[PATCH v2 0/4] Checkpoint/Restore: Show in proc IDs of objects thatcan be shared between tasks

[Pavel Emelyanov]

While doing the checkpoint-restore in the userspace one need to determine
whether various kernel objects (like mm_struct-s of file_struct-s) are shared
between tasks and restore this state.

The 2nd step can for now be solved by using respective CLONE_XXX flags and
the unshare syscall, while there's currently no ways for solving the 1st one.

One of the ways for checking whether two tasks share e.g. an mm_struct is to
provide some mm_struct ID of a task to its proc file. The best from the
performance point of view ID is the object address in the kernel, but showing
them to the userspace is not good for security reasons.

Thus the object address is XOR-ed with a "random" value of the same size and 
then shown in proc. Providing this poison is not leaked into the userspace then
ID seem to be safe. The objects for which the IDs are shown are:

* all namespaces living in /proc/pid/ns/
* open files (shown in /proc/pid/fdinfo/)
* objects, that can be shared with CLONE_XXX flags (except for namespaces)

Changes since
v1: * Tejun worried about the single poison value was a weak side - leaking one
      makes all the IDs vulnerable. To address this several poison values - one
      per object type - are introduced. They are stored in a plain array. Tejun, 
      is this enough from your POV, or you'd like to see them widely scattered 
      over the memory?
    * Pekka proposed to initialized poison values in the late_initcall callback
    * ... and move the code to mm/util.c

Signed-off-by: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>

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