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    SubjectRe: Potenitial security hole in the kernel
    Chris Wedgwood wrote:
    > By the way, the context stored on the stack is entirely a user
    > space context, however it does include some information from the
    > kernel that may be useful to user space, such as a page fault
    > address.
    > I actually (ab)used this for userspace paging with mprotect and
    > friends.... nasty hack :)

    Aye, and it's nice to see that Linux is relatively fast at this.
    (See attached table).

    Various garbage collectors do this to track dirty pages (clisp, Elk and
    Hans Boehm's GC which is now part of the GCC distribution come to mind).
    It's a shame this doesn't work for system calls like read(), as they
    return EFAULT instead of calling the signal handler. (It could be made
    to work with some effort).

    -- Jamie

    Timing measurements

    Here are some rough timings to handle a write protection faults and
    change protection from read-only to writable, page by page in
    sequential order. 2000 pages were faulted, in a region of
    memory-mapped /dev/zero.

    The times given are the total running time divided by the number of
    pages, so they including the time to call mprotect to unprotect each
    page. They are given in microseconds per page, and only the best
    observed time is shown. The first column is the time to dirty a zero
    page for the first time (the OS must process a second page fault and
    COW/allocate a new page in these cases). The second column is the
    time to unprotect a page that is already modified, which is the
    typical scenario for an incremental garbage collector.

    133MHz Pentium Linux 2.3.29 85.52 37.13 (page == 4096).
    233MHz dual Pentium II Linux 2.2.17-4 51.17 23.27 (page == 4096).
    233MHz Pentium II Linux 2.4.4 51.15 16.60 (page == 4096).
    300MHz Pentium II Linux 2.4.4 39.84 13.02 (page == 4096).
    366MHz Intel Celeron Linux 2.4.4 38.83 13.02 (page == 4096).
    400MHz Pentium II Linux 2.4.2 35.83 12.46 (page == 4096).
    600MHz Pentium III Linux 2.4.4 33.60 8.12 (page == 4096).
    600MHz Pentium III Linux 2.2.14-5 27.28 7.75 (page == 4096).
    600MHz Pentium III Linux 2.2.17-4 28.62 9.36 (page == 4096).
    PA-RISC HP-UX (which speed?) 108.44 60.34 (page == 4096).
    Sparc Ultra-1 SunOS 5.6 256.69 209.03 (page == 8192).
    Sparc SunOS 5.7 machine #1 158.26 102.26 (page == 8192).
    Sparc SunOS 5.7 machine #2 190.09 139.25 (page == 8192).
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