The $/$proc file system

The $/$proc file system is a "pseudo" file system that exists only in kernel memory and is used primarily for querying various kernel and per$-$process statistics. This exercise involves designing kernel modules that create additional entries in the $/$proc file system involving both kernel statistics and information related to specific processes. The entire program is included in the figures below.

Figure $2\mathrm{.}15\mathrm{.}2$: The $/$proc file$-$system kernel module, Part $1$

#include

#include

#include

#include

#include

#define BUFFER$\_$SIZE $128$

#define PROC$\_$NAME "hello"

ssize$\_$t proc$\_$read$($struct file $*$file$,$ char $\_\_$user $*$usr$\_$buf,

size$\_$t count, loff$\_$t $*$pos$)$;

static struct file$\_$operations proc$\_$ops $=\{$

$.$owner $=$ THIS$\_$MODULE,

$.$read $=$ proc$\_$read,

$\}$;

$/*$ This function is called when the module is loaded. $*/$

int proc$\_$init$($void$)$

$\{$

$/*$ creates the $/$proc$/$hello entry $*/$

proc$\_$create$($PROC$\_$NAME, $0666,$ NULL, &proc$\_$ops$)$;

return $0$;

$\}$

$/*$ This function is called when the module is removed. $*/$

void proc$\_$exit$($void$)$

$\{$

$/*$ removes the $/$proc$/$hello entry $*/$

remove$\_$proc$\_$entry$($PROC$\_$NAME, NULL$)$;

$\}$

Feedback?

Figure $2\mathrm{.}15\mathrm{.}3$: The $/$proc file system kernel module, Part $2$

$/*$ This function is called each time $/$proc$/$hello is read $*/$

ssize$\_$t proc$\_$read$($struct file $*$file$,$ char $\_\_$user $*$usr$\_$buf,

size$\_$t count, loff$\_$t $*$pos$)$

$\{$

int rv $=0$;

char buffer$[$BUFFER$\_$SIZE$]$;

static int completed $=0$;

if $($completed$)\{$

completed $=0$;

return $0$;

$\}$

completed $=1$;

rv $=$ sprintf$($buffer$,$ "Hello World

$")$;

$/*$ copies kernel space buffer to user space usr$\_$buf $*/$

copy$\_$to$\_$user$($usr$\_$buf, buffer, rv$)$;

return rv;

$\}$

module$\_$init$($proc$\_$init$)$;

module$\_$exit$($proc$\_$exit$)$;

MODULE$\_$LICENSE$("$GPL$")$;

MODULE$\_$DESCRIPTION$("$Hello Module"$)$;

MODULE$\_$AUTHOR$("$SGG$")$;

Feedback?

We begin by describing how to create a new entry in the $/$proc file system. The following program example $($named hello.c and available with the source code for this text$)$ creates a $/$proc entry named $/$proc$/$hello$.$ If a user enters the command

cat $/$proc$/$hello

the infamous Hello World message is returned.

In the module entry point proc$\_$init$(),$ we create the new $/$proc$/$hello entry using the proc$\_$create$()$ function. This function is passed proc$\_$ops, which contains a reference to a struct file$\_$operations. This struct initializes the $.$owner and $.$read members. The value of $.$read is the name of the function proc$\_$read$()$ that is to be called whenever $/$proc$/$hello is read.

Examining this proc$\_$read$()$ function, we see that the string "Hello World

$"$ is written to the variable buffer where buffer exists in kernel memory. Since $/$proc$/$hello can be accessed from user space, we must copy the contents of buffer to user space using the kernel function copy$\_$to$\_$user$().$ This function copies the contents of kernel memory buffer to the variable usr$\_$buf, which exists in user space.

Each time the $/$proc$/$hello file is read, the proc$\_$read$()$ function is called repeatedly until it returns $0,$ so there must be logic to ensure that this function returns $0$ once it has collected the data $($in this case, the string "Hello World

$")$ that is to go into the corresponding $/$proc$/$hello file.

Finally, notice that the $/$proc$/$hello file is removed in the module exit point proc$\_$exit$()$ using the function remove$\_$proc$\_$entry$().$

IV$.$ Assignment

This assignment will involve designing two kernel modules:

Design a kernel module that creates a $/$proc file named $/$proc$/$jiffies that reports the current value of jiffies when the $/$proc$/$jiffies file is read, such as with the command

cat $/$proc$/$jiffies

Be sure to remove $/$proc$/$jiffies when the module is removed.

Design a kernel module that creates a proc file named $/$proc$/$seconds that reports the number of elapsed seconds since the kernel module was loaded. This will involve using the value of jiffies as well as the HZ rate. When a user enters the command

cat $/$proc$/$seconds

your kernel module will report the number of seconds that have elapsed since the kernel module was first loaded. Be sure to remove $/$proc$/$seconds when the module is removed.