linux-kernel - Re: [PATCH v5 3/3] kprobes: arm: enable OPTPROBES for ARM 32

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Date:	Thu, 28 Aug 2014 19:20:15 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Wang Nan <wangnan0@...wei.com>
Cc:	Russell King <linux@....linux.org.uk>,
	"David A. Long" <dave.long@...aro.org>,
	Jon Medhurst <tixy@...aro.org>,
	Taras Kondratiuk <taras.kondratiuk@...aro.org>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	Will Deacon <will.deacon@....com>,
	Pei Feiyue <peifeiyue@...wei.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/3] kprobes: arm: enable OPTPROBES for ARM 32

(2014/08/27 22:02), Wang Nan wrote:
> +/*
> + * ARM can always optimize an instruction when using ARM ISA.
> + */

Hmm, this comment looks not correct anymore :)

> +int arch_prepared_optinsn(struct arch_optimized_insn *optinsn)
> +{
> +	return optinsn->prepared;
> +}

BTW, why don't you check optinsn->insn != NULL ?
If it is not prepared for optimizing,  optinsn->insn always be NULL.

[...]
> +int arch_prepare_optimized_kprobe(struct optimized_kprobe *op)
> +{
> +	u8 *buf;
> +	unsigned long rel_chk;
> +	unsigned long val;
> +
> +	if (!can_optimize(op))
> +		return -EILSEQ;
> +
> +	op->optinsn.insn = get_optinsn_slot();
> +	if (!op->optinsn.insn)
> +		return -ENOMEM;
> +
> +	/*
> +	 * Verify if the address gap is in 32MiB range, because this uses
> +	 * a relative jump.
> +	 *
> +	 * kprobe opt use a 'b' instruction to branch to optinsn.insn.
> +	 * According to ARM manual, branch instruction is:
> +	 *
> +	 *   31  28 27           24 23             0
> +	 *  +------+---+---+---+---+----------------+
> +	 *  | cond | 1 | 0 | 1 | 0 |      imm24     |
> +	 *  +------+---+---+---+---+----------------+
> +	 *
> +	 * imm24 is a signed 24 bits integer. The real branch offset is computed
> +	 * by: imm32 = SignExtend(imm24:'00', 32);
> +	 *
> +	 * So the maximum forward branch should be:
> +	 *   (0x007fffff << 2) = 0x01fffffc =  0x1fffffc
> +	 * The maximum backword branch should be:
> +	 *   (0xff800000 << 2) = 0xfe000000 = -0x2000000
> +	 *
> +	 * We can simply check (rel & 0xfe000003):
> +	 *  if rel is positive, (rel & 0xfe000000) shoule be 0
> +	 *  if rel is negitive, (rel & 0xfe000000) should be 0xfe000000
> +	 *  the last '3' is used for alignment checking.
> +	 */
> +	rel_chk = (unsigned long)((long)op->optinsn.insn -
> +			(long)op->kp.addr + 8) & 0xfe000003;
> +
> +	if ((rel_chk != 0) && (rel_chk != 0xfe000000)) {
> +		__arch_remove_optimized_kprobe(op, 0);
> +		return -ERANGE;
> +	}
> +
> +	buf = (u8 *)op->optinsn.insn;
> +
> +	/* Copy arch-dep-instance from template */
> +	memcpy(buf, &optprobe_template_entry, TMPL_END_IDX);
> +
> +	/* Set probe information */
> +	val = (unsigned long)op;
> +	memcpy(buf + TMPL_VAL_IDX, &val, sizeof(val));
> +
> +	/* Set probe function call */
> +	val = (unsigned long)optimized_callback;
> +	memcpy(buf + TMPL_CALL_IDX, &val, sizeof(val));
> +
> +	flush_icache_range((unsigned long)buf,
> +			   (unsigned long)buf + TMPL_END_IDX);
> +
> +	op->optinsn.prepared = true;
> +	return 0;
> +}
> +

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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