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    SubjectRe: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function
    On Mon, Jun 10, 2019 at 12:52 PM Dave Hansen <dave.hansen@intel.com> wrote:
    >
    > On 6/10/19 12:38 PM, Yu-cheng Yu wrote:
    > >>> When an application starts, its highest stack address is determined.
    > >>> It uses that as the maximum the bitmap needs to cover.
    > >> Huh, I didn't think we ran code from the stack. ;)
    > >>
    > >> Especially given the way that we implemented the new 5-level-paging
    > >> address space, I don't think that expecting code to be below the stack
    > >> is a good universal expectation.
    > > Yes, you make a good point. However, allowing the application manage the bitmap
    > > is the most efficient and flexible. If the loader finds a legacy lib is beyond
    > > the bitmap can cover, it can deal with the problem by moving the lib to a lower
    > > address; or re-allocate the bitmap.
    >
    > How could the loader reallocate the bitmap and coordinate with other
    > users of the bitmap?
    >
    > > If the loader cannot allocate a big bitmap to cover all 5-level
    > > address space (the bitmap will be large), it can put all legacy lib's
    > > at lower address. We cannot do these easily in the kernel.
    >
    > This is actually an argument to do it in the kernel. The kernel can
    > always allocate the virtual space however it wants, no matter how large.
    > If we hide the bitmap behind a kernel API then we can put it at high
    > 5-level user addresses because we also don't have to worry about the
    > high bits confusing userspace.
    >

    That's a fairly compelling argument.

    The bitmap is one bit per page, right? So it's smaller than the
    address space by a factor of 8*2^12 == 2^15. This means that, if we
    ever get full 64-bit linear addresses reserved entirely for userspace
    (which could happen if my perennial request to Intel to split user and
    kernel addresses completely happens), then we'll need 2^48 bytes for
    the bitmap, which simply does not fit in the address space of a legacy
    application.

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