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

[Masami Hiramatsu]

On Mon, 29 Nov 2021 09:40:30 +0800
"liuqi (BA)" <liuqi115@xxxxxxxxxx> wrote:

> 
> 
> On 2021/11/27 20:23, Masami Hiramatsu wrote:
> > On Fri, 26 Nov 2021 18:31:06 +0800
> > "liuqi (BA)" <liuqi115@xxxxxxxxxx> wrote:
> > 
> >>
> >>
> >> On 2021/8/24 18:50, Mark Rutland wrote:
> >>>> diff --git a/arch/arm64/kernel/probes/optprobe_trampoline.S b/arch/arm64/kernel/probes/optprobe_trampoline.S
> >>>> new file mode 100644
> >>>> index 000000000000..24d713d400cd
> >>>> --- /dev/null
> >>>> +++ b/arch/arm64/kernel/probes/optprobe_trampoline.S
> >>>> @@ -0,0 +1,37 @@
> >>>> +/* SPDX-License-Identifier: GPL-2.0 */
> >>>> +/*
> >>>> + * trampoline entry and return code for optprobes.
> >>>> + */
> >>>> +
> >>>> +#include <linux/linkage.h>
> >>>> +#include <asm/asm-offsets.h>
> >>>> +#include <asm/assembler.h>
> >>>> +
> >>>> +	.global optprobe_template_entry
> >>>> +optprobe_template_entry:
> >>> Please use SYM_*(); see arch/arm64/kernel/entry-ftrace.S for examples of
> >>> how to use that for trampolines.
> >>>
> >>> This should be:
> >>>
> >>> SYM_CODE_START(optprobe_template)
> >>>
> >> 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?

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")

Also, the trampoline can be minimized, since what we need is the
probed address (and the address of struct optprobe).
A single trampoline entry will do the following;

1. push lr and a victim register (here, x0)
2. load the address of optprobe to x0
3. call(br) common-optprobe asm code
4. pop lr and x0
5. jump back to (next to) the original place

Here the common-optprobe asm code does;

c1. push all registers on the stack (like save_all_base_regs) for making
  struct pt_regs.
c2. set the pt_regs address to x1.
c3. call optimized_callback()
c4. return

Since arm64 will emulate the probed instruction, we can do this.
(On the other hand, x86 needs to run the probed insn in trampoline
 code, it will do that between step 4 and 5)

The trampoline entry code is just 5 instructions (but may need an
immediate value (&optprobe) needs to be embedded).

Thank you,

> 
> Thanks,
> Qi
> 
> >>
> >> So how to reuse SYM_CODE_START  in this situation, does anyone has a
> >> good idea?
> >>
> >> Thanks,
> >> Qi
> >>> ... and note the matching end below.
> >>>
> >>>> +	sub sp, sp, #PT_REGS_SIZE
> >>>> +	save_all_base_regs
> >>>> +	/* Get parameters to optimized_callback() */
> >>>> +	ldr	x0, 1f
> >>>> +	mov	x1, sp
> >>>> +	/* Branch to optimized_callback() */
> >>>> +	.global optprobe_template_call
> >>>> +optprobe_template_call:
> >>> SYM_INNER_LABEL(optprobe_template_call, SYM_L_GLOBAL)
> >>>
> >>> ...and likewise for all the other labels.
> >>>
> >>>> +	nop
> >>>> +	restore_all_base_regs
> >>>> +	ldr lr, [sp, #S_LR]
> >>>> +        add sp, sp, #PT_REGS_SIZE
> >>>> +	.global optprobe_template_restore_orig_insn
> >>>> +optprobe_template_restore_orig_insn:
> >>>> +	nop
> >>>> +	.global optprobe_template_restore_end
> >>>> +optprobe_template_restore_end:
> >>>> +	nop
> >>>> +	.global optprobe_template_end
> >>>> +optprobe_template_end:
> >>>> +	.global optprobe_template_val
> >>>> +optprobe_template_val:
> >>>> +	1:	.long 0
> >>>> +		.long 0
> >>>> +	.global optprobe_template_max_length
> >>>> +optprobe_template_max_length:
> >>> SYM_INNER_LABEL(optprobe_template_end, SYM_L_GLOBAL)
> >>> SYM_CODE_END(optprobe_template)
> >>>
> >>> Thanks,
> >>> Mark.
> >>>
> >>>> -- 
> > 
> > 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>