Re: [PATCH 2.6.25-rc2 3/9] Kconfig: Improve init/Kconfig help descriptions - NAMESPACES

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Nick Andrew (nick@nick-andrew.net):
> Rewrite the help descriptions for clarity, accuracy and consistency.
> 
> Kernel config options affected:
> 
>   - NAMESPACES
>   - UTS_NS
>   - IPC_NS
>   - USER_NS
>   - PID_NS
> 
> Signed-off-by: Nick Andrew <nick@nick-andrew.net>
> ---
> Try #3.
> 
> --- a/init/Kconfig	2008-02-20 09:34:48.000000000 +1100
> +++ b/init/Kconfig	2008-02-22 09:01:09.000000000 +1100
> @@ -414,31 +414,71 @@ config NAMESPACES
>  	bool "Namespaces support" if EMBEDDED
>  	default !EMBEDDED
>  	help
> -	  Provides the way to make tasks work with different objects using
> -	  the same id. For example same IPC id may refer to different objects
> -	  or same user id or pid may refer to different tasks when used in
> -	  different namespaces.
> +	  Select various namespace options.
> +
> +	  Namespaces allow different kernel objects (such as processes
> +	  or sockets) to have the same ID in different namespaces.
> +	  Identifiers like process IDs, which historically were globally
> +	  unique, will now be unique only within each PID namespace.
> +	  Each task can refer only to PIDs within the same namespace
> +	  as the task itself.
> +
> +	  Namespaces are used by container systems (i.e. vservers)
> +	  to provide isolation between the containers.
> +
> +	  This option does not affect any kernel code directly; it merely
> +	  allows you to select namespace options below.
> +
> +	  Answer Y if you will be using a container system, and you
> +	  will probably want to enable all the namespace options
> +	  below.
> 
>  config UTS_NS
>  	bool "UTS namespace"
>  	depends on NAMESPACES
>  	help
> -	  In this namespace tasks see different info provided with the
> -	  uname() system call
> +	  Enable support for multiple UTS system attributes.
> +
> +	  Each UTS namespace provides an individual view of the
> +	  information returned by the uname() system call including
> +	  hostname, kernel version and domain name.
> +
> +	  This is used by container systems (e.g. vservers) so that
> +	  each container has its own hostname and other attributes.
> +	  Tasks in the container are placed in the UTS namespace
> +	  corresponding to the container.

Hi Nick,

this paragraph reads a little weird...  really what happens is that
each forked task is in the same UTS namespace as its parent, unless
it was cloned with CLONE_NEWUTS or has done unshare(CLONE_NEWUTS),
in which case it receives a copy.

> +
> +	  Answer Y if you will be using a container system.
> 
>  config IPC_NS
>  	bool "IPC namespace"
>  	depends on NAMESPACES && SYSVIPC
>  	help
> -	  In this namespace tasks work with IPC ids which correspond to
> -	  different IPC objects in different namespaces
> +	  Enable support for namespace-specific IPC IDs.
> +
> +	  IPC IDs will be unique only within each IPC namespace.
> +
> +	  This is used by container systems (e.g. vservers).
> +	  Tasks in the container are placed in the IPC namespace
> +	  corresponding to the container.

Same as with UTS, except that upon CLONE_NEWIPC the task receives a
blank new ipc namespace, not a copy of the original.

> +	  Answer Y if you will be using a container system.
> 
>  config USER_NS
>  	bool "User namespace (EXPERIMENTAL)"
>  	depends on NAMESPACES && EXPERIMENTAL
>  	help
> -	  This allows containers, i.e. vservers, to use user namespaces
> -	  to provide different user info for different servers.
> +	  Enable experimental support for user namespaces.
> +
> +	  This is a function used by container-based virtualisation systems
> +	  (e.g. vservers). User namespaces are intended to ensure that
> +	  processes with the same uid which are in different containers are
> +	  isolated from each other.
> +
> +	  Currently user namespaces provide separate accounting, while
> +	  isolation must be provided using SELinux or a custom security
> +	  module.
> +
>  	  If unsure, say N.
> 
>  config PID_NS
> @@ -446,12 +486,16 @@ config PID_NS
>  	default n
>  	depends on NAMESPACES && EXPERIMENTAL
>  	help
> -	  Suport process id namespaces.  This allows having multiple
> -	  process with the same pid as long as they are in different
> -	  pid namespaces.  This is a building block of containers.
> +	  Enable experimental support for hierarchical process id namespaces.
> 
> -	  Unless you want to work with an experimental feature
> -	  say N here.
> +	  This is a function used by container-based virtualisation
> +	  systems (e.g. vservers).  Each process will have a distinct
> +	  Process ID in each PID namespace which the process is in.
> +	  Processes in the container are placed in the PID namespace
> +	  corresponding to the container, and cannot see or affect
> +	  processes in any parent PID namespace.

A cloned process inherits the pid namespace hierarchy from its
parent.  If it was cloned with CLONE_NEWPID, the hierarchy is
topped with one additional newly created pid namespace.  This
is the only pid namespace in which the new process will be able
to see processes, while it will be visible in all namespaces in
its pidns hierarchy.

A pid namespace cannot be unshared.

> +
> +	  If unsure, say N.
> 
>  config BLK_DEV_INITRD
>  	bool "Initial RAM filesystem and RAM disk (initramfs/initrd) support"