1================= 2Symbol Namespaces 3================= 4 5The following document describes how to use Symbol Namespaces to structure the 6export surface of in-kernel symbols exported through the family of 7EXPORT_SYMBOL() macros. 8 9.. Table of Contents 10 11 === 1 Introduction 12 === 2 How to define Symbol Namespaces 13 --- 2.1 Using the EXPORT_SYMBOL macros 14 --- 2.2 Using the DEFAULT_SYMBOL_NAMESPACE define 15 === 3 How to use Symbols exported in Namespaces 16 === 4 Loading Modules that use namespaced Symbols 17 === 5 Automatically creating MODULE_IMPORT_NS statements 18 191. Introduction 20=============== 21 22Symbol Namespaces have been introduced as a means to structure the export 23surface of the in-kernel API. It allows subsystem maintainers to partition 24their exported symbols into separate namespaces. That is useful for 25documentation purposes (think of the SUBSYSTEM_DEBUG namespace) as well as for 26limiting the availability of a set of symbols for use in other parts of the 27kernel. As of today, modules that make use of symbols exported into namespaces, 28are required to import the namespace. Otherwise the kernel will, depending on 29its configuration, reject loading the module or warn about a missing import. 30 312. How to define Symbol Namespaces 32================================== 33 34Symbols can be exported into namespace using different methods. All of them are 35changing the way EXPORT_SYMBOL and friends are instrumented to create ksymtab 36entries. 37 382.1 Using the EXPORT_SYMBOL macros 39================================== 40 41In addition to the macros EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL(), that allow 42exporting of kernel symbols to the kernel symbol table, variants of these are 43available to export symbols into a certain namespace: EXPORT_SYMBOL_NS() and 44EXPORT_SYMBOL_NS_GPL(). They take one additional argument: the namespace as a 45string constant. Note that this string must not contain whitespaces. 46E.g. to export the symbol ``usb_stor_suspend`` into the 47namespace ``USB_STORAGE``, use:: 48 49 EXPORT_SYMBOL_NS(usb_stor_suspend, "USB_STORAGE"); 50 51The corresponding ksymtab entry struct ``kernel_symbol`` will have the member 52``namespace`` set accordingly. A symbol that is exported without a namespace will 53refer to ``NULL``. There is no default namespace if none is defined. ``modpost`` 54and kernel/module/main.c make use the namespace at build time or module load 55time, respectively. 56 572.2 Using the DEFAULT_SYMBOL_NAMESPACE define 58============================================= 59 60Defining namespaces for all symbols of a subsystem can be very verbose and may 61become hard to maintain. Therefore a default define (DEFAULT_SYMBOL_NAMESPACE) 62is been provided, that, if set, will become the default for all EXPORT_SYMBOL() 63and EXPORT_SYMBOL_GPL() macro expansions that do not specify a namespace. 64 65There are multiple ways of specifying this define and it depends on the 66subsystem and the maintainer's preference, which one to use. The first option 67is to define the default namespace in the ``Makefile`` of the subsystem. E.g. to 68export all symbols defined in usb-common into the namespace USB_COMMON, add a 69line like this to drivers/usb/common/Makefile:: 70 71 ccflags-y += -DDEFAULT_SYMBOL_NAMESPACE='"USB_COMMON"' 72 73That will affect all EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL() statements. A 74symbol exported with EXPORT_SYMBOL_NS() while this definition is present, will 75still be exported into the namespace that is passed as the namespace argument 76as this argument has preference over a default symbol namespace. 77 78A second option to define the default namespace is directly in the compilation 79unit as preprocessor statement. The above example would then read:: 80 81 #define DEFAULT_SYMBOL_NAMESPACE "USB_COMMON" 82 83within the corresponding compilation unit before the #include for 84<linux/export.h>. Typically it's placed before the first #include statement. 85 863. How to use Symbols exported in Namespaces 87============================================ 88 89In order to use symbols that are exported into namespaces, kernel modules need 90to explicitly import these namespaces. Otherwise the kernel might reject to 91load the module. The module code is required to use the macro MODULE_IMPORT_NS 92for the namespaces it uses symbols from. E.g. a module using the 93usb_stor_suspend symbol from above, needs to import the namespace USB_STORAGE 94using a statement like:: 95 96 MODULE_IMPORT_NS("USB_STORAGE"); 97 98This will create a ``modinfo`` tag in the module for each imported namespace. 99This has the side effect, that the imported namespaces of a module can be 100inspected with modinfo:: 101 102 $ modinfo drivers/usb/storage/ums-karma.ko 103 [...] 104 import_ns: USB_STORAGE 105 [...] 106 107 108It is advisable to add the MODULE_IMPORT_NS() statement close to other module 109metadata definitions like MODULE_AUTHOR() or MODULE_LICENSE(). Refer to section 1105. for a way to create missing import statements automatically. 111 1124. Loading Modules that use namespaced Symbols 113============================================== 114 115At module loading time (e.g. ``insmod``), the kernel will check each symbol 116referenced from the module for its availability and whether the namespace it 117might be exported to has been imported by the module. The default behaviour of 118the kernel is to reject loading modules that don't specify sufficient imports. 119An error will be logged and loading will be failed with EINVAL. In order to 120allow loading of modules that don't satisfy this precondition, a configuration 121option is available: Setting MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS=y will 122enable loading regardless, but will emit a warning. 123 1245. Automatically creating MODULE_IMPORT_NS statements 125===================================================== 126 127Missing namespaces imports can easily be detected at build time. In fact, 128modpost will emit a warning if a module uses a symbol from a namespace 129without importing it. 130MODULE_IMPORT_NS() statements will usually be added at a definite location 131(along with other module meta data). To make the life of module authors (and 132subsystem maintainers) easier, a script and make target is available to fixup 133missing imports. Fixing missing imports can be done with:: 134 135 $ make nsdeps 136 137A typical scenario for module authors would be:: 138 139 - write code that depends on a symbol from a not imported namespace 140 - ``make`` 141 - notice the warning of modpost telling about a missing import 142 - run ``make nsdeps`` to add the import to the correct code location 143 144For subsystem maintainers introducing a namespace, the steps are very similar. 145Again, ``make nsdeps`` will eventually add the missing namespace imports for 146in-tree modules:: 147 148 - move or add symbols to a namespace (e.g. with EXPORT_SYMBOL_NS()) 149 - ``make`` (preferably with an allmodconfig to cover all in-kernel 150 modules) 151 - notice the warning of modpost telling about a missing import 152 - run ``make nsdeps`` to add the import to the correct code location 153 154You can also run nsdeps for external module builds. A typical usage is:: 155 156 $ make -C <path_to_kernel_src> M=$PWD nsdeps 157