kbuild: improved modversioning support for external modules

With following patch a second option is enabled to obtain
symbol information from a second external module when a
external module is build.
The recommended approach is to use a common kbuild file but
that may be impractical in certain cases.
With this patch one can copy over a Module.symvers from one
external module to make symbols (and symbol versions) available
for another external module.

Updated documentation in Documentation/kbuild/modules.txt

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

1 files changed, 80 insertions, 7 deletions

diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt
index 87d858df4e34..fcccf2432f98 100644
--- a/Documentation/kbuild/modules.txt
+++ b/Documentation/kbuild/modules.txt
@@ -23,7 +23,10 @@ In this document you will find information about:
         === 6. Module installation
            --- 6.1 INSTALL_MOD_PATH
            --- 6.2 INSTALL_MOD_DIR
-        === 7. Module versioning
+        === 7. Module versioning & Module.symvers
+           --- 7.1 Symbols fron the kernel (vmlinux + modules)
+           --- 7.2 Symbols and external modules
+           --- 7.3 Symbols from another external module
         === 8. Tips & Tricks
            --- 8.1 Testing for CONFIG_FOO_BAR
 
@@ -89,7 +92,8 @@ when building an external module.
         make -C $KDIR M=$PWD modules_install
                 Install the external module(s).
                 Installation default is in /lib/modules/<kernel-version>/extra,
-                but may be prefixed with INSTALL_MOD_PATH - see separate chapter.
+                but may be prefixed with INSTALL_MOD_PATH - see separate
+                chapter.
 
         make -C $KDIR M=$PWD clean
                 Remove all generated files for the module - the kernel
@@ -433,7 +437,7 @@ External modules are installed in the directory:
                 => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf
 
 
-=== 7. Module versioning
+=== 7. Module versioning & Module.symvers
 
 Module versioning is enabled by the CONFIG_MODVERSIONS tag.
 
@@ -443,11 +447,80 @@ when a module is loaded/used then the CRC values contained in the kernel are
 compared with similar values in the module. If they are not equal then the
 kernel refuses to load the module.
 
-During a kernel build a file named Module.symvers will be generated. This
-file includes the symbol version of all symbols within the kernel. If the 
-Module.symvers file is saved from the last full kernel compile one does not
-have to do a full kernel compile to build a module version's compatible module.
+Module.symvers contains a list of all exported symbols from a kernel build.
 
+--- 7.1 Symbols fron the kernel (vmlinux + modules)
+
+        During a kernel build a file named Module.symvers will be generated.
+        Module.symvers contains all exported symbols from the kernel and
+        compiled modules. For each symbols the corresponding CRC value
+        is stored too.
+
+        The syntax of the Module.symvers file is:
+                <CRC>       <Symbol>           <module>
+        Sample:
+                0x2d036834  scsi_remove_host   drivers/scsi/scsi_mod
+
+        For a kernel build without CONFIG_MODVERSIONING enabled the crc
+        would read: 0x00000000
+
+        Module.symvers serve two purposes.
+        1) It list all exported symbols both from vmlinux and all modules
+        2) It list CRC if CONFIG_MODVERSION is enabled
+
+--- 7.2 Symbols and external modules
+
+        When building an external module the build system needs access to
+        the symbols from the kernel to check if all external symbols are
+        defined. This is done in the MODPOST step and to obtain all
+        symbols modpost reads Module.symvers from the kernel.
+        If a Module.symvers file is present in the directory where
+        the external module is being build this file will be read too.
+        During the MODPOST step a new Module.symvers file will be written
+        containing all exported symbols that was not defined in the kernel.
+        
+--- 7.3 Symbols from another external module
+
+        Sometimes one external module uses exported symbols from another
+        external module. Kbuild needs to have full knowledge on all symbols
+        to avoid spitting out warnings about undefined symbols.
+        Two solutions exist to let kbuild know all symbols of more than
+        one external module.
+        The method with a top-level kbuild file is recommended but may be
+        impractical in certain situations.
+
+        Use a top-level Kbuild file
+                If you have two modules: 'foo', 'bar' and 'foo' needs symbols
+                from 'bar' then one can use a common top-level kbuild file so
+                both modules are compiled in same build.
+
+                Consider following directory layout:
+                ./foo/ <= contains the foo module
+                ./bar/ <= contains the bar module
+                The top-level Kbuild file would then look like:
+                
+                #./Kbuild: (this file may also be named Makefile)
+                        obj-y := foo/ bar/
+
+                Executing:
+                        make -C $KDIR M=`pwd`
+
+                will then do the expected and compile both modules with full
+                knowledge on symbols from both modules.
+
+        Use an extra Module.symvers file
+                When an external module is build a Module.symvers file is
+                generated containing all exported symbols which are not
+                defined in the kernel.
+                To get access to symbols from module 'bar' one can copy the
+                Module.symvers file from the compilation of the 'bar' module
+                to the directory where the 'foo' module is build.
+                During the module build kbuild will read the Module.symvers
+                file in the directory of the external module and when the
+                build is finished a new Module.symvers file is created
+                containing the sum of all symbols defined and not part of the
+                kernel.
+                
 === 8. Tips & Tricks
 
 --- 8.1 Testing for CONFIG_FOO_BAR