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Message-ID: <20140403163219.GA4882@linux.vnet.ibm.com>
Date: Thu, 3 Apr 2014 22:02:19 +0530
From: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Ingo Molnar <mingo@...e.hu>,
Ananth N Mavinakayanahalli <ananth@...ibm.com>,
David Long <dave.long@...aro.org>,
Denys Vlasenko <dvlasenk@...hat.com>,
"Frank Ch. Eigler" <fche@...hat.com>,
Jim Keniston <jkenisto@...ibm.com>,
Jonathan Lebon <jlebon@...hat.com>,
Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] uprobes: Kill UPROBE_SKIP_SSTEP and can_skip_sstep()
> UPROBE_COPY_INSN, UPROBE_SKIP_SSTEP, and uprobe->flags must die. This
> patch kills UPROBE_SKIP_SSTEP. I never understood why it was added;
> not only it doesn't help, it harms.
>
> It can only help to avoid arch_uprobe_skip_sstep() if it was already
> called before and failed. But this is ugly, if we want to know whether
> we can emulate this instruction or not we should do this analysis in
> arch_uprobe_analyze_insn(), not when we hit this probe for the first
> time.
>
> And in fact this logic is simply wrong. arch_uprobe_skip_sstep() can
> fail or not depending on the task/register state, if this insn can be
> emulated but, say, put_user() fails we need to xol it this time, but
> this doesn't mean we shouldn't try to emulate it when this or another
> thread hist this bp next time.
>
> And this is the actual reason for this change. We need to emulate the
> "call" insn, but push(return-address) can obviously fail.
>
> Per-arch notes:
>
> x86: __skip_sstep() can only emulate "rep;nop". With this
> change it will be called every time and most probably
> for no reason.
>
> This will be fixed by the next changes. We need to
> change this suboptimal code anyway.
>
> arm: Should not be affected. It has its own "bool simulate"
> flag checked in arch_uprobe_skip_sstep().
>
> ppc: Looks like, it can emulate almost everything. Does it
> actually needs to record the fact that emulate_step()
> failed? Hopefully not. But if yes, it can add the ppc-
> specific flag into arch_uprobe.
>
> TODO: rename arch_uprobe_skip_sstep() to arch_uprobe_emulate_insn(),
>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Acked-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
--
Thanks and Regards
Srikar Dronamraju
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