Messages in this thread | | | Date | Thu, 17 Jul 2008 16:26:34 -0400 | From | "Frank Ch. Eigler" <> | Subject | Re: [RFC] systemtap: begin the process of using proper kernel APIs (part1: use kprobe symbol_name/offset instead of address) |
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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.
> 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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