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Re: [PATCH] Linux Kernel Markers
- From: Karim Yaghmour <karim at opersys dot com>
- To: Martin Bligh <mbligh at google dot com>
- Cc: "Frank Ch. Eigler" <fche at redhat dot com>, Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>, prasanna at in dot ibm dot com, Andrew Morton <akpm at osdl dot org>, Ingo Molnar <mingo at elte dot hu>, Mathieu Desnoyers <mathieu dot desnoyers at polymtl dot ca>, Paul Mundt <lethal at linux-sh dot org>, linux-kernel <linux-kernel at vger dot kernel dot org>, Jes Sorensen <jes at sgi dot com>, Tom Zanussi <zanussi at us dot ibm dot com>, Richard J Moore <richardj_moore at uk dot ibm dot com>, Michel Dagenais <michel dot dagenais at polymtl dot ca>, Christoph Hellwig <hch at infradead dot org>, Greg Kroah-Hartman <gregkh at suse dot de>, Thomas Gleixner <tglx at linutronix dot de>, William Cohen <wcohen at redhat dot com>, ltt-dev at shafik dot org, systemtap at sources dot redhat dot com, Alan Cox <alan at lxorguk dot ukuu dot org dot uk>
- Date: Wed, 20 Sep 2006 15:43:48 -0400
- Subject: Re: [PATCH] Linux Kernel Markers
- Organization: Opersys inc.
- References: <4510151B.5070304@google.com> <20060919093935.4ddcefc3.akpm@osdl.org> <45101DBA.7000901@google.com> <20060919063821.GB23836@in.ibm.com> <45102641.7000101@google.com> <20060919070516.GD23836@in.ibm.com> <451030A6.6040801@google.com> <45105B5E.9080107@opersys.com> <451141B1.40803@hitachi.com> <451178B0.9030205@opersys.com> <20060920180808.GI18646@redhat.com> <451186F2.3060702@google.com> <45118D63.8070604@opersys.com> <451194DA.40300@google.com>
- Reply-to: karim at opersys dot com
Martin Bligh wrote:
> Do we even need the filler padding? I thought we could insert kprobes
> at the beginning of any function without that ... it was only a
> requirement for mid-function (sometimes). If we copy the whole function,
> we don't even need that any more ...
>
> if kprobes can do it, I don't see why djprobes can't ... after all, it
> just seems to use kprobes to insert a jump, AFAICS.
I guess I must not be explaining myself properly.
The padding is for one purpose and one purpose only: having
a know-to-be-good location at the beginning of the
uninstrumented function for later using djprobes on. Once
you've got that, then you can indeed copy the entire
function and do whatever you want *without* using djprobes
or kprobes, but using direct calls.
If you don't have the padding, then you might yourself in
a case where you're replacing bytes from multiple instructions
where something somewhere may have an IP within the replaced
range. And to get around that you have to pull a few magic
tricks *and* make a few assumptions. But if you replace a
5 bytes instruction (or the equivalent as in Hiramatsu-san's
proposla) with another 5 bytes instruction, none of that is
needed and djprobes can be used *today* to do that.
Using this, you've got an arguably non-existent penalty
for the function with the filler and a very fast jump to
the instrumented function. The best of both worlds
actually.
Let me know if I'm still not being clear.
Karim