linux-kernel - Re: [RFC] systemtap: begin the process of using proper kernel APIs (part1: use kprobe symbol

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Message-ID: <20080717202634.GI18295@redhat.com>
Date:	Thu, 17 Jul 2008 16:26:34 -0400
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	systemtap@...rceware.org
Subject: Re: [RFC] systemtap: begin the process of using proper kernel APIs (part1: use kprobe symbol_name/offset instead of address)

Hi -

On Thu, Jul 17, 2008 at 03:12:26PM -0500, James Bottomley wrote:
> [...]
> > Can you explain in detail how you believe this is materially
> > different from offsetting from _stext?
> 
> Basically because _stext is an incredibly dangerous symbol; being linker
> generated it doesn't actually get put in the right place if you look:

Thank you for your response.

> jejb@...rkweed> nm vmlinux |egrep -w '_stext|_text'
> ffffffff80209000 T _stext
> ffffffff80200000 A _text
> 
> Since we can't do negative offsets

Actually, "we" as in systemtap could do it just fine if that were
desired.  And really _stext is therefore an arbitrary choice - it
could be any other reference.

My point is that the proposed effort to identify a nearby function
symbol to use as a base for each probe's symbol+offset calculation is
wasted.

> you've lost access to the symbols in the sections that start before _stext.  

What's between _text and _stext appears to consist of kernel boot-time
functions that are unmapped the time anything like systemtap could
run.

> Assuming you meant _text (which is dangerous because it's a define
> in the kernel linker script and could change).

By "dangerous" do you only mean that it may require a one-liner
catch-up patch in systemtap if the kernel linker scripts change?

> Then you can't offset into other sections, like init sections or
> modules.

Kernel init sections are unprobeable by definition, so that doesn't
matter.  Modules are also irrelevant, since their addresses are
relative to their relocation bases / sections, not to a kernel
(vmlinux) symbol.

- FChE
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