2 files changed, 41 insertions, 0 deletions

diff --git a/Documentation/00-INDEX b/Documentation/00-INDEX
index 299615d821ac..c3014df066c4 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.
 mutex-design.txt
         - info on the generic mutex subsystem.
+namespaces/
+        - directory with various information about namespaces
 nbd.txt
         - info on a TCP implementation of a network block device.
 netlabel/
diff --git a/Documentation/namespaces/compatibility-list.txt b/Documentation/namespaces/compatibility-list.txt
new file mode 100644
index 000000000000..defc5589bfcd
--- /dev/null
+++ b/Documentation/namespaces/compatibility-list.txt
@@ -0,0 +1,39 @@
+        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       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. Intentionally, 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, belonging to user 10 in another
+   namespace.
+
+   The same is true for the IPC namespaces being shared - two users
+   from different user namespaces should not access the same IPC objects
+   even having equal UIDs.
+
+   But currently this is not so.
+