Re: [PATCH -tip 0/4 V3] tracing: kprobe-based event tracer

[Ingo Molnar <mingo@elte.hu>]

* Masami Hiramatsu <mhiramat@redhat.com> wrote:

> 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

looks cool. Needs to be put somewhere in arch/x86/lib/, provided as 
a generic facility, with a Kconfig variable that says that the 
architecture supports it and then the kprobes-tracer could make 
immediate use of it, right?

> 
> > 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.

Not if it's under a never-true (not provable to the compiler) branch 
condition but i digress :)

> > Can you see any fundamental reason why this couldnt be done?
> 
> Nope, because we've done :-)

Cool :)

	Ingo