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    SubjectRe: [PATCH v10 3.3-rc2 1/9] uprobes: Install and remove breakpoints.
    (2012/02/09 15:37), Srikar Dronamraju wrote:
    >>
    >> No, that is a meaningless operation.
    >> As I originally said,
    >>
    >>> 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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