(2012/02/09 15:37), Srikar Dronamraju wrote:

quoted

No, that is a meaningless operation.
As I originally said,

quoted

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

I am confused by why we need to call insn_get_length(insn) before
checking insn->rex_prefix.nbytes? Is it needed.

Ah, certainly, no, if the insn is already decoded.

Denys and Masami, can you please confirm if below is fine.


#ifdef CONFIG_X86_64
/*
 * If uprobe->insn doesn't use rip-relative addressing, return
 * immediately.  Otherwise, rewrite the instruction so that it accesses
 * its memory operand indirectly through a scratch register.  Set
 * uprobe->arch_info.fixups and uprobe->arch_info.rip_rela_target_address
 * accordingly.  (The contents of the scratch register will be saved
 * before we single-step the modified instruction, and restored
 * afterward.)
 *
 * We do this because a rip-relative instruction can access only a
 * relatively small area (+/- 2 GB from the instruction), and the XOL
 * area typically lies beyond that area.  At least for instructions
 * that store to memory, we can't execute the original instruction
 * and "fix things up" later, because the misdirected store could be
 * disastrous.
 *
 * Some useful facts about rip-relative instructions:
 * - There's always a modrm byte.
 * - There's never a SIB byte.
 * - The displacement is always 4 bytes.
 */
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;

Here, I think it is better to add a comment that
insn_rip_relative() decodes until modrm. :)

	/* Clear REX.b bit (extension of MODRM.rm field):
	 * we want to encode rax/rcx, not r8/r9.
	 */
	if (insn->rex_prefix.nbytes) {
		cursor = uprobe->insn + insn_offset_rex_prefix(insn);
		*cursor &= 0xfe;
	}

	/*
	 * 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;
}

Confirmed, this looks good to me ;)

Thanks!


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

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