How to make a pseudo-file As a follow-up to our first lab, we examine the steps needed to create our own ‘/proc’ file

The ‘extensibility’ imperative A modern OS needs the ability to evolve It will need to support new devices It will need to allow ‘bugs’ to be repaired It will need to permit performance gains Otherwise: suffer early obsolescence!

Two Extensibility Mechanisms ‘Open Source’ programming ‘Installable’ kernel modules

‘Open Source’ Source text for the Linux kernel is on disk (usual directory-location is ‘/usr/src/linux’) Majority of the sources are written in ‘C’ (so ‘portable’ across CPU architectures) Some are written in assembly languages (for nearly twenty different architectures) Files are grouped into major categories (kernel, mm, fs, net, ipc, lib, drivers, init) You could edit and recompile your kernel

Installable kernel modules Great mechanism for kernel ‘extensibility’ Neat tool for studying how kernel works Kernel can be modified while it’s running Unnecessary to recompile and then reboot But inherently unsafe: programming bugs in the kernel can cause system crashes!

Some ‘superuser’ privileges Since modifying a running kernel is ‘risky’, only authorized ‘system administrators’ are normally allowed to install kernel modules But our systems are specially configured to permit you to install or remove modules This is purely for ‘educational’ purposes (so you should use this privilege wisely)

Linux module structure Two ‘module administration’ functions are mandatory components in every module plus Appropriate ‘module service’ functions and their supporting kernel data-structures are optional components in particular modules also Recent kernels require a Module License!

A minimal module-template #include int init_module( void ) { // code here gets called during module installation } void cleanup_module( void ) { // code here gets called during module removal } MODULE_LICENSE(“GPL”);

How to compile a module You could directly invoke the C-compiler: $ gcc –c –O –D__KERNEL__ -DMODULE –I/lib/modules/2.4.26/build/include mod.c OR: you can use the ‘make’ utility: $ make mod.o

The ‘printk()’ function Kernel modules cannot call any functions in the C runtime library (e.g., ‘printf()’) But similar kernel versions of important functions are provided (e.g., ‘printk()’) Syntax and semantics are slightly different (e.g., priority and message-destination) Capabilities may be somewhat restricted (e.g., printk() can’t show floating-point)

Simple module example #include int init_module( void ) { printk( “ Hello, world!\n” ); return 0;// SUCCESS } void cleanup_module( void ) { printk( “ Goodbye everyone\n” ); } MODULE_LICENSE(“GPL”);

How to install and remove root# /sbin/insmod hello.o root# /sbin/rmmod hello

A non-trivial module-example Let’s see how we can use kernel functions to create our own ‘/proc’ file Easy if we use ‘create_proc_read_entry()’ during module-initialization (and then use ‘remove_proc_entry()’ during cleanup) Prototypes in the header We’ll show the current value of a volatile kernel variable, named ‘jiffies’

jiffies unsigned long volatile jiffies; global kernel variable (used by scheduler) Initialized to zero when system reboots Gets incremented when timer interrupts So it counts ‘clock-ticks’ since cpu restart ‘tick-frequency’ is architecture dependent

Writing the ‘jiffies.c’ module We need to declare a name for pseudo-file static char modname[ ] = “jiffies”; We need to define a ‘proc_read’ function (see code on the following slide) We need to ‘register’ our filename, along with its ‘read’ method, in ‘init_module()’ We need to ‘unregister’ our pseudo-file in ‘cleanup_module()’

Our module’s ‘read’ method static int my_proc_read( char *buf, char **start, off_t off, int count, int *eof, void *data ) { return sprintf( buf, “jiffies=%lu\n”, jiffies ); }

Our ‘initialization’ function int init_module( void ) { create_proc_read_entry( modname, 0, NULL, my_proc_read, NULL ); return 0; }

Our ‘cleanup’ function void cleanup_module( void ) { remove_proc_entry( modname, NULL ); }

In-class exercise #1 Use an editor (e.g., ‘vi’) to create a source file in C (named ‘jiffies.c’) for your module Use the ‘make’ command to compile your module’s source-file into an object-file Use the ‘insmod’ command to install your module object-file into the running kernel Use the ‘cat’ command to display ‘jiffies’ Then use ‘rmmod’ to remove your module

Makefile We need it to automate module compiles Otherwise we type a VERY long command To compile ‘jiffies.c’ type: $ make jiffies.o Our Makefile defines some ‘implicit rules’ ‘make’ works by doing pattern-matching

Rule syntax targets … : dependencies … commands...

Pattern Rule example CC = gcc INCLUDE = /lib/modules/2.4.26/build/include CFLAGS = -c –O CFLAGS += -D__KERNEL__ -DMODULE # primary pattern rule %.o : %.c %.h $(CC) -I$(INCLUDE) $CFLAGS $< # fallback pattern rule %.o : %.c $(CC) -I$(INCLUDE) $CFLAGS $<

‘Makefile’ is on class website You can download the ‘Makefile’ from our website: Put the ‘Makefile’ in your current working directory (along with your module source) Then you can compile by typing this short command:$ make jiffies.o

In-class exercise #2 Use your knowledge of standard C library functions (e.g., open(), read(), close() ) to write an application-program (name it ‘showjifs.cpp’) which reads the information from your ‘proc/jiffies’ pseudo-file and then prints it on the screen Use a program-loop to re-read the pseudo file multiple times How rapidly does ‘jiffies’ value increase?