linux-kernel - Re: [PATCH v4 6/8] riscv/kprobe: Add code to check if kprobe can be optimized

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Date:   Mon, 07 Nov 2022 17:56:04 +0100
From:   Björn Töpel <bjorn@...nel.org>
To:     Chen Guokai <chenguokai17@...ls.ucas.ac.cn>,
        paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, rostedt@...dmis.org, mingo@...hat.com,
        sfr@...b.auug.org.au
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        liaochang1@...wei.com, Chen Guokai <chenguokai17@...ls.ucas.ac.cn>
Subject: Re: [PATCH v4 6/8] riscv/kprobe: Add code to check if kprobe can be
 optimized

Chen Guokai <chenguokai17@...ls.ucas.ac.cn> writes:

[...]

> diff --git a/arch/riscv/kernel/probes/opt.c b/arch/riscv/kernel/probes/opt.c
> index 6d23c843832e..876bec539554 100644
> --- a/arch/riscv/kernel/probes/opt.c
> +++ b/arch/riscv/kernel/probes/opt.c

[...]

>  /*
> - * If two free registers can be found at the beginning of both
> - * the start and the end of replaced code, it can be optimized
> - * Also, in-function jumps need to be checked to make sure that
> - * there is no jump to the second instruction to be replaced
> + * The kprobe can be optimized when no in-function jump reaches to the
> + * instructions replaced by optimized jump instructions(AUIPC/JALR).
>   */
>  static bool can_optimize(unsigned long paddr, struct optimized_kprobe *op)
>  {
> -	return false;
> +	int ret;
> +	unsigned long addr, size = 0, offset = 0;
> +	struct kprobe *kp = get_kprobe((kprobe_opcode_t *)paddr);
> +
> +	/*
> +	 * Skip optimization if kprobe has been disarmed or instrumented
> +	 * instruction support XOI.
> +	 */
> +	if (!kp || (riscv_probe_decode_insn(&kp->opcode, NULL) != INSN_GOOD))
> +		return false;
> +
> +	/*
> +	 * Find a instruction window large enough to contain a pair
> +	 * of AUIPC/JALR, and ensure each instruction in this window
> +	 * supports XOI.
> +	 */
> +	ret = search_copied_insn(paddr, op);
> +	if (ret)
> +		return false;
> +
> +	if (!kallsyms_lookup_size_offset(paddr, &size, &offset))
> +		return false;
> +
> +	/* Check there is enough space for relative jump(AUIPC/JALR) */
> +	if (size - offset <= op->optinsn.length)
> +		return false;
> +
> +	/*
> +	 * Decode instructions until function end, check any instruction
> +	 * don't jump into the window used to emit optprobe(AUIPC/JALR).
> +	 */

Don't the fixup tables need to be checked, similar to the x86 code?


Björn