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    SubjectRe: [RFC] prevent "dd if=/dev/mem" crash
    On Fri, Oct 17, 2003 at 05:49:55PM -0700, Andrew Morton wrote:
    > David Mosberger <davidm@napali.hpl.hp.com> wrote:
    > >
    > > 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.
    >
    > An access to an illegal address should generate a fault, period. This puts
    > the processing into the hands of software. If software chooses to silently
    > ignore the fault (ie: "silent data corruption") then it is poorly designed.

    It *does* signal a fault, in the form of a machine check. On other
    architectures I'm familiar with this is usually implemented as an
    interrupt, but the idea is similar - when the system bus controller
    detects a bad address on the bus, it returns all 1s (for a read) and
    signals an interrupt. Usually you can turn this interrupt off (and
    most likely you can on Itanium chipsets too) but that is not a good
    idea.

    The problem is that this interrupt is not synchronous with respect to
    the instruction stream, and this makes it difficult for software to
    recover from, particularly in a monolithic system like Linux where you
    can't just terminate the faulting driver. The best you can usually do
    is to print the details and hope that it's a once-off. It is not
    something that you can sensibly use to abort copy_*_user.

    In any case touching random addresses is just plain bad. What if
    there's a device mapped there which happens to have read side effects
    like clearing the interrupt cause, so e.g. every time you read /dev/mem
    you cause a timeout on your SCSI bus :)

    Matt

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