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    SubjectRe: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)
    Frederic Weisbecker wrote:
    > On Tue, Jan 19, 2010 at 09:47:45AM -0800, Jim Keniston wrote:
    >>> Do you have plans for a variant
    >>> that's completely in userspace?
    >> I don't know of any such plans, but I'd be interested to read more of
    >> your thoughts here. As I understand it, you've suggested replacing the
    >> probed instruction with a jump into an instrumentation vma (the XOL
    >> area, or something similar). Masami has demonstrated -- through his
    >> djprobes enhancement to kprobes -- that this can be done for many x86
    >> instructions.
    >> What does the code in the jumped-to vma do? Is the instrumentation code
    >> that corresponds to the uprobe handlers encoded in an ad hoc .so?
    > Once the instrumentation is requested by a process that is not the
    > instrumented one, this looks impossible to set a uprobe without a
    > minimal voluntary collaboration from the instrumented process
    > (events sent through IPC or whatever). So that looks too limited,
    > this is not anymore a true dynamic uprobe.

    Agreed. Since uprobe's handler must be running in kernel,
    we need to jump into kernel space anyway. "Booster" (just skips
    a single-stepping(trap) exception) may be useful for
    improving uprobe performance.

    And also as Andi said, using jump instead of int3 in userspace
    has 2GB address space limitation. It's not a problem for kernel
    inside, but a big problem in userspace.

    Thank you,

    Masami Hiramatsu

    Software Engineer
    Hitachi Computer Products (America), Inc.
    Software Solutions Division


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