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Re^4: [OT] Stats problem

by BrowserUk (Pope)
on Feb 27, 2015 at 12:04 UTC ( #1118067=note: print w/replies, xml ) Need Help??


in reply to Re^3: [OT] Stats problem
in thread [OT] Stats problem

non-periodic function that can be easily created and checked?

Any time you squeeze a 64-bit value (address) into a 32-bit pot, you are going to get repeats.

The good thing with using the offset directly is that you know that the repeats are always going to be 4 billion bytes apart. And thus only occur if the program uses more than 4GB of heap; and on my 8GB only occur twice. (Not strictly true if I allowed my machine to go into swapping!)

With any non-periodic function, the repeats will (must) still occur, the only difference is that the spacing will vary, and be less. It could even put then in adjacent memory slots; or certainly a lot closer together.

Intuitively -- though as I observed elsewhere, there is nothing much that is intuitive about this -- the danger of the copy-over problem seems more likely the closer together they are.


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Re^5: [OT] Stats problem
by QM (Parson) on Feb 27, 2015 at 13:32 UTC
    Yes, yes, all valid points. I was just trying to remove one more weakness, which is the 4GB offsets matching.

    Consider that anything that hits the 4GB+x weakness will be undetectable, regardless of the length of the overrun. (OK, within reason, as a long enough overrun will surely break something else.)

    Under an MD5 hash scheme, the chances of a 32bit slot being overwritten with the correct magic data is 1/4G, the same as with the offset method. But for the offset method, if the from/to addresses are 4GB apart, a run will generate the correct data, regardless of the length of run. For MD5 hash, the probabilities are independent, even for a malloc overrun as in the example, because consecutive hash values are not dependent on the neighboring hash values in any simple way.

    Still, 1/4G is quite small.

    -QM
    --
    Quantum Mechanics: The dreams stuff is made of

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