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

[Masami Hiramatsu]

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.
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.

[...]
> +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.

> +	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,



-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>