symbol-namespaces.rst (6944B)
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. 45Please note that due to macro expansion that argument needs to be a 46preprocessor symbol. E.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.c make use the namespace at build time or module load time, 55respectively. 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 #undef DEFAULT_SYMBOL_NAMESPACE 82 #define DEFAULT_SYMBOL_NAMESPACE USB_COMMON 83 84within the corresponding compilation unit before any EXPORT_SYMBOL macro is 85used. 86 873. How to use Symbols exported in Namespaces 88============================================ 89 90In order to use symbols that are exported into namespaces, kernel modules need 91to explicitly import these namespaces. Otherwise the kernel might reject to 92load the module. The module code is required to use the macro MODULE_IMPORT_NS 93for the namespaces it uses symbols from. E.g. a module using the 94usb_stor_suspend symbol from above, needs to import the namespace USB_STORAGE 95using a statement like:: 96 97 MODULE_IMPORT_NS(USB_STORAGE); 98 99This will create a ``modinfo`` tag in the module for each imported namespace. 100This has the side effect, that the imported namespaces of a module can be 101inspected with modinfo:: 102 103 $ modinfo drivers/usb/storage/ums-karma.ko 104 [...] 105 import_ns: USB_STORAGE 106 [...] 107 108 109It is advisable to add the MODULE_IMPORT_NS() statement close to other module 110metadata definitions like MODULE_AUTHOR() or MODULE_LICENSE(). Refer to section 1115. for a way to create missing import statements automatically. 112 1134. Loading Modules that use namespaced Symbols 114============================================== 115 116At module loading time (e.g. ``insmod``), the kernel will check each symbol 117referenced from the module for its availability and whether the namespace it 118might be exported to has been imported by the module. The default behaviour of 119the kernel is to reject loading modules that don't specify sufficient imports. 120An error will be logged and loading will be failed with EINVAL. In order to 121allow loading of modules that don't satisfy this precondition, a configuration 122option is available: Setting MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS=y will 123enable loading regardless, but will emit a warning. 124 1255. Automatically creating MODULE_IMPORT_NS statements 126===================================================== 127 128Missing namespaces imports can easily be detected at build time. In fact, 129modpost will emit a warning if a module uses a symbol from a namespace 130without importing it. 131MODULE_IMPORT_NS() statements will usually be added at a definite location 132(along with other module meta data). To make the life of module authors (and 133subsystem maintainers) easier, a script and make target is available to fixup 134missing imports. Fixing missing imports can be done with:: 135 136 $ make nsdeps 137 138A typical scenario for module authors would be:: 139 140 - write code that depends on a symbol from a not imported namespace 141 - ``make`` 142 - notice the warning of modpost telling about a missing import 143 - run ``make nsdeps`` to add the import to the correct code location 144 145For subsystem maintainers introducing a namespace, the steps are very similar. 146Again, ``make nsdeps`` will eventually add the missing namespace imports for 147in-tree modules:: 148 149 - move or add symbols to a namespace (e.g. with EXPORT_SYMBOL_NS()) 150 - ``make`` (preferably with an allmodconfig to cover all in-kernel 151 modules) 152 - notice the warning of modpost telling about a missing import 153 - run ``make nsdeps`` to add the import to the correct code location 154 155You can also run nsdeps for external module builds. A typical usage is:: 156 157 $ make -C <path_to_kernel_src> M=$PWD nsdeps