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

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Date:	Thu, 17 Jul 2008 15:12:26 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	"Frank Ch. Eigler" <fche@...hat.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)

On Thu, 2008-07-17 at 14:30 -0400, Frank Ch. Eigler wrote:
> James Bottomley <James.Bottomley@...senPartnership.com> writes:
> 
> > [...]
> > Just by way of illustration, this is systemtap fixed up according to
> > suggestion number 1.  You can see now using your test case that we get:
> >
> > # probes
> > kernel.function("do_open@...block_dev.c:929") /* pc=<lookup_bdev+0x90> */ /* <- kernel.function("do_open") */
> > kernel.function("do_open@...nfsctl.c:24") /* pc=<sys_nfsservctl+0x6a> */ /* <- kernel.function("do_open") */
> > kernel.function("do_open@.../mqueue.c:642") /* pc=<sys_mq_unlink+0x130> */ /* <- kernel.function("do_open") */
> > [...]
> 
> 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:

jejb@...rkweed> nm vmlinux |egrep -w '_stext|_text'
ffffffff80209000 T _stext
ffffffff80200000 A _text

Since we can't do negative offsets, you've lost access to the symbols in
the sections that start before _stext.  Assuming you meant _text (which
is dangerous because it's a define in the kernel linker script and could
change).  Then you can't offset into other sections, like init sections
or modules.

James


James


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