Dynamic Kernels: Modularized Device Drivers

This is the first in a series of four articles co-authored by Alessandro Rubini and Georg Zezchwitz which present a practical approach to writing Linux device drivers as kernel loadable modules. This installment presents and introduction to thte topic, preparing the reader to understand next month's installment.
What is a module?

A module is a code segment which registers itself with the kernel as a device driver, is called by the kernel in order to communicate with the device, and in turn invokes other kernel functions to accomplish its tasks. Modules utilize a clean interface between the “kernel proper” and the device, which both makes the modules easy to write and keeps the kernel source code from being cluttered.

The module must be compiled to object code (no linking; leave the compiled code in .o files), and then loaded into the running kernel with insmod. The insmod program is a runtime linker, which resolves any undefined symbols in the module to addresses in the running kernel by means of the kernel symbol table.

This means that you can write a module much like a conventional C-language program, and you can call functions you don't define, in the same way you usually call printf() and fopen() in your application. However, you can count only on a minimal set of external functions, which are the public functions provided by the kernel. insmod will put the right kernel-space addresses in your compiled module wherever your code calls a kernel function, then insert the module into the running Linux kernel.

If you are in doubt whether a kernel-function is public or not, you can look for its name either in the source file /usr/src/linux/kernel/ksyms.c or in the run-time table /proc/ksyms.

To use make to compile your module, you'll need a Makefile as simple as the following one:

TARGET = myname

ifdef DEBUG
  # -O is needed, because of "extern inline"
  # Add -g if your gdp is patched and can use it
  CFLAGS = -O3 -D__KERNEL__ -fomit-frame-pointer

all: $(TARGET).o

As you see, no special rule is needed to build a module, only the correct value for CFLAGS. I recommend that you include debugging support in your code, because without patches, gdb isn't able to take advantage of the symbol information provided by the -g flag to a module while it is part of the running kernel.

Debugging support will usually mean extra code to print messages from within the driver. Using printk() for debugging is powerful, and the alternatives are running a debugger on the kernel, peeking in /dev/mem, and other extremely low-level techniques. There are a few tools available on the Internet to help use these other techniques, but you need to be conversant with gdb and be able to read real kernel code in order to benefit from them. The most interesting tool at time of writing is kdebug-1.1, which lets you use gdb on a running kernel, examining or even changing kernel data structures (including those in loaded kernel modules) while the kernel is running. Kdebug is available for ftp from sunsite.unc.edu and its mirrors under /pub/Linux/kernel.

Just to make things a little harder, the kernel equivalent of the standard printf() function is called printk(), because it does not work exactly the same as printf(). Before 1.3.37, conventional printk()'s generated lines in /var/adm/messages, while later kernels will dump them to the console. If you want quiet logging (only within the messages file, via syslogd) you must prepend the symbol KERN_DEBUG to the format string. KERN_DEBUG and similar symbols are simply strings, which get concatenated to your format string by the compiler. This means that you must not put a comma between KERN_DEBUG and the format string. These symbols can be found in <linux/kernel.h>, and are documented there. Also, printk() does not support floating point formats.

Remember, that syslog write to the messages file as soon as possible, in order to save all messages on disk in case of a system crash. This means that an over-printk-ed module will slow down perceptibly, and will fill your disk in a short time.

Almost any module misbehaviour will generate an [cw]Oops[ecw] message from the kernel. An Oops is what happens when the kernel gets an exception from kernel code. In other words, Oopses are the equivalent of segmentation faults in user space, though no core file is generated. The result is usually the sudden destruction of the responsible process, and a few lines of low-level information in the messages file. Most Oops messages are the result of dereferencing a NULL pointer.

This way to handle disasters is a friendly one, and you'll enjoy it whenever your code is faulty: most other Unices produce a kernel panic instead. This does not mean that Linux never panics. You must be prepared to generate panics whenever you write functions that operate outside of a process context, such as within interrupt handlers and timer callbacks.

The scarce, nearly unintelligible information included with the [cw]Oops[ecw] message represents the processor state when the code faulted, and can be used to understand where the error is. A tool called ksymoops is able to print more readable information out of the oops, provided you have a kernel map handy. The map is what is left in /usr/src/linux/System.map after a kernel compilation. Ksymoops was distributed within util-linux-2.4, but was removed in 2.5 because it has been included in the kernel distribution during the linux-1.3 development.

If you really understand the Oops message, you can use it as you want, like invoking gdb off-line to disassemble the whole responsible function. if you understand neither the Oops nor the ksymoops output, you'd better add some more debugging printk() code, recompile, and reproduce the bug.

The following code can ease management of debugging messages. It must reside in the module's public include file, and will work for both kernel code (the module) and user code (applications). Note however that this code is gcc-specific. Not too big a problem for a kernel module, which is gcc-dependent anyway. This code was suggested by Linus Torvalds, as an enhancement over my previous ansi-compliant approach.

#ifndef PDEBUG
#  ifdef DEBUG_modulename
#    ifdef __KERNEL__
#      define PDEBUG(fmt, args...) printk (KERN_DEBUG fmt , ## args)
#    else
#      define PDEBUG(fmt, args...) fprintf (stderr, fmt , ## args)
#    endif
#  else
#    define PDEBUG(fmt, args...)
#  endif

#ifndef PDEBUGG
#  define PDEBUGG(fmt, args...)

After this code, every PDEBUG("any %i or %s...\n", i, s); in the module will result in a printed message only if the code is compiled with -DDEBUG_modulename, while PDEBUGG() with the same arguments will expand to nothing. In user mode applications, it works the same, except that the message is printed to stderr instead of the messages file.

Using this code, you can enable or disable any message by removing or adding a single G character.