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    SubjectRe: [RFC, PATCH 7/24] i386 Vmi memory hole
    Gerd Hoffmann wrote:
    >>> The complications in my patch come
    >>> from the fact that the vsyscall page has to be relocated dynamically,
    >>> requiring, basically run time linking on the page and some tweaks to get
    >>> sysenter to work. If you don't use vsyscall (say, non-TLS glibc), then
    >>> you don't need that complexity. But I think it might be needed now,
    >>> even for Xen.
    >> I believe both Xen and execshield move vsyscall out of fixmap, and then
    >> map into userspace as normal vma.
    > Yep, my patch (attached below for reference) moves the vsyscall page
    > into user address space, just below PAGE_OFFSET. Works basically the
    > same way the vsyscall page is mapped in the ia32 emulation of the x86_64
    > architecture. Address stays fixed, thus the relocation magic isn't needed.
    > Once the vsyscall page is moved out of fixmap it's easy to make fixmap
    > movable and thus have a runtime-resizable address space hole at the top
    > of address space. Patch is attached too, although that one is more
    > proof-of-concept, it doesn't make much sense as-is. It has a kernel
    > command line option to specify the top of address space so you can play
    > around with it ...
    > Both patches are against -rc3 and most likely still apply just fine,
    > havn't tested that though.

    Your patch looks a lot cleaner and less hackish than mine. But I wonder
    if it still works with kernels that support the sysenter method of
    calling into the kernel. Look at the following code:

    movl TSS_sysenter_esp0(%esp),%esp
    pushl $(__USER_DS)
    pushl %ebp
    pushl $(__USER_CS)

    SYSENTER_RETURN is a link time constant that is defined based on the
    location of the vsyscall page. If the vsyscall page can move, this can
    not be a constant. The reason is, this "fake" exception frame is used
    to return back to the EIP of the call site, and sysenter does not record
    the EIP of the call site.

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      Last update: 2006-03-15 09:39    [W:0.031 / U:14.488 seconds]
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