Documentation/core-api/symbol-namespaces.rst

=================
Symbol Namespaces
=================

The following document describes how to use Symbol Namespaces to structure the
export surface of in-kernel symbols exported through the family of
EXPORT_SYMBOL() macros.

Introduction
============

Symbol Namespaces have been introduced as a means to structure the export
surface of the in-kernel API. It allows subsystem maintainers to partition
their exported symbols into separate namespaces. That is useful for
documentation purposes (think of the SUBSYSTEM_DEBUG namespace) as well as for
limiting the availability of a set of symbols for use in other parts of the
kernel. As of today, modules that make use of symbols exported into namespaces,
are required to import the namespace. Otherwise the kernel will, depending on
its configuration, reject loading the module or warn about a missing import.

Additionally, it is possible to put symbols into a module namespace, strictly
limiting which modules are allowed to use these symbols.

How to define Symbol Namespaces
===============================

Symbols can be exported into namespace using different methods. All of them are
changing the way EXPORT_SYMBOL and friends are instrumented to create ksymtab
entries.

Using the EXPORT_SYMBOL macros
------------------------------

In addition to the macros EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL(), that allow
exporting of kernel symbols to the kernel symbol table, variants of these are
available to export symbols into a certain namespace: EXPORT_SYMBOL_NS() and
EXPORT_SYMBOL_NS_GPL(). They take one additional argument: the namespace as a
string constant. Note that this string must not contain whitespaces.
E.g. to export the symbol ``usb_stor_suspend`` into the
namespace ``USB_STORAGE``, use::

	EXPORT_SYMBOL_NS(usb_stor_suspend, "USB_STORAGE");

The corresponding ksymtab entry struct ``kernel_symbol`` will have the member
``namespace`` set accordingly. A symbol that is exported without a namespace will
refer to ``NULL``. There is no default namespace if none is defined. ``modpost``
and kernel/module/main.c make use the namespace at build time or module load
time, respectively.

Using the DEFAULT_SYMBOL_NAMESPACE define
-----------------------------------------

Defining namespaces for all symbols of a subsystem can be very verbose and may
become hard to maintain. Therefore a default define (DEFAULT_SYMBOL_NAMESPACE)
is been provided, that, if set, will become the default for all EXPORT_SYMBOL()
and EXPORT_SYMBOL_GPL() macro expansions that do not specify a namespace.

There are multiple ways of specifying this define and it depends on the
subsystem and the maintainer's preference, which one to use. The first option
is to define the default namespace in the ``Makefile`` of the subsystem. E.g. to
export all symbols defined in usb-common into the namespace USB_COMMON, add a
line like this to drivers/usb/common/Makefile::

	ccflags-y += -DDEFAULT_SYMBOL_NAMESPACE='"USB_COMMON"'

That will affect all EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL() statements. A
symbol exported with EXPORT_SYMBOL_NS() while this definition is present, will
still be exported into the namespace that is passed as the namespace argument
as this argument has preference over a default symbol namespace.

A second option to define the default namespace is directly in the compilation
unit as preprocessor statement. The above example would then read::

	#define DEFAULT_SYMBOL_NAMESPACE "USB_COMMON"

within the corresponding compilation unit before the #include for
<linux/export.h>. Typically it's placed before the first #include statement.

Using the EXPORT_SYMBOL_FOR_MODULES() macro
-------------------------------------------

Symbols exported using this macro are put into a module namespace. This
namespace cannot be imported. These exports are GPL-only as they are only
intended for in-tree modules.

The macro takes a comma separated list of module names, allowing only those
modules to access this symbol. Simple tail-globs are supported.

For example::

  EXPORT_SYMBOL_FOR_MODULES(preempt_notifier_inc, "kvm,kvm-*")

will limit usage of this symbol to modules whose name matches the given
patterns.

How to use Symbols exported in Namespaces
=========================================

In order to use symbols that are exported into namespaces, kernel modules need
to explicitly import these namespaces. Otherwise the kernel might reject to
load the module. The module code is required to use the macro MODULE_IMPORT_NS
for the namespaces it uses symbols from. E.g. a module using the
usb_stor_suspend symbol from above, needs to import the namespace USB_STORAGE
using a statement like::

	MODULE_IMPORT_NS("USB_STORAGE");

This will create a ``modinfo`` tag in the module for each imported namespace.
This has the side effect, that the imported namespaces of a module can be
inspected with modinfo::

	$ modinfo drivers/usb/storage/ums-karma.ko
	[...]
	import_ns:      USB_STORAGE
	[...]

For modules that are currently loaded, imported namespaces are also available
via sysfs::

	$ cat /sys/module/ums_karma/import_ns
	USB_STORAGE

It is advisable to add the MODULE_IMPORT_NS() statement close to other module
metadata definitions like MODULE_AUTHOR() or MODULE_LICENSE().

Loading Modules that use namespaced Symbols
===========================================

At module loading time (e.g. ``insmod``), the kernel will check each symbol
referenced from the module for its availability and whether the namespace it
might be exported to has been imported by the module. The default behaviour of
the kernel is to reject loading modules that don't specify sufficient imports.
An error will be logged and loading will be failed with EINVAL. In order to
allow loading of modules that don't satisfy this precondition, a configuration
option is available: Setting MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS=y will
enable loading regardless, but will emit a warning.

Automatically creating MODULE_IMPORT_NS statements
==================================================

Missing namespaces imports can easily be detected at build time. In fact,
modpost will emit a warning if a module uses a symbol from a namespace
without importing it.
MODULE_IMPORT_NS() statements will usually be added at a definite location
(along with other module meta data). To make the life of module authors (and
subsystem maintainers) easier, a script and make target is available to fixup
missing imports. Fixing missing imports can be done with::

	$ make nsdeps

A typical scenario for module authors would be::

	- write code that depends on a symbol from a not imported namespace
	- ``make``
	- notice the warning of modpost telling about a missing import
	- run ``make nsdeps`` to add the import to the correct code location

For subsystem maintainers introducing a namespace, the steps are very similar.
Again, ``make nsdeps`` will eventually add the missing namespace imports for
in-tree modules::

	- move or add symbols to a namespace (e.g. with EXPORT_SYMBOL_NS())
	- ``make`` (preferably with an allmodconfig to cover all in-kernel
	  modules)
	- notice the warning of modpost telling about a missing import
	- run ``make nsdeps`` to add the import to the correct code location

You can also run nsdeps for external module builds. A typical usage is::

	$ make -C <path_to_kernel_src> M=$PWD nsdeps

Note: it will happily generate an import statement for the module namespace;
which will not work and generates build and runtime failures.