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    SubjectRe: [PATCH] Subject: kprobes-x86: correct post-eip value in post_hander()
    Ananth N Mavinakayanahalli wrote:
    > On Mon, Mar 17, 2008 at 12:59:05PM +0200, Yakov Lerner wrote:
    >> On Mon, Mar 17, 2008 at 7:19 AM, Ananth N Mavinakayanahalli
    >> <> wrote:
    >>> On Sun, Mar 16, 2008 at 03:21:21AM -0500, Yakov Lerner wrote:
    >>> >
    >>> > I was trying to get the address of instruction to be executed
    >>> > next after the kprobed instruction. But regs->eip in post_handler()
    >>> > contains value which is useless to the user. It's pre-corrected value.
    >>> > This value is difficult to use without access to resume_execution(), which
    >>> > is not exported anyway.
    >>> > I moved the invocation of post_handler() to *after* resume_execution().
    >>> > Now regs->eip contains meaningful value in post_handler().
    >>> >
    >>> > I do not think this change breaks any backward-compatibility.
    >>> > To make meaning of the old value, post_handler() would need access to
    >>> > resume_execution() which is not exported. I have difficulty to believe
    >>> > that previous, uncorrected, regs->eip can be meaningfully used in
    >>> > post_handler().
    >>> resume_execution() exists not just for the program counter fixups after
    >>> out-of-line singlestepping, but is also as an insurance to put the
    >>> program counter back to the correct address in case the user's
    >>> post_handler() mucks around with it. That isn't possible with this
    >>> change :-(
    >> I see your point. This can be prevented by saving and restoring regs->ip
    >> around the post_handler() call, no ? Current code is beautiful. Saving and
    >> restoring regs->ip would make this place look ugly.
    >> Otoh, if the post_handler() wants to crash the kernel, it can do it
    >> in thousand ways, not just by trashing regs->ip, no ?
    > Of course, there still are other ways to shoot yourself in the foot with
    > the post_handler(), but, atleast for cases we can control, we need to do
    > the right thing.

    Ananth, I think we can not prevent it even if resume_execution() is called
    after post_handler, because resume_execution() refers reg->ip...:-(

    And Yakov, I think you might need to make a patchset against all arch which
    support kprobes, because this patch modifies expected behavior of kprobes
    only on x86.

    IMHO, Yakov's suggestion will be also good for resume_execution(), because
    it only has to clean up after expectable-single-stepping. (user code is
    unexpectable... we can not control all of that)


    Masami Hiramatsu

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


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