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

[Denys Vlasenko]

On Tue, Feb 7, 2012 at 6:17 PM, Srikar Dronamraju
<srikar@xxxxxxxxxxxxxxxxxx> 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

        /* 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;

-- 
vda
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