|From:||Ingo Molnar <mingo-AT-elte.hu>|
|To:||Linus Torvalds <torvalds-AT-linux-foundation.org>, Steven Rostedt <rostedt-AT-goodmis.org>, Fr??d??ric Weisbecker <fweisbec-AT-gmail.com>, Arnaldo Carvalho de Melo <acme-AT-redhat.com>, Li Zefan|
|Subject:||Re: linux-next: add utrace tree|
|Date:||Sat, 23 Jan 2010 07:04:01 +0100|
|Cc:||"Frank Ch. Eigler" <fche-AT-redhat.com>, Andrew Morton <akpm-AT-linux-foundation.org>, Stephen Rothwell <sfr-AT-canb.auug.org.au>, Ananth N Mavinakayanahalli <ananth-AT-in.ibm.com>, Peter Zijlstra <a.p.zijlstra-AT-chello.nl>, Peter Zijlstra <peterz-AT-infradead.org>, Fr??d??ric Weisbecker <fweisbec-AT-gmail.com>, LKML <linux-kernel-AT-vger.kernel.org>, Steven Rostedt <rostedt-AT-goodmis.org>, Arnaldo Carvalho de Melo <acme-AT-redhat.com>, linux-next-AT-vger.kernel.org, "H. Peter Anvin" <hpa-AT-zytor.com>, utrace-devel-AT-redhat.com, Thomas Gleixner <tglx-AT-linutronix.de>|
* Linus Torvalds <firstname.lastname@example.org> 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
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