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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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