon.com>,
	<robin.murphy@arm.com>, <f.fangjian@huawei.com>,
	<linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 2/2] arm64: kprobe: Enable OPTPROBE for arm64
Date: Wed, 1 Dec 2021 10:50:01 +0900	[thread overview]
Message-ID: <20211201105001.5164f98ba783e7207df1229c@kernel.org> (raw)
In-Reply-To: <9fdb2ad3-539f-fb4d-7a96-c737fec967f7@huawei.com>

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>

  reply	other threads:[~2021-12-01  1:53 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-18  7:33 [PATCH v4 0/2] arm64: " Qi Liu
2021-08-18  7:33 ` [PATCH v4 1/2] Make save_all_base_regs and restore_all_base_regs as common macro Qi Liu
2021-08-18  7:33 ` [PATCH v4 2/2] arm64: kprobe: Enable OPTPROBE for arm64 Qi Liu
2021-08-18 16:27   ` Masami Hiramatsu
2021-08-24 10:50   ` Mark Rutland
2021-08-24 11:50     ` Barry Song
2021-08-24 12:11       ` Mark Rutland
2021-08-24 12:42         ` Barry Song
2021-08-25  2:13     ` Masami Hiramatsu
2021-08-25  3:12       ` Barry Song
2021-09-07  3:14     ` liuqi (BA)
2021-11-26 10:31     ` liuqi (BA)
2021-11-27 12:23       ` Masami Hiramatsu
2021-11-29  1:40         ` liuqi (BA)
2021-11-29  5:00           ` Masami Hiramatsu
2021-11-29  6:50             ` liuqi (BA)
2021-11-29 14:35               ` Masami Hiramatsu
2021-11-30  6:48                 ` liuqi (BA)
2021-12-01  1:50                   ` Masami Hiramatsu [this message]
2021-12-01  2:55                     ` liuqi (BA)

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