linux-kernel - Re: [PATCH] module interface improvement for kprobes

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Date:	Sat, 5 Aug 2006 12:35:38 +0100
From:	Christoph Hellwig <hch@...radead.org>
To:	David Smith <dsmith@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, rusty@...tcorp.com.au,
	prasanna@...ibm.com, ananth@...ibm.com,
	anil.s.keshavamurthy@...el.com, davem@...emloft.net
Subject: Re: [PATCH] module interface improvement for kprobes

On Fri, Aug 04, 2006 at 01:30:39PM -0500, David Smith wrote:
> Why shouldn't I put a probe into a module other than at a symbol I can
> find with kallsyms?  For example, I'm interested when a particular
> module hits an error condition that occurs.  I don't want to probe how
> many times the function gets called - just when the error condition
> occurs.

How do you find that offset?  You'll probably mention the S-Word but
we really want something that works with the latest kernel, not just
the vendor trees.

> With the existing interface, if I use kallsysms to find the value of a
> symbol, the module can be unloaded between the time I use kallsyms and
> register the kprobe.  The patch I included fixes that race condition by
> incrementing the module reference count.

Yes, and that's a good thing.  But the interface for doing it is wrong.
You don't really want the users to do all that by itself.  For the typical
case of putting a probe at the usual points you want an interface that
puts in the probe given a name and does the right thing for you.  For example
the interface I proposed in my last mail.  Adding another field to struct
kprobe to specify an offset into the symbol would be the logical extension
of that.

> Your example works for a very small number of symbols, but with a large
> number it could take a long time to register the kprobes.  Plus, that
> would need to be done every time the kprobe was registered.  With my
> patch, the symbol lookup can be done once, then all those symbols can be
> turned into offsets from the base address of the module.

Registering a kprobe is everything but a fastpath, and you definitly should
not have a lot of probes anyway.  It's far more worthwhile to have a sane
interface that the user can't get wrong then a small speedup in something
that's not a fastpath.  I think Rusty even has a paper or talk about why
this is absolutely nessecary :)
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