Subject: Re: linux-next: add utrace tree

From: Ingo Molnar <[email-masked]>
Date: Sat, 23 Jan 2010 07:04:01 +0100

* Linus Torvalds <[email-masked]> wrote:

> On Thu, 21 Jan 2010, Frank Ch. Eigler wrote:
> 
> > Less passionate analysis would identify a long history of contribution by 
> > the the greater affiliated team, including via merged code and by and 
> > passing on requirements and experiences.
> 
> The reason I'm so passionate is that I dislike the turn the discussion was 
> taking, as if "utrace" was somehow _good_ because it allowed various other 
> interfaces to hide behind it. And I'm not at all convinced that is true.
> 
> And I really didn't want to single out system tap, I very much feel the same 
> way abotu some seccomp-replacement "security model that the kernel doesn't 
> even need to know about" thing.
> 
> So don't take the systemtap part to be the important part, it's the bigger 
> issue of "I'd much rather have explicit interfaces than have generic hooks 
> that people can then use in any random way".
> 
> I realize that my argument is very anti-thetical to the normal CS teaching 
> of "general-purpose is good". I often feel that very specific code with very 
> clearly defined (and limited) applicability is a good thing - I'd rather 
> have just a very specific ptrace layer that does nothing but ptrace, than a 
> "generic plugin layer that can be layered under ptrace and other things".

( I think to a certain degree it mirrors the STEAMS hooks situation from a 
  decade ago - and while there were big flamewars back then we never regretted 
  not taking the STREAMS opaque hooks upstream. )

> In one case, you know exactly what the users are, and what the semantics are 
> going to be. In the other, you don't.
> 
> So I really want to see a very big and immediate upside from utrace. Because 
> to me, the "it's a generic layer with any application you want to throw at 
> it" is a _downside_.

One component of the whole utrace/systemtap codebase that i think would make 
sense upstreaming in the near term is the concept of user-space probes. We are 
actively looking into it from a 'perf probe' angle, and PeterZ suggested a few 
ideas already. Allowing apps to transparently improve the standard set of 
events is a plus. (From a pure Linux point of view it's probably more 
important than any kernel-only instrumentation.)

Also, if any systemtap person is interested in helping us create a more 
generic filter engine out of the current ftrace filter engine (which is really 
a precursor of a safe, sandboxed in-kernel script engine), that would be 
excellent as well. Right now we support simple C-syntax expressions like:

   perf record -R -f -e irq:irq_handler_entry --filter 'irq==18 || irq==19'

More could be done - a simple C-like set of function perhaps - some minimal 
per probe local variable state, etc. (perhaps even looping as well, with a 
limit on number of predicament executions per filter invocation.)

( _Such_ a facility, could then perhaps be used to allow applications access 
  to safe syscall sandboxing techniques: i.e. a programmable seccomp concept 
  in essence, controlled via ASCII space filter expressions [broken down into
  predicaments for fast execution], syscall driven and inherited by child 
  tasks so that security restrictions percolate down automatically.

  IMHO that would be a superior concept for security modules too: there's no 
  reason why all the current somewhat opaque security hooks couldnt be 
  expressed in terms of more generic filter expressions, via a facility that
  can be used both for security and for instrumentation. That's all what 
  SELinux boils down to in the end: user-space injected policy rules. )

The opaque hookery all around the core kernel just to push everything outside 
of mainline is one of the biggest downsides of utrace/systemtap - and neither 
uprobes nor the concept of user-defined scripting around existing events is 
affected much by that.

So lots of work is left and all that work is going to be rather utilitarian 
with little downside: specific functionality with an immediately visible 
upside, with no need for opaque hooks.

	Ingo