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>

1 files changed, 22 insertions, 0 deletions

diff --git a/Documentation/core-api/symbol-namespaces.rst b/Documentation/core-api/symbol-namespaces.rst
index 12e4aecdae94..29875e25e376 100644
--- 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.