Transcript Kernel Module

Lab 4
Kernel Module
Operating System Lab
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What is a Kernel Module?
• Modules are pieces of code that can be loaded and unloaded
into the kernel upon demand.
• They extend the functionality of the kernel without the need to
recompile and reboot the system.
• Example
– One type of module is the device driver, which allows the kernel to
access hardware connected to the system.
– Without modules, we would have to build monolithic kernels and add
new functionality directly into the kernel image.
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Files relate to modules
• /etc/modprobe.conf
– Kernel will mount modules according to this file when boot on.
• /lib/modules/2.6.22.***/modules.dep
– If other modules must be loaded before the requested module may be
loaded.
• /lib/modules/2.6.22.***/kernel
– All modules of a kernel are in it.
• /proc/modules
– what modules are already loaded into the kernel by reading this file
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How Do Modules Get Into The
Kernel? (1/4)
• lsmod
– what modules are already loaded into the kernel by reading
the file /proc/modules.
[root@localhost~ ]#lsmod
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How Do Modules Get Into The
Kernel? (2/4)
• modprobe
– This instruction can load the designated specific
module , or a group of interdependent module .
• according to /lib/modules/2.6.22.***/modules.dep.
[root@localhost~ ]#modprobe [-lcfr] modules_name
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How Do Modules Get Into The
Kernel? (3/4)
• depmod [-aens]
– the file /lib/modules/version/modules.dep is created by depmod −a.
[root@localhost~ ]#depmod -a
• modinfo
– show the information of a module.
[root@localhost~ ]#modinfo module_name
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How Do Modules Get Into The Kernel?
(4/4)
• insmod
– similar to modprobe, but it can load the modules that aren’t in
/lib/modules/2.6.22.***/kernel
[root@localhost~ ]#insmod module_name.ko
• rmmod
– remove modules
[root@localhost~ ]#rmmod module_name
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Hello, World: The Simplest Module
/*
* hello.c − The simplest kernel module.
*/
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_INFO */
int init_module(void)
{
printk("Hello world 1.\n");
return 0;
}
void cleanup_module(void)
{
printk("Goodbye world 1.\n");
}
MODULE_LICENSE(“GPL")
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Makefile for a Basic Kernel Module
(1/2)
obj-m +=testfunction.o
all:
make –C /lib/modules/linux-2.6.22.6/build M=$(PWD) modules
clean:
make –C /lib/modules/linux-2.6.22.6/build M=$(PWD) clean
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Makefile for a Basic Kernel Module
(2/2)
[root@localhost~ ]#make
make −C /lib/modules/2.6.22.6/build M=/root/Desktop/
make[1]: Entering directory `/usr/src/2.6.22.6
CC [M] /root/Desktop/hello.o
Building modules, stage 2.
MODPOS
CC /root/Desktop/hello.mod.o
LD [M] /root/Desktop/hello.ko
make[1]: Leaving directory `/usr/src/2.6.22.6'
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Exercise
• We want to `spy' on a certain user, and to printk() a message
whenever that user opens a file.
• Towards this end, we replace the system call to open a file
with our own function, called our_sys_open.
• This function checks the uid (user's id) of the current process,
and if it's equal to the uid we spy on, it calls printk() to display
the name of the file to be opened.
• Then, either way, it calls the original open() function with the
same parameters, to actually open the file.
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Reference
• The Linux Kernel Module Programming Guide
– http://www.faqs.org/docs/kernel/
• Google
– Keyword “kernel module”
• Linux kernel module and TCP/IP program design
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Exercise (1/8)
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Exercise(2/8)
• [root@localhost~ ]#vim /usr/src/linux2.6.22.***/net/core/dev.c
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Exercise (3/8)
• From previous two slides, we can use
command “ping” to show some message.
• If we want to change the message showed, we
should re-edit the kernel, re-compile it, and
reboot each times.
– There is no efficiency.
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Exercise(4/8)
• Use kernel module to avoid recompiling kernel when
we edit the kernel each times.
• So, we define a back door procedure in the kernel.
– In other words, we make a hole in the kernel, and we can
put the kernel module in it.
• Write a kernel module that can make the “ping”
instruction show “hello world”
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Exercise(5/8)
•
[root@localhost~ ]#vim /usr/src/linux2.6.22.***/net/core/dev.c
– dev_queue_xmit()
int (*testfunction)(struct sk_buff *)=0;
if(testfunction){
testfunction(skb);
}
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Exercise(6/8)
EXPORT_SYMBOL(testfunction);
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Exercise(7/8)
• Include file
–
–
–
–
<linux/module.h>
<linux/kernel.h>
<linux/skbuff.h>
<linux/ip.h>
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Exercise(8/8)
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Reference
• The Linux Kernel Module Programming Guide
– http://www.faqs.org/docs/kernel/
• Google
– Keyword “kernel module”
• Linux kernel module and TCP/IP program design
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