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    SubjectRe: A question about PROT_NONE on ARM and ARM26
    On Wed, Jun 30, 2004 at 11:59:22PM +0100, Russell King wrote:
    > Ok, to fill in for just this bit, the domain covering user space mappings
    > always remains in "client" mode, so page protections are always checked.
    > PAGE_NONE does not have the "user" bit set, so both user space accesses
    > and ldrt/strt instructions will be unable to access the pages, which is
    > the desired behaviour.
    > However, plain ldr and str instructions will access the page, but
    > get_user/put_user doesn't use them, and copy_from_user/copy_to_user
    > are carefully crafted to ensure that we hit the necessary permission
    > checks for each page it touches on the first access.

    What if CONFIG_PREEMPT is enabled, and you get preempted after that
    first access, and another thread unmaps the page before you're
    finished with it?

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