Re: [RFC] systemtap: begin the process of using proper kernel APIs (part1: use kprobe symbol_name/offset instead of address)

James Bottomley
2008-07-17T21:06:23Z

On Thu, 2008-07-17 at 16:26 -0400, Frank Ch. Eigler wrote:
> 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@sparkweed> 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.

It's not exactly wasted ... the calculations have to be done anyway for
modules.

> > 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.

Well, no, they're the head code. It's actually used in CPU boot and
tear down, one of the things it's useful to probe, I think.

> > 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?

Dangerous as in it's not necessarily part of the kernel linker scripts.
Some arches have it defined as a symbol, some have it as a linker script
definition ... that's why it's location is strange.

The point, really, is to remove some of the fragile dependencies between
systemtap and the kernel.

> > 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.

Then the definition needs altering. I can see that the industrial
customers aren't interested but kernel developers are ... a lot of
problems occur in the init sections.

I think you'll find that systemtap will run quite happily from a shell
in an initramfs before the init sections are discarded. Plus there's
always module init sections which can appear at any time.

James