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Subject: Re: [RFC] [PATCH 2.6.37-rc5-tip 10/20] 10: uprobes: task specific
 information.
From: Peter Zijlstra <peterz@infradead.org>
To: Josh Stone <jistone@redhat.com>
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Date: Tue, 25 Jan 2011 20:56:35 +0100
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On Tue, 2011-01-25 at 10:38 -0800, Josh Stone wrote:
> On 01/25/2011 05:56 AM, Peter Zijlstra wrote:
> > Ah, I think I found it while reading patch 13, you need the pre/post_xol
> > callbacks, can't you simply synthesize their effect into the replacement
> > sequence?
> > 
> >   push %rax
> >   mov $vaddr, %rax
> >   $INSN
> >   pop %rax
> >   jmp $next_insn
> > 
> > like replacements would obviate the need for the pre/post callbacks and
> > allow you to run straight through.
> 
> For this particular example, you'd better be sure that $INSN doesn't
> need %rsp intact.

Well, either that of fix up the %rsp offset, but yes I had not
considered this.

> Control flow in general also makes this challenging.  If $INSN is a
> call, then any inline fixups won't get a chance until after return.  If
> $INSN is a jump, then its target must be modified so that both taken and
> not-taken paths land in respective fixup locations.  I'm sure there are
> more cases that I'm not thinking of.

Right.

> > It would also remove the whole single-step need since they're proper
> > boosted probes.
> 
> Kprobes has boosting, but it doesn't apply to all opcodes.  I would
> guess that the same could be done for uprobes, where certain opcodes get
> a fixup sequence like you suggest, but the pre/post_xol mechanism is
> still needed in general.

Bummer..