12.1.1. Linking Done In Kernel
The biggest change to LKMs between Linux 2.4 and Linux 2.6 is an
internal one: LKMs get loaded much differently. Most people
won't see any difference except that the suffix on a file containing
an LKM has changed, because they use high level tools to manage
the LKMs and the interface to those tools hasn't changed.
Before Linux 2.6, a user space program would interpret the ELF object
(.o) file and do all the work of linking it to the running kernel,
generating a finished binary image. The program would pass that image
to the kernel and the kernel would do little more than stick it in
memory. In Linux 2.6, the kernel does the linking. A user space
program passes the contents of the ELF object file directly to the
kernel. For this to work, the ELF object image must contain
additional information. To identify this particular kind of ELF
object file, we name the file with suffix ".ko"
("kernel object") instead of ".o" For example,
the serial device driver that in Linux 2.4 lived in the file
serial.o in Linux 2.6 lives in the file
So there is a whole new modutils package
for use with Linux 2.6. In it, insmod is a trivial
program, as compared to the full blown linker of the Linux 2.4
Also, the procedure to build an LKM is somewhat harder. To make a .ko
file, you start with a regular .o file. You run the program modpost (which comes with the Linux source code) on it to
create a C source file that describes the additional sections the .ko
file needs. We'll call this the .mod file because you conventionally
include ".mod" in the file name.
You compile the .mod file and link the result with the original .o
file to make a .ko file.
The .mod object file contains the name that the LKM instance will have
when you load the LKM. You set that name with a -D compile option
(when you compile the .mod file) that sets the KBUILD_MODNAME macro.
This change means some things are decidedly harder -- choosing the name for
the LKM instance, for example. In Linux 2.4, the name was one of the
inputs to the kernel. insmod decided on the name
and passed it to the kernel. insmod's -o option
told it explicitly what to use for the LKM instance name. But in 2.6,
there is no such parameter on the system call and hence no -o option
on insmod. The name is part of the ELF object (.o
file) that you pass to the kernel. The default name is built into the
ELF object, but if you want to load it with some other name, you must
edit the ELF image before passing it to insmod.