Re: [PATCH v2] arm64: kprobe: Enable OPTPROBE for arm64

[liuqi (BA)]

Hi Masami,

On 2021/8/5 9:54, Masami Hiramatsu wrote:
> On Wed, 4 Aug 2021 14:02:09 +0800
> Qi Liu <liuqi115@xxxxxxxxxx> wrote:
>
> > This patch introduce optprobe for ARM64. In optprobe, probed
> > instruction is replaced by a branch instruction to detour
> > buffer. Detour buffer contains trampoline code and a call to
> > optimized_callback(). optimized_callback() calls opt_pre_handler()
> > to execute kprobe handler.
> >
> > Limitations:
> > - We only support !CONFIG_RANDOMIZE_MODULE_REGION_FULL case to
> > guarantee the offset between probe point and kprobe pre_handler
> > is not larger than 128MiB.
> >
> > Performance of optprobe on Hip08 platform is test using kprobe
> > example module[1] to analyze the latency of a kernel function,
> > and here is the result:
> >
> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/samples/kprobes/kretprobe_example.c
> >
> > kprobe before optimized:
> > [280709.846380] do_empty returned 0 and took 1530 ns to execute
> > [280709.852057] do_empty returned 0 and took 550 ns to execute
> > [280709.857631] do_empty returned 0 and took 440 ns to execute
> > [280709.863215] do_empty returned 0 and took 380 ns to execute
> > [280709.868787] do_empty returned 0 and took 360 ns to execute
> > [280709.874362] do_empty returned 0 and took 340 ns to execute
> > [280709.879936] do_empty returned 0 and took 320 ns to execute
> > [280709.885505] do_empty returned 0 and took 300 ns to execute
> > [280709.891075] do_empty returned 0 and took 280 ns to execute
> > [280709.896646] do_empty returned 0 and took 290 ns to execute
> > [280709.902220] do_empty returned 0 and took 290 ns to execute
> > [280709.907807] do_empty returned 0 and took 290 ns to execute
> >
> > optprobe:
> > [ 2965.964572] do_empty returned 0 and took 90 ns to execute
> > [ 2965.969952] do_empty returned 0 and took 80 ns to execute
> > [ 2965.975332] do_empty returned 0 and took 70 ns to execute
> > [ 2965.980714] do_empty returned 0 and took 60 ns to execute
> > [ 2965.986128] do_empty returned 0 and took 80 ns to execute
> > [ 2965.991507] do_empty returned 0 and took 70 ns to execute
> > [ 2965.996884] do_empty returned 0 and took 70 ns to execute
> > [ 2966.002262] do_empty returned 0 and took 80 ns to execute
> > [ 2966.007642] do_empty returned 0 and took 70 ns to execute
> > [ 2966.013020] do_empty returned 0 and took 70 ns to execute
> > [ 2966.018400] do_empty returned 0 and took 70 ns to execute
> > [ 2966.023779] do_empty returned 0 and took 70 ns to execute
> > [ 2966.029158] do_empty returned 0 and took 70 ns to execute
> >
> > Signed-off-by: Qi Liu <liuqi115@xxxxxxxxxx>
> >
> > ---
> >
> > Changes since V1:
> > - Address the comments from Masami, checks for all branch instructions, and
> > use aarch64_insn_patch_text_nosync() instead of aarch64_insn_patch_text()
> > in each probe.
>
> Is it safe for the multicore system? If it is safe because it modifies
> just one instruction (modifying 32bit in atomic), I understand it.

Seems raw_spin_lock_irqsave is used in aarch64_insn_patch_text_nosync() 
and spinlock could support a protection in multicore system.
> BTW, anyway, you should use _nosync() variant in arch_prepare_optimized_kprobe()
> too, beacause the optprobe insn buffer is not touched until the probed instruction
> is optimized by br.
Yes, sounds resonable.
> [...]
> > +int arch_prepare_optimized_kprobe(struct optimized_kprobe *op, struct kprobe *orig)
> > +{
> > +	kprobe_opcode_t *code;
> > +	u32 insn;
> > +	int ret, i;
> > +	void *addrs[TMPL_END_IDX];
> > +	void *addr;
> > +
> > +	code = get_optinsn_slot();
> > +	if (!code)
> > +		return -ENOMEM;
> > +
> > +	if (!is_offset_in_range((unsigned long)code,
> > +				(unsigned long)orig->addr + 8))
> > +		goto error;
> > +
> > +	if (!is_offset_in_range((unsigned long)code + TMPL_CALL_BACK,
> > +				(unsigned long)optimized_callback))
> > +		goto error;
> > +
> > +	if (!is_offset_in_range((unsigned long)&code[TMPL_RESTORE_END],
> > +				(unsigned long)op->kp.addr + 4))
> > +		goto error;
> > +
> > +	/* Setup template */
> > +	for (i = 0; i < TMPL_END_IDX; i++)
> > +		addrs[i] = code + i;
> > +
> > +	ret = aarch64_insn_patch_text(addrs, optprobe_template_entry,
> > +				      TMPL_END_IDX);
>
> You should use aarch64_insn_patch_text_nosync() here (and all the
> aarch64_insn_patch_text() in this function too), because the insn
> buffer must not executed until the probe point is optimized.
aarch64_insn_patch_text() could patch multi instructions to code[] each 
time and aarch64_insn_patch_text_nosync() could only patch one 
instruction each time, so maybe aarch64_insn_patch_text() is better here.

I'll replace other aarch64_insn_patch_text() in this function.

Thanks,
Qi

> > +	if (ret < 0)
> > +		goto error;
> > +
> > +	/* Set probe information */
> > +	addr = code + TMPL_VAL_IDX;
> > +	insn =  (unsigned long long)op & 0xffffffff;
> > +	aarch64_insn_patch_text(&addr, &insn, 1);
> > +
> > +	addr = addr + 4;
> > +	insn = ((unsigned long long)op & GENMASK_ULL(63, 32)) >> 32;
> > +	aarch64_insn_patch_text(&addr, &insn, 1);
> > +
> > +	addr = code + TMPL_CALL_BACK;
> > +	insn =  aarch64_insn_gen_branch_imm((unsigned long)addr,
> > +				(unsigned long)optimized_callback,
> > +				AARCH64_INSN_BRANCH_LINK);
> > +	aarch64_insn_patch_text(&addr, &insn, 1);
> > +
> > +	/* The original probed instruction */
> > +	addr = code + TMPL_RESTORE_ORIGN_INSN;
> > +	insn =  orig->opcode;
> > +	aarch64_insn_patch_text(&addr, &insn, 1);
> > +
> > +	/* Jump back to next instruction */
> > +	addr = code + TMPL_RESTORE_END;
> > +	insn = aarch64_insn_gen_branch_imm(
> > +				(unsigned long)(&code[TMPL_RESTORE_END]),
> > +				(unsigned long)(op->kp.addr) + 4,
> > +				AARCH64_INSN_BRANCH_NOLINK);
> > +	aarch64_insn_patch_text(&addr, &insn, 1);
> > +
> > +	flush_icache_range((unsigned long)code,
> > +			   (unsigned long)(&code[TMPL_END_IDX]));
> > +	/* Set op->optinsn.insn means prepared. */
> > +	op->optinsn.insn = code;
> > +
> > +	return 0;
>
> Thank you,