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 <peterz@infradead.org>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

diff --git a/Documentation/core-api/symbol-namespaces.rst b/Documentation/core-api/symbol-namespaces.rst
index 06f766a6aab2446c6218e80fe2573aa79dc59ace..c6f59c5e25648efd261b970fbb0d93cc68abc354 100644 (file)
--- a/Documentation/core-api/symbol-namespaces.rst
+++ b/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.

diff --git a/include/linux/export.h b/include/linux/export.h
index a8c23d945634b3c29b6e76aa894c51d0fe91d250..f35d03b4113b19798036d2993d67eb932ad8ce6f 100644 (file)
--- a/include/linux/export.h
+++ b/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 */