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    SubjectRe: [PATCH -tip 0/4 V3] tracing: kprobe-based event tracer
    Ingo Molnar wrote:
    > * Masami Hiramatsu <> wrote:
    >> Ingo Molnar wrote:
    >>> * Masami Hiramatsu <> 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.
    > 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.


    >>> 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


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