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    This patch merges the rework that has been in my 2.4 pool for the last month
    or so. I'm going to describe what happened in some detail since it hasn't
    been discussed on lkml at all, and there are some generic kernel changes
    involved which may be of wider interest.

    The design of UML has had as its main points:
    every UML process has a corresponding host process
    the UML kernel is mapped into the top .5G of each process' address space
    there is a special thread, the tracing thread, which ptraces all the
    other threads, managing their transitions between the kernel and userspace.

    This is insecure, because protecting UML kernel data from its processes is
    hard to do right, and impossible to do quickly. UML does have a 'jail' mode
    which implements this, which is many times slower than non-jail.

    It's also slow, because entry to userspace involves a signal delivery to
    the process entering the kernel and a signal return when leaving the kernel.

    To fix these problems, I followed up an observation by Ingo a few months ago
    that a full process context switch is several times faster than an in-process
    signal delivery.

    I implemented a new mode which puts the UML kernel into a completely separate
    address space from its processes. skas (== "separate kernel address space" -
    the traditional mode is now called tt (== "tracing thread")) mode has these
    main points:
    the kernel is in a separate process and address space from its processes
    UML processes share a single host process
    each UML process has its own host address space
    thus, the "userspace" process hops between address spaces on each UML
    context switch

    skas mode has a number of advantages over the traditional tt mode:

    better security - since the kernel is in a different address space, processes
    can't even see, let alone modify, kernel data, since they can't form a kernel

    better performance - it is significantly faster. Kernel builds in skas mode
    approach twice the speed of tt mode, and ~30% slower than the host (compared
    to ~100% slower with tt mode).

    better debuggability - it is now possible to 'gdb linux' and have it do what
    you expect. Tools like gprof, gcov, and ddd should now just work, without
    needing special support inside UML (although gprof currently needs the removal
    of some of that support in order to work). It is possible to build UML as
    a normal dynamically linked binary, which will make it possible to valgrind
    the kernel (although valgrind is currently bothered by UML's use of clone).

    cleaner code - process creation, switching, and destruction are far simpler,
    cleaner, and faster in the skas code than in the tt code.

    miscellaneous - UML process address spaces are now identical to those on the
    host - this is advantageous for applications such as honeypots, as well as
    possibly for applications which use the full 3G address space. The kernel
    now has a full 3G of virtual address space.

    There is one major disadvantage to skas mode - it can't be implemented given
    the support currently in the stock kernel.

    I've added some stuff into the generic and i386 code to make skas mode
    possible. This support includes:
    /proc/mm, which allows address spaces to be created independently of
    a number of ptrace extensions

    /proc/mm has the following semantics -
    open creates a new, empty address space - the file descriptor returned
    is used as a handle to that address space
    write modifies the address space according to the structure that is
    passed in as the buffer argument. The possible operations are map, unmap,
    protect, and copy segments. The first three are identical to mmap, munmap,
    and mprotect. The last is used to copy the arch-specific data associated
    with the mm_struct as part of cloning the address space.
    close drops the reference count of the address space, which normally
    frees it since UML will not have any processes running in it

    The ptrace extensions are:
    PTRACE_FAULTINFO - returns the information assoicated with the child's
    most recent segfault
    PTRACE_SIGPENDING - returns the child's pending signal mask
    PTRACE_LDT - performs a modify_ldt on the child - this is really an
    address space operation and will be moved to /proc/mm at some point
    PTRACE_SWITCH_MM - switches the child from its current address space
    to the one associated with the file descriptor pass in with this call

    The host support patch is available with all the other UML downloads at

    I welcome any comments on it. The /proc/mm write semantics are less than
    ideal - I especially would like suggestions for improvements.

    The 2.5.49 UML patch is available at

    For the other UML mirrors and other downloads, see

    Other links of interest:

    The UML project home page :
    The UML Community site :


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