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



    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;

    /* 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;
    }



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