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user_ns: Introduce user_nsmap_uid and user_ns_map_gid.

Define what happens when a we view a uid from one user_namespace
in another user_namepece.

- If the user namespaces are the same no mapping is necessary.

- For most cases of difference use overflowuid and overflowgid,
the uid and gid currently used for 16bit apis when we have a 32bit uid
that does fit in 16bits. Effectively the situation is the same,
we want to return a uid or gid that is not assigned to any user.

- For the case when we happen to be mapping the uid or gid of the
creator of the target user namespace use uid 0 and gid as confusing
that user with root is not a problem.

Signed-off-by: Eric W. Biederman <>
Acked-by: Serge E. Hallyn <>
Signed-off-by: David S. Miller <>

Eric W. Biederman Eric W. Biederman 2010-06-13

David S. Miller David S. Miller 2010-06-16

changed include
changed include/linux
changed include/linux/user_namespace.h
changed kernel
changed kernel/user_namespace.c
include/linux/user_namespace.h Diff Switch to side-by-side view
kernel/user_namespace.c Diff Switch to side-by-side view