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157 lines
6.8 KiB
157 lines
6.8 KiB
================= |
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Symbol Namespaces |
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================= |
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The following document describes how to use Symbol Namespaces to structure the |
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export surface of in-kernel symbols exported through the family of |
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EXPORT_SYMBOL() macros. |
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.. Table of Contents |
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=== 1 Introduction |
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=== 2 How to define Symbol Namespaces |
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--- 2.1 Using the EXPORT_SYMBOL macros |
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--- 2.2 Using the DEFAULT_SYMBOL_NAMESPACE define |
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=== 3 How to use Symbols exported in Namespaces |
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=== 4 Loading Modules that use namespaced Symbols |
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=== 5 Automatically creating MODULE_IMPORT_NS statements |
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1. Introduction |
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=============== |
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Symbol Namespaces have been introduced as a means to structure the export |
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surface of the in-kernel API. It allows subsystem maintainers to partition |
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their exported symbols into separate namespaces. That is useful for |
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documentation purposes (think of the SUBSYSTEM_DEBUG namespace) as well as for |
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limiting the availability of a set of symbols for use in other parts of the |
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kernel. As of today, modules that make use of symbols exported into namespaces, |
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are required to import the namespace. Otherwise the kernel will, depending on |
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its configuration, reject loading the module or warn about a missing import. |
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2. How to define Symbol Namespaces |
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================================== |
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Symbols can be exported into namespace using different methods. All of them are |
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changing the way EXPORT_SYMBOL and friends are instrumented to create ksymtab |
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entries. |
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2.1 Using the EXPORT_SYMBOL macros |
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================================== |
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In addition to the macros EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL(), that allow |
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exporting of kernel symbols to the kernel symbol table, variants of these are |
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available to export symbols into a certain namespace: EXPORT_SYMBOL_NS() and |
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EXPORT_SYMBOL_NS_GPL(). They take one additional argument: the namespace. |
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Please note that due to macro expansion that argument needs to be a |
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preprocessor symbol. E.g. to export the symbol ``usb_stor_suspend`` into the |
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namespace ``USB_STORAGE``, use:: |
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EXPORT_SYMBOL_NS(usb_stor_suspend, USB_STORAGE); |
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The corresponding ksymtab entry struct ``kernel_symbol`` will have the member |
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``namespace`` set accordingly. A symbol that is exported without a namespace will |
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refer to ``NULL``. There is no default namespace if none is defined. ``modpost`` |
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and kernel/module/main.c make use the namespace at build time or module load |
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time, respectively. |
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2.2 Using the DEFAULT_SYMBOL_NAMESPACE define |
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============================================= |
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Defining namespaces for all symbols of a subsystem can be very verbose and may |
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become hard to maintain. Therefore a default define (DEFAULT_SYMBOL_NAMESPACE) |
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is been provided, that, if set, will become the default for all EXPORT_SYMBOL() |
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and EXPORT_SYMBOL_GPL() macro expansions that do not specify a namespace. |
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There are multiple ways of specifying this define and it depends on the |
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subsystem and the maintainer's preference, which one to use. The first option |
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is to define the default namespace in the ``Makefile`` of the subsystem. E.g. to |
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export all symbols defined in usb-common into the namespace USB_COMMON, add a |
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line like this to drivers/usb/common/Makefile:: |
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ccflags-y += -DDEFAULT_SYMBOL_NAMESPACE=USB_COMMON |
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That will affect all EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL() statements. A |
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symbol exported with EXPORT_SYMBOL_NS() while this definition is present, will |
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still be exported into the namespace that is passed as the namespace argument |
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as this argument has preference over a default symbol namespace. |
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A second option to define the default namespace is directly in the compilation |
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unit as preprocessor statement. The above example would then read:: |
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#undef DEFAULT_SYMBOL_NAMESPACE |
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#define DEFAULT_SYMBOL_NAMESPACE USB_COMMON |
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within the corresponding compilation unit before any EXPORT_SYMBOL macro is |
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used. |
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3. How to use Symbols exported in Namespaces |
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============================================ |
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In order to use symbols that are exported into namespaces, kernel modules need |
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to explicitly import these namespaces. Otherwise the kernel might reject to |
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load the module. The module code is required to use the macro MODULE_IMPORT_NS |
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for the namespaces it uses symbols from. E.g. a module using the |
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usb_stor_suspend symbol from above, needs to import the namespace USB_STORAGE |
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using a statement like:: |
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MODULE_IMPORT_NS(USB_STORAGE); |
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This will create a ``modinfo`` tag in the module for each imported namespace. |
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This has the side effect, that the imported namespaces of a module can be |
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inspected with modinfo:: |
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$ modinfo drivers/usb/storage/ums-karma.ko |
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[...] |
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import_ns: USB_STORAGE |
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[...] |
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It is advisable to add the MODULE_IMPORT_NS() statement close to other module |
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metadata definitions like MODULE_AUTHOR() or MODULE_LICENSE(). Refer to section |
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5. for a way to create missing import statements automatically. |
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4. Loading Modules that use namespaced Symbols |
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============================================== |
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At module loading time (e.g. ``insmod``), the kernel will check each symbol |
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referenced from the module for its availability and whether the namespace it |
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might be exported to has been imported by the module. The default behaviour of |
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the kernel is to reject loading modules that don't specify sufficient imports. |
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An error will be logged and loading will be failed with EINVAL. In order to |
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allow loading of modules that don't satisfy this precondition, a configuration |
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option is available: Setting MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS=y will |
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enable loading regardless, but will emit a warning. |
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5. Automatically creating MODULE_IMPORT_NS statements |
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===================================================== |
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Missing namespaces imports can easily be detected at build time. In fact, |
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modpost will emit a warning if a module uses a symbol from a namespace |
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without importing it. |
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MODULE_IMPORT_NS() statements will usually be added at a definite location |
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(along with other module meta data). To make the life of module authors (and |
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subsystem maintainers) easier, a script and make target is available to fixup |
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missing imports. Fixing missing imports can be done with:: |
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$ make nsdeps |
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A typical scenario for module authors would be:: |
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- write code that depends on a symbol from a not imported namespace |
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- ``make`` |
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- notice the warning of modpost telling about a missing import |
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- run ``make nsdeps`` to add the import to the correct code location |
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For subsystem maintainers introducing a namespace, the steps are very similar. |
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Again, ``make nsdeps`` will eventually add the missing namespace imports for |
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in-tree modules:: |
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- move or add symbols to a namespace (e.g. with EXPORT_SYMBOL_NS()) |
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- ``make`` (preferably with an allmodconfig to cover all in-kernel |
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modules) |
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- notice the warning of modpost telling about a missing import |
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- run ``make nsdeps`` to add the import to the correct code location |
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You can also run nsdeps for external module builds. A typical usage is:: |
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$ make -C <path_to_kernel_src> M=$PWD nsdeps
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