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    SubjectRe: Re: [PATCH v10 3.3-rc2 1/9] uprobes: Install and remove breakpoints.
    (2012/02/08 18:40), Denys Vlasenko wrote:
    > On Tue, Feb 7, 2012 at 6:17 PM, Srikar Dronamraju
    > <srikar@linux.vnet.ibm.com> wrote:
    >> Changelog: (Since v10): Add code to clear REX.B prefix pointed out by Denys Vlasenko
    >> and fix suggested by Masami Hiramatsu.
    > ...
    >> + /*
    >> + * 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);
    >> + if (insn->rex_prefix.nbytes)
    >> + *cursor &= 0xfe; /* Clearing REX.B bit */
    >
    > It looks like cursor points to mod/reg/rm byte, not rex byte.
    > Comment above says it too. You seem to be clearing a bit
    > in a wrong byte. I think it should be

    Right, but...

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

    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 */
    > }

    You have to move the cursor itself, since the .rex_prefix
    is just a copied data...

    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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