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    SubjectRe: ld-Script needed OR (predicted) Architecture of Kernel 3.0 ;-)
    On Wed, 15 Oct 2003, Eric W. Biederman wrote:

    > "Richard B. Johnson" <> writes:
    > >
    > > Also, we have a module loader and unloader that allows modules
    > > to be inserted and removed from a running system. There are
    > > even experimental systems that allow the whole kernel to be
    > > changed without (apparent) re-booting.
    > Any pointers. I have heard rumors but I have never seen an actual
    > implementation. Even one that failed in a lot of cases.
    > Eric

    I heard rumors, too. Something going on, on source-forge, but
    I haven't looked. I surmise that, if it works, it uses hooks
    for the "suspend-to-disk" code that is also supposed to work
    but I haven't ever seen either.

    In principle, it should not be too hard to do on Intel.

    (1) User-mode code opens a module and writes the new kernel
    into kernel space using ordinary read or write module function
    calls. The module kernel code saves the data in a temporary
    kmalloc()ed buffer.

    (2) User mode code kills everybody except itself and dismounts
    all file-systems.

    (3) User-mode code, via an ioctl() signals the module code
    to restart.

    (4) The kernel-mode code disables all interrupts, then writes
    the saved data to a physical location, outside the normal kernel

    (5) The kernel-mode code then copies some code to low RAM where
    there is a 1:1 virtual to physical translation below 1 megabyte.
    It then jumps there. The code needs to be relocatable, not
    a problem for the simple stuff it needs to do.

    (6) Since the virtual:physical transation is 1:1, it can now
    disable paging.

    (7) With paging disabled, it copies the saved data from the
    physical location used in (4) to offset 1 megabyte, the normal
    relocation address.

    (8) It jumps to the kernel startup code at the normal relocated

    FYI, I did something like this to non-distructively find the
    physical address of bad RAM a few years ago. I made a module
    that transitioned to 1:1 translation with paging disabled to
    find the real bad RAM (the test program had written some tokens
    to both sides of the failed area). It then transitioned back
    to paged mode and, ultimately, back to user-mode code, with
    the answer.

    Dick Johnson
    Penguin : Linux version 2.4.22 on an i686 machine (797.90 BogoMips).
    Note 96.31% of all statistics are fiction.

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