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    SubjectDoes this work? "dcprobes" an x86-hack simple djprobes-equivalent?

    Richard, Hiramatsu-san,

    First, I _don't know if the following works_, I just got a
    hunch, and I thought I'd run this by you since you've both
    done extensive work on binary patching on x86. If this works,
    though, I think this would provide much of the same advantages
    as djprobes without any of its disadvantages. Note: this is
    very much an x86 hack, but my understanding is that djrobes'
    difficulties specifically stem from the variable-length
    instruction set found on the x86, so other archs should be
    easier to adapt.

    The problem I'm trying to solve:

    Since its inception, djprobes has run into quite a few problems
    in trying to replace arbitrary instruction spans because of
    the inherent problem of squashing an instruction to which
    someone/somewhere may sometime in the future directly jump
    (say insert 5 byte jump over a sequence of bytes having multiple
    instructions.) Solving this requires making quite a few
    assumptions about the program. Let's just say that, as I said
    elsewhere, I'm not too comfortable with the idea of letting
    djprobes replace any instruction that is less than 5 bytes --
    but as I explained elsewhere, it can be very useful in cases
    where the replaced instruction stream is tailored for being
    replaced by djprobes; see my earlier "bprobes" proposal.

    So here's what I got in mind:

    There is, though, one single case that I can think of where
    replacing a multi-byte span will just work (in as far as my
    current analysis allows me to see) without caring if anything
    anywhere will ever jump back into the span -- regardless of
    whether preemption is active or not. If fact, there would be
    no need to freeze any process or try to look up any process'
    stack while doing this.

    As you mentioned earlier Richard, we can just go ahead and
    stick an int3 almost anywhere. Now, the opcode for that is
    CC. So what if we tried this:
    - For address starting at A and spanning S bytes (where S cannot
    be smaller than 7), create entry in hash table for that
    replacement target, which includes a copy of the bytes found
    in that span.
    - Starting at A, insert S int3s. That is if these are 7 bytes,
    we now have:
    CC CC CC CC CC CC CC
    - Replace first byte with "far call" OP:
    9A CC CC CC CC CC CC

    Hence the name, dcprobes: dynamic call probes.

    So now, we've got a far call to 0xCCCC,0xCCCCCCCC at every probe point.
    And since this is a call, the caller's EIP is on the stack, and
    we can then mux based on that EIP to direct to the appropriate
    callback, possibly using the hash table. And because every
    component of the address is actually int3, we don't really care
    if anybody has a reference to any byte within A+1, A+2, A+3, etc.
    because as soon as that thing runs, it'll trigger an int3 where
    we'll be able to detect that using the hash table, fix things
    up and the system continue running. That fixes one of the major
    problems with djprobes since we would be able to arbitrarily
    insert probe points anywhere without having to make any
    assumptions about the code.

    Of course, this means hardwiring a multiplexing function at
    0xCCCC,0xCCCCCCCC, if that makes any sense (offset 0xCCCCCCCC
    of code segment entry 7,099 of the LDT with an RPL of 1).

    [ If the far call thing didn't work, we could resort to a
    "standard" call (E8 instead of 9A and 5 bytes instead of 7) and
    then hack up the destination address based on the current
    location to make sure that at offset current+0xCCCCCCCC
    something meaningful is there ... with the appropriate
    protection ... this may be simpler than the far call, but may
    require a specially-built kernel that changes the layout of
    the memory map for user-space ... ]

    For removal, things are slightly more complicated. We can go
    ahead and replace the old bytecode back so long as we do it
    operands first and opcodes last starting by the opcode of the
    last instruction in the stream. In that way, no freezing of
    tasks is necessary since no instruction will actually execute
    until the opcode is back. And since the addresses where valid
    opcodes were expected did, at all times, have valid opcodes, then
    there is no need to make any educated guesses about the system
    or do anything about references tasks/threads may have -- making
    this mechanism workable even for assembly and preemptable kernels.

    If replacing the bytecode back as explained above is too
    complicated (since it requires some disassembly), we could just
    suspend all processes, force all CPUs to rendez-vous, modify the
    byte span, and let everything run. Again, without needing to look
    up any tasks' stack or anything like that.

    Also, because this is a call, we can keep track of threads that
    go in the multiplexer and therefore implement some form of
    locking in order to preserve the ability to remove the probe
    sanely.

    If this works, djprobes should be easily modified for this,
    and I'm sure the djprobes team has got ample experience to do
    this.

    Of course, if it works. Hence my question: does this make sense?

    [ Obviously, this is not a replacement for what Mathieu is working
    on right now, but it would be useful for significantly speeding up
    the generic SystemTap probes, which was one of djprobes' much
    anticipated goals. ]

    Karim
    --
    President / Opersys Inc.
    Embedded Linux Training and Expertise
    www.opersys.com / 1.866.677.4546

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