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    Subject[PATCH][DOCUMENTATION] The namespaces compatibility list doc
    From time to time people begin discussions about how the
    namespaces are working/going-to-work together.

    Ted T'so proposed to create some document that describes what
    problems user may have when he/she creates some new namespace,
    but keeps others shared. I liked this idea, so here's the
    initial version of such a document with the problems I currently
    have in mind and can describe somewhat audibly - the "namespaces
    compatibility list".

    The Documentation/namespaces/ directory is about to contain more
    docs about the namespaces stuff.

    Thanks to Cedirc for notes and spell checks on the doc.

    Signed-off-by: Pavel Emelyanov <>


    commit 83061c56e1c4dcd54d48a62b108d219a7f5279a0
    Author: Pavel <>
    Date: Fri Nov 16 12:25:53 2007 +0300

    Namespaces compatibility list

    diff --git a/Documentation/00-INDEX b/Documentation/00-INDEX
    index 910e511..3ead06b 100644
    --- a/Documentation/00-INDEX
    +++ b/Documentation/00-INDEX
    @@ -262,6 +262,8 @@ mtrr.txt
    - how to use PPro Memory Type Range Registers to increase performance.
    - info on the generic mutex subsystem.
    + - directory with various information about namespaces
    - info on a TCP implementation of a network block device.
    diff --git a/Documentation/namespaces/compatibility-list.txt b/Documentation/namespaces/compatibility-list.txt
    new file mode 100644
    index 0000000..9c9e5c1
    --- /dev/null
    +++ b/Documentation/namespaces/compatibility-list.txt
    @@ -0,0 +1,33 @@
    + Namespaces compatibility list
    +This document contains the information about the problems user
    +may have when creating tasks living in different namespaces.
    +Here's the summary. This matrix shows the known problems, that
    +occur when tasks share some namespace (the columns) while living
    +in different other namespaces (the rows):
    + UTS IPC VFS PID User Net
    +UTS X
    +IPC X 1
    +VFS X
    +PID 1 1 X
    +User 2 X
    +Net X
    +1. Both the IPC and the PID namespaces provide IDs to address
    + object inside the kernel. E.g. semaphore with ipcid or
    + process group with pid.
    + In both cases, tasks shouldn't try exposing this id to some
    + other task living in a different namespace via a shared filesystem
    + or IPC shmem/message. The fact is that this ID is only valid
    + within the namespace it was obtained in and may refer to some
    + other object in another namespace.
    +2. Intentionnaly, two equal user ids in different user namespaces
    + should not be equal from the VFS point of view. In other
    + words, user 10 in one user namespace shouldn't have the same
    + access permissions to files, beloging to user 10 in another
    + namespace. But currently this is not so.
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