module: Provide EXPORT_SYMBOL_GPL_FOR_MODULES() helper

Helper macro to more easily limit the export of a symbol to a given
list of modules.

Eg:

  EXPORT_SYMBOL_GPL_FOR_MODULES(preempt_notifier_inc, "kvm");

will limit the use of said function to kvm.ko, any other module trying
to use this symbol will refure to load (and get modpost build
failures).

Requested-by: Masahiro Yamada <masahiroy@kernel.org>
Requested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Author: Peter Zijlstra

Documentation/core-api/symbol-namespaces.rst

@@ -28,6 +28,9 @@ 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.
+
 2. How to define Symbol Namespaces
 ==================================
 
@@ -83,6 +86,22 @@ unit as preprocessor statement. The above example would then read::
 within the corresponding compilation unit before the #include for
 <linux/export.h>. Typically it's placed before the first #include statement.
 
+2.3 Using the EXPORT_SYMBOL_GPL_FOR_MODULES() macro
+===================================================
+
+Symbols exported using this macro are put into a module namespace. This
+namespace cannot be imported.
+
+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_GPL_FOR_MODULES(preempt_notifier_inc, "kvm,kvm-*")
+
+will limit usage of this symbol to modules whoes name matches the given
+patterns.
+
 3. How to use Symbols exported in Namespaces
 ============================================
 
@@ -154,3 +173,6 @@ in-tree modules::
 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.

include/linux/export.h

@@ -24,11 +24,17 @@
 	.long sym
 #endif
 
-#define ___EXPORT_SYMBOL(sym, license, ns)		\
+/*
+ * LLVM integrated assembler cam merge adjacent string literals (like
+ * C and GNU-as) passed to '.ascii', but not to '.asciz' and chokes on:
+ *
+ *   .asciz "MODULE_" "kvm" ;
+ */
+#define ___EXPORT_SYMBOL(sym, license, ns...)		\
 	.section ".export_symbol","a"		ASM_NL	\
 	__export_symbol_##sym:			ASM_NL	\
 		.asciz license			ASM_NL	\
-		.asciz ns			ASM_NL	\
+		.ascii ns "\0"			ASM_NL	\
 		__EXPORT_SYMBOL_REF(sym)	ASM_NL	\
 	.previous
 
@@ -85,4 +91,6 @@
 #define EXPORT_SYMBOL_NS(sym, ns)	__EXPORT_SYMBOL(sym, "", ns)
 #define EXPORT_SYMBOL_NS_GPL(sym, ns)	__EXPORT_SYMBOL(sym, "GPL", ns)
 
+#define EXPORT_SYMBOL_GPL_FOR_MODULES(sym, mods) __EXPORT_SYMBOL(sym, "GPL", "module:" mods)
+
 #endif /* _LINUX_EXPORT_H */