CSCI-3753: Operating SystemsFall 2019

Anh NguyenDepartment of Computer Science University of Colorado Boulder

Week 2: System Calls vs

Loadable Kernel Modules (LKMs)

CSCI 3753 Fall 2019 2

What is a System Call?

•A programmatic way in which a computer programrequests a service from the kernel of the operatingsystem it is executed on

•Each system call

corresponds to a number

defined in a syscalls table.

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syscall(number, …)

Do we use system calls at all?

•Example 1:• If you open a file using fopen() in the library stdio.h,

it gets translated into the open() system call.• In the standard library, the user-space implementation

of the open() system call executes and passes thesystem call number.

•Example 2:• In Unix-like systems, fork() and exec() are C-

library functions that in turn execute instructions thatinvoke the fork() and exec() system calls.

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EXECUTing a System Call

1. A user space program invokes the syscall

2. A (typically) software interrupt called a trap is triggered (INT)

3. Mode bit is flipped from user to kernel (1 to 0)

4. The interrupt tells the kernel which syscall was called

1. Requisite data may be passed in 2. The kernel verifies if all parameters are legal before

executing the system call

5. After execution, mode bit flips and user program resumes

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ADDing a System Call

1. Write the system call source code

arch/x86/kernel/newSysCall.c

2. Add the syscall prototype to the syscalls header file

include/linux/syscalls.h

3. Add the new syscall to the Makefile

arch/x86/kernel/Makefile

4. Add the syscall to the syscalls table

arch/x86/entry/syscalls/syscall_64.tbl

5. Re-compile the kernel

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Protection of Passing Values Between Spaces

• copy_from_user()

•get_user()

• copy_to_user()

•put_user()

•printk()

• cat /var/log/syslog

•dmesg

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How to Extend a Kernel?

•Method 1: System calls

Recompile the kernel !!!

è It takes a long time to compile the kernel source code (2-3 hours)

•Method 2:

Loadable Kernel Module (LKM)

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Loadable Kernel Module (LKM)

• LKM is an object file that contains a chunk of codeto add to the base kernel of an operating systemwhile it is RUNNING.

•Typically, it is used to add support for •New hardware as device drivers• Filesystem drivers

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LKM Pros & Cons

•Pros:• DON’T have to REBUILD the kernel• Save memory• Unloaded in order to free memory and other resources when

it is no longer required

• Be MUCH faster to maintain and debug

•Cons• FRAGMENTATION penalty• Security

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LKM Utilities

• insmod: Insert an LKM into the kernel.

• rmmod: Remove an LKM from the kernel.

• lsmod: List currently loaded LKMs.

•modprobe: Insert/remove an LKM or set of LKMs intelligently. • e.g., if you must load A before loading B, modprobe will

automatically load A when you tell it to load B.

• kerneld: Kernel daemon program• allows kernel modules to be loaded automatically rather than

manually with insmod/modprobe

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LKM Utilities

• depmod: Determine interdependencies between LKMs.

• ksyms: Display symbols that are exported by the kernel for use by new LKMs.

•modinfo: Display contents of .modinfo section in an LKM object file.

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Week 2 – Checklist

q Submit PS1 by 6PM today !!!

q Start PA1

q Read more about LKMs

q Quizo Every weeko Open book BUT NO use of device to interact with

otherso Only ONE attempt

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