The header file linux/version.h is empty from MRG version 2 realtime kernel

Root Cause

The example below shows building a simple kernel module in the affected MRG kernel.

• Module initialization for our custom module below uses an explicit variable LINUX_VERSION_CODE which is usually included in version.h

  static int hello_init(void)
  {
      printk(KERN_ALERT "version code %d\n", LINUX_VERSION_CODE); 
          return 0;
  }
  

• Because version.h is empty, compilation fails due to the missing symbol LINUX_VERSION_CODE

  # make -C /lib/modules/3.10.18-rt14.7.el6rt.x86_64/build M=/root/test-module modules
  make[1]: Entering directory `/usr/src/kernels/3.10.18-rt14.7.el6rt.x86_64'
    CC [M]  /root/test-module/hello.o
  /root/test-module/hello.c: In function ‘hello_init’:
  /root/test-module/hello.c:11: error: ‘LINUX_VERSION_CODE’ undeclared (first use in this function)
  /root/test-module/hello.c:11: error: (Each undeclared identifier is reported only once
  /root/test-module/hello.c:11: error: for each function it appears in.)
  make[2]: *** [/root/test-module/hello.o] Error 1
  

• When building out-of-tree kernel modules, the kernel can include a default search include path to gcc. This is usually hidden to reduce the complexity visible to module developers.

• Internally, the system is adding the -I parameter to gcc for the /lib/modules/3.10.18-rt14.7.el6rt.x86_64/build directory, along with a few other directories to search. One such directory is generated/uapi/linux/version.h which is generated during compilation.

• The -I directive can specify multiple include directories and the first to match successfully is the included file that gcc accepts. In this circumstance it is the wrong version.h.

• If we remove or delete the empty version.h file and then attempt to build, the following should occur:

  # make
  make -C /lib/modules/3.10.18-rt14.7.el6rt.x86_64/build M=/root/test-module modules
  make[1]: Entering directory `/usr/src/kernels/3.10.18-rt14.7.el6rt.x86_64'
    CC [M]  /root/test-module/hello.o
    Building modules, stage 2.
    MODPOST 1 modules
    CC      /root/test-module/hello.mod.o
    LD [M]  /root/test-module/hello.ko
  make[1]: Leaving directory `/usr/src/kernels/3.10.18-rt14.7.el6rt.x86_64'
  

• The correct versions.h is used and the module builds. The discrepancy in which version.h to use was tracked in BZ983603 and fixed in RHSA-2013-1264

• The empty version.h file can be safely removed as part of the "make" if required (although this does mean you run make with sufficient privileges), or you can alternatively specify the FULL path to version.h as shown below:

  #ifdef VERSION_H_WORKAROUND
  #include <generated/uapi/linux/version.h>
  #endif
  
  #include <linux/version.h>  // this should include an empty file, which.. doesn't matter.
  

• And then set it as a define at compile time, (likely through the makefile).

  # gcc -Wall -DVERSION_H_WORKAROUND module_name.c [...]