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

[Masami Hiramatsu]

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,

> 
> 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>