(2012/02/08 18:40), Denys Vlasenko wrote:

On Tue, Feb 7, 2012 at 6:17 PM, Srikar Dronamraju
[off-list ref] wrote:

quoted

Changelog: (Since v10): Add code to clear REX.B prefix pointed out by Denys Vlasenko
and fix suggested by Masami Hiramatsu.

...

quoted

+       /*
+        * 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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