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 31Additionally, it is possible to put symbols into a module namespace, strictly 32limiting which modules are allowed to use these symbols. 33 342. How to define Symbol Namespaces 35================================== 36 37Symbols can be exported into namespace using different methods. All of them are 38changing the way EXPORT_SYMBOL and friends are instrumented to create ksymtab 39entries. 40 412.1 Using the EXPORT_SYMBOL macros 42================================== 43 44In addition to the macros EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL(), that allow 45exporting of kernel symbols to the kernel symbol table, variants of these are 46available to export symbols into a certain namespace: EXPORT_SYMBOL_NS() and 47EXPORT_SYMBOL_NS_GPL(). They take one additional argument: the namespace as a 48string constant. Note that this string must not contain whitespaces. 49E.g. to export the symbol ``usb_stor_suspend`` into the 50namespace ``USB_STORAGE``, use:: 51 52 EXPORT_SYMBOL_NS(usb_stor_suspend, "USB_STORAGE"); 53 54The corresponding ksymtab entry struct ``kernel_symbol`` will have the member 55``namespace`` set accordingly. A symbol that is exported without a namespace will 56refer to ``NULL``. There is no default namespace if none is defined. ``modpost`` 57and kernel/module/main.c make use the namespace at build time or module load 58time, respectively. 59 602.2 Using the DEFAULT_SYMBOL_NAMESPACE define 61============================================= 62 63Defining namespaces for all symbols of a subsystem can be very verbose and may 64become hard to maintain. Therefore a default define (DEFAULT_SYMBOL_NAMESPACE) 65is been provided, that, if set, will become the default for all EXPORT_SYMBOL() 66and EXPORT_SYMBOL_GPL() macro expansions that do not specify a namespace. 67 68There are multiple ways of specifying this define and it depends on the 69subsystem and the maintainer's preference, which one to use. The first option 70is to define the default namespace in the ``Makefile`` of the subsystem. E.g. to 71export all symbols defined in usb-common into the namespace USB_COMMON, add a 72line like this to drivers/usb/common/Makefile:: 73 74 ccflags-y += -DDEFAULT_SYMBOL_NAMESPACE='"USB_COMMON"' 75 76That will affect all EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL() statements. A 77symbol exported with EXPORT_SYMBOL_NS() while this definition is present, will 78still be exported into the namespace that is passed as the namespace argument 79as this argument has preference over a default symbol namespace. 80 81A second option to define the default namespace is directly in the compilation 82unit as preprocessor statement. The above example would then read:: 83 84 #define DEFAULT_SYMBOL_NAMESPACE "USB_COMMON" 85 86within the corresponding compilation unit before the #include for 87<linux/export.h>. Typically it's placed before the first #include statement. 88 892.3 Using the EXPORT_SYMBOL_GPL_FOR_MODULES() macro 90=================================================== 91 92Symbols exported using this macro are put into a module namespace. This 93namespace cannot be imported. 94 95The macro takes a comma separated list of module names, allowing only those 96modules to access this symbol. Simple tail-globs are supported. 97 98For example: 99 100 EXPORT_SYMBOL_GPL_FOR_MODULES(preempt_notifier_inc, "kvm,kvm-*") 101 102will limit usage of this symbol to modules whoes name matches the given 103patterns. 104 1053. How to use Symbols exported in Namespaces 106============================================ 107 108In order to use symbols that are exported into namespaces, kernel modules need 109to explicitly import these namespaces. Otherwise the kernel might reject to 110load the module. The module code is required to use the macro MODULE_IMPORT_NS 111for the namespaces it uses symbols from. E.g. a module using the 112usb_stor_suspend symbol from above, needs to import the namespace USB_STORAGE 113using a statement like:: 114 115 MODULE_IMPORT_NS("USB_STORAGE"); 116 117This will create a ``modinfo`` tag in the module for each imported namespace. 118This has the side effect, that the imported namespaces of a module can be 119inspected with modinfo:: 120 121 $ modinfo drivers/usb/storage/ums-karma.ko 122 [...] 123 import_ns: USB_STORAGE 124 [...] 125 126 127It is advisable to add the MODULE_IMPORT_NS() statement close to other module 128metadata definitions like MODULE_AUTHOR() or MODULE_LICENSE(). Refer to section 1295. for a way to create missing import statements automatically. 130 1314. Loading Modules that use namespaced Symbols 132============================================== 133 134At module loading time (e.g. ``insmod``), the kernel will check each symbol 135referenced from the module for its availability and whether the namespace it 136might be exported to has been imported by the module. The default behaviour of 137the kernel is to reject loading modules that don't specify sufficient imports. 138An error will be logged and loading will be failed with EINVAL. In order to 139allow loading of modules that don't satisfy this precondition, a configuration 140option is available: Setting MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS=y will 141enable loading regardless, but will emit a warning. 142 1435. Automatically creating MODULE_IMPORT_NS statements 144===================================================== 145 146Missing namespaces imports can easily be detected at build time. In fact, 147modpost will emit a warning if a module uses a symbol from a namespace 148without importing it. 149MODULE_IMPORT_NS() statements will usually be added at a definite location 150(along with other module meta data). To make the life of module authors (and 151subsystem maintainers) easier, a script and make target is available to fixup 152missing imports. Fixing missing imports can be done with:: 153 154 $ make nsdeps 155 156A typical scenario for module authors would be:: 157 158 - write code that depends on a symbol from a not imported namespace 159 - ``make`` 160 - notice the warning of modpost telling about a missing import 161 - run ``make nsdeps`` to add the import to the correct code location 162 163For subsystem maintainers introducing a namespace, the steps are very similar. 164Again, ``make nsdeps`` will eventually add the missing namespace imports for 165in-tree modules:: 166 167 - move or add symbols to a namespace (e.g. with EXPORT_SYMBOL_NS()) 168 - ``make`` (preferably with an allmodconfig to cover all in-kernel 169 modules) 170 - notice the warning of modpost telling about a missing import 171 - run ``make nsdeps`` to add the import to the correct code location 172 173You can also run nsdeps for external module builds. A typical usage is:: 174 175 $ make -C <path_to_kernel_src> M=$PWD nsdeps 176 177Note: it will happily generate an import statement for the module namespace; 178which will not work and generates build and runtime failures. 179