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From: Denys Vlasenko <vda.linux@googlemail.com>
Date: Wed, 8 Feb 2012 10:40:30 +0100
Message-ID: <CAK1hOcOd3hd31vZYw1yAVGK=gMV=vQotL1mRZkVgM=7M8mbMyw@mail.gmail.com>
Subject: Re: [PATCH v10 3.3-rc2 1/9] uprobes: Install and remove breakpoints.
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@elte.hu>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, Linux-mm <linux-mm@kvack.org>,
        Andi Kleen <andi@firstfloor.org>,
        Christoph Hellwig <hch@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Roland McGrath <roland@hack.frob.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Anton Arapov <anton@redhat.com>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Jim Keniston <jkenisto@linux.vnet.ibm.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>
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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

        /* 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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