linux-kernel - Re: [PATCH v10 3.3-rc2 1/9] uprobes: Install and remove breakpoints.

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Date:	Fri, 03 Feb 2012 21:01:40 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Roland McGrath <roland@...k.frob.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Anton Arapov <anton@...hat.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH v10 3.3-rc2 1/9] uprobes: Install and remove breakpoints.

(2012/02/02 23:18), Srikar Dronamraju wrote:
> 
> Changelog: (Since v9) : Use insn_offset_modrm as suggested by Masami Hiramatsu.

Would you add REX.B clearing code to handle_riprel_insn() too?
Of course, that might not happen because it's a non-effective bit,
however user can program it and pass it to uprobes.

> +static void handle_riprel_insn(struct mm_struct *mm, struct uprobe *uprobe,
> +							struct insn *insn)
> +{
> +	u8 *cursor;
> +	u8 reg;
> +
> +	if (mm->context.ia32_compat)
> +		return;
> +
> +	uprobe->arch_info.rip_rela_target_address = 0x0;
> +	if (!insn_rip_relative(insn))
> +		return;
> +

So, here you need a REX.B clearing, like below.

insn_get_length(insn);
if (insn->rex_prefix.nbytes) {
	cursor = uprobe->insn + insn_offset_rex_prefix(insn);
	*cursor &= 0xfe;	/* Clearing REX.B bit */
}


> +	/*
> +	 * Point cursor at the modrm byte.  The next 4 bytes are the
> +	 * displacement.  Beyond the displacement, for some instructions,
> +	 * is the immediate operand.
> +	 */
> +	cursor = uprobe->insn + insn_offset_modrm(insn);
> +	insn_get_length(insn);
> +
> +	/*
> +	 * Convert from rip-relative addressing to indirect addressing
> +	 * via a scratch register.  Change the r/m field from 0x5 (%rip)
> +	 * to 0x0 (%rax) or 0x1 (%rcx), and squeeze out the offset field.
> +	 */
> +	reg = MODRM_REG(insn);
> +	if (reg == 0) {
> +		/*
> +		 * The register operand (if any) is either the A register
> +		 * (%rax, %eax, etc.) or (if the 0x4 bit is set in the
> +		 * REX prefix) %r8.  In any case, we know the C register
> +		 * is NOT the register operand, so we use %rcx (register
> +		 * #1) for the scratch register.
> +		 */
> +		uprobe->arch_info.fixups = UPROBES_FIX_RIP_CX;
> +		/* Change modrm from 00 000 101 to 00 000 001. */
> +		*cursor = 0x1;
> +	} else {
> +		/* Use %rax (register #0) for the scratch register. */
> +		uprobe->arch_info.fixups = UPROBES_FIX_RIP_AX;
> +		/* Change modrm from 00 xxx 101 to 00 xxx 000 */
> +		*cursor = (reg << 3);
> +	}
> +
> +	/* Target address = address of next instruction + (signed) offset */
> +	uprobe->arch_info.rip_rela_target_address = (long)insn->length
> +					+ insn->displacement.value;
> +	/* Displacement field is gone; slide immediate field (if any) over. */
> +	if (insn->immediate.nbytes) {
> +		cursor++;
> +		memmove(cursor, cursor + insn->displacement.nbytes,
> +						insn->immediate.nbytes);
> +	}
> +	return;
> +}

Thank you,

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