Messages in this thread | | | Date | Wed, 01 Apr 2009 10:09:08 -0400 | From | Masami Hiramatsu <> | Subject | Re: [PATCH -tip 0/4 V3] tracing: kprobe-based event tracer |
| |
Ingo Molnar wrote: > * Masami Hiramatsu <mhiramat@redhat.com> wrote: > >> Hi, >> >> Here are the patches of kprobe-based event tracer for x86, version >> 3. Since this feature seems to attract some developers, I'd like >> to push these basic patches into -tip tree so that they can easily >> play it. >> >> This version supports only x86(-32/-64) (If someone is interested >> in porting this to other architectures, I'd happy to help :)), and >> no respawn-able probe support (this would be better to push -mm >> tree.) >> >> This can be applied on the linux-2.6-tip tree. > > This bit: > >> Future items: >> - Check insertion point safety by using instruction decoder. > > is i believe a must-fix-before-merge item.
Hi Ingo,
I agreed. Fortunately, Jim Keniston and I wrote an x86 instruction decoder :-) which has been made originally for uprobe andd kprobes jump-optimizer.
https://www.redhat.com/archives/utrace-devel/2009-March/msg00031.html
> The functionality is genuinely useful, and if used dynamically on > the host it can be a lot more versatile and a lot more accessible > than a KGDB session - but code patching safety is a must-have. > > It does not have to be a full decoder, just a simplified decoding > run that starts from a known function-symbol address, and works its > way down in the function looking at instruction boundaries, and > figuring out whether the code patching is safe. If it sees anything > it cannot deal with it bails out.
Yeah, that is what I'll do.
> I suspect you could get very good practical results by supporting > just a small fraction of the x86 instruction set architecture. If > failures to insert a probe safely are printed out in clear terms: > > Could not insert probe at address 0xc01231234 due to: > Unknown instruction: 48 8d 15 db ff ff ff 00 00 00 > > People will fill in the missing ISA bits quickly :-) > > And people doing: > > asm(" .byte 0x00, 0x01, 0x02, 0x03;"); /* hehe, I broke the decoder! */ > > ... in kernel .text functions will be talked to in private :)
Aha, that function will get illegal instruction exception :-) even without kprobe.
> > So please lets do this now, it needs to happen.
Sure.
> Not having this was the main design failure of original kprobes, > this fragility is what isolated kprobes from the rest of the > instrumentation world and made it essentially a SystemTap-only > special. And this problem is fixable. > > It does not have to be a full solution, but it has to be a pretty > safe one. If it's safe and there are no showstopper objections from > others we can apply it to -tip > > Can you see any fundamental reason why this couldnt be done?
Nope, because we've done :-)
Thanks!
> > Thanks, > > Ingo
-- Masami Hiramatsu
Software Engineer Hitachi Computer Products (America) Inc. Software Solutions Division
e-mail: mhiramat@redhat.com
| |