Re: 2.4 Ready list - Kernel Hooks

From: Karim Yaghmour (
Date: Wed Oct 23 2002 - 11:21:05 EST

Werner Almesberger wrote:
> Richard J Moore wrote:
> > This is nothing more than a call-back mechanism such as could be used by
> > LSM or LTT.
> Hmm, Greg has already voiced some violent disagreement regarding
> LSM :-) That leaves LTT. Given the more exploratory nature of LTT,
> I wonder if [dk]probes wouldn't be quite sufficient there, too.

The whole point of tracing is that the system's behavior should not
be modified but only recorded. Generating int3 won't do.

> Oh, you could probably have some "fast" probes by just checking
> for a certain "anchor" pattern (e.g. a sequence of 5 nops on
> i386), which could then be replaced with a direct call. This
> optimization would have to be optional, in case some code yields
> the anchor pattern such that it isn't also a basic block.

If I remember correctly, the optimized arch-dependent code in kernel
hooks uses "compare immediate" and the value of the immediate is
edited to enable/disable hooking. Given modern branch-prediction the
cost should be quite close to an unconditional jump.


                 Karim Yaghmour
      Embedded and Real-Time Linux Expert
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