module: Provide EXPORT_SYMBOL_GPL_FOR_MODULES() helper

[ Upstream commit 707f853d7fa3ce323a6875487890c213e34d81a0 ]

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>
Stable-dep-of: cbe4134ea4bc ("fs: export anon_inode_make_secure_inode() and fix secretmem LSM bypass")
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/Documentation/core-api/symbol-namespaces.rst b/Documentation/core-api/symbol-namespaces.rst
index 12e4aecdae94522c30929c427410505bef6f3c4b..29875e25e376f66c4ee07943313c118554ad60de 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
 ==================================
 
@@ -84,6 +87,22 @@ unit as preprocessor statement. The above example would then read::
 within the corresponding compilation unit before any EXPORT_SYMBOL macro is
 used.
 
+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
 ============================================
 
@@ -155,3 +174,6 @@ 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 9911508a9604fb048c2587730520331ac621fd80..06f7a4eb6492860ecaaf4ffaa225382420e5275f 100644 (file)
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -42,11 +42,17 @@ extern struct module __this_module;
        .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
 
@@ -88,4 +94,6 @@ extern struct module __this_module;
 #define EXPORT_SYMBOL_NS(sym, ns)      __EXPORT_SYMBOL(sym, "", __stringify(ns))
 #define EXPORT_SYMBOL_NS_GPL(sym, ns)  __EXPORT_SYMBOL(sym, "GPL", __stringify(ns))
 
+#define EXPORT_SYMBOL_GPL_FOR_MODULES(sym, mods) __EXPORT_SYMBOL(sym, "GPL", "module:" mods)
+
 #endif /* _LINUX_EXPORT_H */