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    SubjectRe: [RFC] prevent "dd if=/dev/mem" crash
    >>>>> On Fri, 17 Oct 2003 16:55:43 -0700, Andrew Morton <> said:

    >> If we really believe copy_*_user() must correctly handle *all* faults,
    >> isn't the "p >= __pa(high_memory)" test superfluous?

    Andrew> This code was conceived before my time and I don't recall seeing much
    Andrew> discussion, so this is all guesswork..

    Andrew> I'd say that the high_memory test _is_ superfluous and that
    Andrew> if anyone cared, we would remove it and establish a
    Andrew> temporary pte against the address if it was outside the
    Andrew> direct-mapped area. But nobody cares enough to have done
    Andrew> anything about it.

    What about memory-mapped device registers? Isn't all memory
    physically contiguous on x86 and that's why the "p >=
    __pa(high_memory)" test saves you from that?

    >> On ia64, a read to non-existent physical memory causes the processor
    >> to time out and take a machine check. I'm not sure it's even possible
    >> to recover from that.

    Andrew> ick. That would be very poor form.

    Reasonable people can disagree on that. One philosophy states that if
    your kernel touches random addresses, it's better to signal a visible
    error (machine-check) than to risk silent data corruption.

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