[CRIU] [PATCH 2/3] vdso: Proxify vDSO code on restore

[Cyrill Gorcunov]

When tasks are restored in a kernel which version differ from the one
they were being dumped on, the vDSO content may has been changed thus
any call to vDSO service routine will lead to SIGBUS in a better case
or in wrong results without notice in a worst one.

So to work this situation out we need a that named vDSO proxy, where
any call to original vDSO service routine shall be redirected to
a valid runtime vDSO supplied by the kernel we're restoring on.

In terms of ugly ascii gprahics it might be represented as following.

Original vDSO in image
======================

    +---------------------------+
    | __vdso_getcpu             |
    |       ...                 |
    |       function body here  |
    +---------------------------+
    | __vdso_time               |
    |       ...                 |
    |       function body here  |
    +---------------------------+
    ...

Run-time proxified calls
========================

    Mapped from image and then patched

    +---------------------------+
    | __vdso_getcpu             |
    |       redirect            +----+
    |       int3                |    |
    +---------------------------+    |
    | __vdso_time               |    |
    |       redirect            +-+  |
    |       int3                | |  |
    +---------------------------+ |  |
    ...                           |  |
                                  |  |  New memory area with valid vDSO content
                                  |  |  +---------------------------+
                                  |  |  | __vdso_getcpu             |
                                  |  +->|       ...                 |
                                  |     |       function body here  |
                                  |     +---------------------------+
                                  |     | __vdso_time               |
                                  +---->|       ...                 |
                                        |       function body here  |
                                        +---------------------------+
                                        ...

To achieve this we

 1) Read vDSO from memory the kernel provides us and remember
    its structure and functions addresses.

 2) Analyze VMAs read from images, if vdso is met -- remember where
    this area is premaped.

 3) Once the vdso page data read from disk -- parse it and fill symbols
    table, we will use it to patch the vdso calls. Then mark it with
    special vdso signature. Thus on next dump session it might be
    recognized as vdso area.

 4) When we start unmapping self vmas, if runtime vdso met (from [1])
    we remap it at some new place.

 5) Patch old vDSO entry points to redirect calls to new vDSO entries.

 6) Mark new vDSO with special sign thus in future patch the dumper will
    find that the second vDSO should be omitted from writting on disk.

Signed-off-by: Cyrill Gorcunov <gorcunov at openvz.org>
---
 cr-restore.c       | 83 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
 include/restorer.h | 12 ++++++++
 pie/restorer.c     | 54 +++++++++++++++++++++++++++++++++++
 3 files changed, 146 insertions(+), 3 deletions(-)

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