Messages in this thread | | | Date | Wed, 01 Apr 2009 13:40:24 -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: > >> 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?
Yeah, I'd rather add a safety checker in kprobes-x86 itself, because sometimes it has to fixup instructions modified by previous kprobes.
Thanks,
>>> 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
-- Masami Hiramatsu
Software Engineer Hitachi Computer Products (America) Inc. Software Solutions Division
e-mail: mhiramat@redhat.com
| |