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    SubjectRe: ptrace and pfn mappings
    On Tue, Oct 10, 2006 at 12:47:46PM +1000, Benjamin Herrenschmidt wrote:
    >
    > > Switch the mm and do a copy_from_user? (rather than the GUP).
    > > Sounds pretty ugly :P
    > >
    > > Can you do a get_user_pfns, and do a copy_from_user on the pfn
    > > addresses? In other words, is the memory / mmio at the end of a
    > > given address the same from the perspective of any process? It
    > > is for physical memory of course, which is why get_user_pages
    > > works...
    >
    > Doesn't help with the racyness.

    I don't understand what the racyness is that you can solve by accessing
    it from the target process's mm?

    > > What if you hold your per-object lock over the operation? (I guess
    > > it would have to nest *inside* mmap_sem, but that should be OK).
    >
    > Over the ptrace operation ? how so ?

    You just have to hold it over access_process_vm, AFAIKS. Once it
    is copied into the kernel buffer that's done. Maybe I misunderstood
    what the race is?
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