Operating Systems 2017F Lecture 12 - Revision history

Rquaium: /* Additional Notes */

2017-10-21T15:00:11Z

Additional Notes

Rquaium: /* Additional Notes */

2017-10-21T14:58:45Z

Additional Notes

Rquaium: /* Additional Notes */

2017-10-21T14:58:05Z

Additional Notes

Rquaium at 14:57, 21 October 2017

2017-10-21T14:57:12Z

← Older revision		Revision as of 14:57, 21 October 2017
Line 96:		Line 96:
	- e.g., when was it last accessed		- e.g., when was it last accessed
	</pre>		</pre>


			== Additional Notes ==

			Announcements
			You may want to use Openstack after midterm
			Semaphore tutorial is like kernel code
			Tutorial similar to assignment!

			Additional Notes
			Segmentation
			Inode fields
			Page table entries
			Scheduling

			Paging - fixed sized memory allocations of usually 4k or 8k blocks (cab have some larger ones)

			Paging only works well if you have hardware to help you - specifically Memory Management Unit
			Need page tables, TLB (Translation Lookaside Buffer: memory cache that speeds up access time to a user memory lcoation)
			Do "table lookup" on every memory access

			If you do not have full paging support how would you manage memory?
			Basic approach: put everything at a fixed place in memory
			- Useful if you have limited RAM

			Classic Way: Put code, data into "segments" instead of pages
			- Semantic units of memory (stack, heap, "text")
			- Execve loads segments into memory
			- Still use this terminology when referring to parts of a binary program

			Segments:
			Variable sized block of memory
			Base: starting address
			Bound: how long is it
			Key idea is segments can be placed in different parts of the address space:
			- to move, you just change the base
			- Code inside a segment refers to rest of segment via offsets
			- On each memory access, CPU adds segment register to offset to get physical address
			- Having a base address with offsets is still used in userspace

			16-bit x86 (8086) is a segmented architecture
			- Memory access is msotly relative toa segment base address
			Standard modes for 32-bit x86 (80386+) and 64-bit x86 (amd64) is "flat" things can be wherever
			- Segmented memory can be a pain because it can "wrap around" - going past the last address can put you in the first, or it just gives error

			Inode fields:
			Stat, lstat gives you details on an inode
			Inodes allows access to contents of files and have metadata such as indoe number, user id, stuff about time
			- Size
			- "Mode" - rwx (he shoes us how to change read write permissions the normal way and also using chmod number to set individual bits and also through GUI), others
			- Owner, group

			Xdg-open foo.js same as double clicking knows what preferred program to open a file

			Users, groups
			Users and group are used to restrict access
			- each file has a user and group associated with it, permissions
			- Each process has a user and group
			What if file ownership/permissions are "wrong"
			- Use "whoami" command to find user class
			In UNIX root user has all power
			- Can bypass all permission checks (Equivalent to Administrator on Windows)
			- Dangerous because you can become another user and do anything
			- You can view permission model in System Monitor and see processes owned by root, anilclass, etc. Regular users users cannot usually send kill commands to other processes. If you are the root no one else can send you a signal but you can have access to everything so very dangerous if attacker takes over root
			- Id command calls getuid
			- Kernel does not know where username is stored
			- Hash function returns fixed size input over variable input and are used to securely store passwords. Even the hashes are stored in shadow files because you can password breach by guessing the password and hashing it and comparing it to the hashed password values. Lot of work but can be successful if you have a weak common password. There are rainbow tables that store some of the hashed passwords. Hashed password stored in etc\shadow

			Page tavle entries
			Page tables are like tables (actually more like very wide trees) used to map virtual adrdress to phsyical address
			Contain top bits of physical address along with metadata
			- Readable?, writable? - executable (contains binary code)?
			Most pages are either r+x(code) or r+w(data)

Rquaium at 14:56, 21 October 2017

2017-10-21T14:56:05Z

← Older revision		Revision as of 14:56, 21 October 2017
Line 96:		Line 96:
	- e.g., when was it last accessed		- e.g., when was it last accessed
	</pre>		</pre>

	~~Announcements~~
	~~You may want to use Openstack after midterm~~
	~~Semaphore tutorial is like kernel code~~
	~~Tutorial similar to assignment!~~

	~~Additional Notes~~
	~~Segmentation~~
	~~Inode fields~~
	~~Page table entries~~
	~~Scheduling~~

	~~Paging - fixed sized memory allocations of usually 4k or 8k blocks (cab have some larger ones)~~

	~~Paging only works well if you have hardware to help you - specifically Memory Management Unit~~
	~~Need page tables, TLB (Translation Lookaside Buffer: memory cache that speeds up access time to a user memory lcoation)~~
	~~Do "table lookup" on every memory access~~

	~~If you do not have full paging support how would you manage memory?~~
	~~Basic approach: put everything at a fixed place in memory~~
	~~- Useful if you have limited RAM~~

	~~Classic Way: Put code, data into "segments" instead of pages~~
	~~- Semantic units of memory (stack, heap, "text")~~
	~~- Execve loads segments into memory~~
	~~- Still use this terminology when referring to parts of a binary program~~

	~~Segments:~~
	~~Variable sized block of memory~~
	~~Base: starting address~~
	~~Bound: how long is it~~
	~~Key idea is segments can be placed in different parts of the address space:~~
	~~- to move, you just change the base~~
	~~- Code inside a segment refers to rest of segment via offsets~~
	~~- On each memory access, CPU adds segment register to offset to get physical address~~
	~~- Having a base address with offsets is still used in userspace~~

	~~16-bit x86 (8086) is a segmented architecture~~
	~~- Memory access is msotly relative toa segment base address~~
	~~Standard modes for 32-bit x86 (80386+) and 64-bit x86 (amd64) is "flat" things can be wherever~~
	~~- Segmented memory can be a pain because it can "wrap around" - going past the last address can put you in the first, or it just gives error~~

	~~Inode fields:~~
	~~Stat, lstat gives you details on an inode~~
	~~Inodes allows access to contents of files and have metadata such as indoe number, user id, stuff about time~~
	~~- Size~~
	~~- "Mode" - rwx (he shoes us how to change read write permissions the normal way and also using chmod number to set individual bits and also through GUI), others~~
	~~- Owner, group~~

	~~Xdg-open foo.js same as double clicking knows what preferred program to open a file~~

	~~Users, groups~~
	~~Users and group are used to restrict access~~
	~~- each file has a user and group associated with it, permissions~~
	~~- Each process has a user and group~~
	~~What if file ownership/permissions are "wrong"~~
	~~- Use "whoami" command to find user class~~
	~~In UNIX root user has all power~~
	~~- Can bypass all permission checks (Equivalent to Administrator on Windows)~~
	~~- Dangerous because you can become another user and do anything~~
	- You can view permission model in System Monitor and see processes owned by root, anilclass, etc. Regular users users cannot usually send kill commands to other processes. If you are the root no one else can send you a signal but you can have access to everything so very dangerous if attacker takes over root
	~~- Id command calls getuid~~
	~~- Kernel does not know where username is stored~~
	- Hash function returns fixed size input over variable input and are used to securely store passwords. Even the hashes are stored in shadow files because you can password breach by guessing the password and hashing it and comparing it to the hashed password values. Lot of work but can be successful if you have a weak common password. There are rainbow tables that store some of the hashed passwords. Hashed password stored in etc\shadow

	~~Page tavle entries~~
	~~Page tables are like tables (actually more like very wide trees) used to map virtual adrdress to phsyical address~~
	~~Contain top bits of physical address along with metadata~~
	~~- Readable?, writable? - executable (contains binary code)?~~
	~~Most pages are either r+x(code) or r+w(data)~~

Rquaium: /* Additional Student Notes */

2017-10-21T14:55:22Z

Additional Student Notes

@@ Line 97: / Line 97: @@
 </pre>
-===Additional Student Notes===
+Announcements
-	- Hash function returns fixed size input over variable input and are used to securely store passwords. Even the hashes are stored in shadow files because you can password breach by guessing the password and hashing it and comparing it to the hashed password values. Lot of work but can be successful if you have a weak common password. There are rainbow tables that store some of the hashed passwords. Hashed password stored in etc\shadow
+You may want to use Openstack after midterm

Rquaium: /* Notes */

2017-10-21T14:55:10Z

Notes

← Older revision		Revision as of 14:55, 21 October 2017
Line 96:		Line 96:
	- e.g., when was it last accessed		- e.g., when was it last accessed
	</pre>		</pre>

			===Additional Student Notes===
			- Hash function returns fixed size input over variable input and are used to securely store passwords. Even the hashes are stored in shadow files because you can password breach by guessing the password and hashing it and comparing it to the hashed password values. Lot of work but can be successful if you have a weak common password. There are rainbow tables that store some of the hashed passwords. Hashed password stored in etc\shadow

Soma: Created page with "==Video== Video from the lecture given on October 17, 2017 [http://homeostasis.scs.carleton.ca/~soma/os-2017f/lectures/comp3000-2017f-lec12-17Oct2017.mp4 is now available]...."

2017-10-17T20:20:50Z

Created page with "==Video== Video from the lecture given on October 17, 2017 [http://homeostasis.scs.carleton.ca/~soma/os-2017f/lectures/comp3000-2017f-lec12-17Oct2017.mp4 is now available]...."

New page

==Video==

Video from the lecture given on October 17, 2017 [http://homeostasis.scs.carleton.ca/~soma/os-2017f/lectures/comp3000-2017f-lec12-17Oct2017.mp4 is now available].

==Notes==

===In-Class===

<pre>
Lecture 12
----------

* segmentation
* inode fields
* page table entries
* scheduling

paging, blocks - fixed-sized memory allocations
- typically 4K or 8K (can have some huge ones)

paging only works well if you have MMU
- need page tables, TLB
- do "table lookup" on every memory access

How do you manage memory without paging?

* put everything at a fixed place in memory
- useful if you have very little RAM

* put code, data into "segments"
- semantic units of memory (stack, heap, "text")
- still use this terminology when referring to parts of a binary program

Segments:
* a variable-sized block of memory
* base: starting address
* bound: how long is it

Key idea is segments can be placed in different parts of the address space
- to move, you just change the base
- code inside a segment refers to rest of segment via offsets
- on each memory access, CPU adds segment register to offset to get
physical address
- having a base address with offsets is still used in userspace

16-bit x86 (8086) is a segmented architecture
- memory accesses are mostly relative to a segment base address

standard modes for 32-bit x86 (80386+) and 64-bit x86 (amd64) is "flat"
- segmented memory can be a pain because it can "wrap around" - going past
the last address can put you in the first, or it just gives you an error

inode fields
------------
- stat, lstat gives you details on an inode
- inodes allow access to contents of files and have the key metadata
- size
- "mode" - rwx, others
- owner, group

Users, groups
-------------
users and groups are used to restrict access
- each file has a user and group associated with it, permissions
- each process has a user and group

What if file ownership/permissions are "wrong"?

In UNIX, the "root" user has all power
- can bypass all permission checks
- (unless you are on a mandatory access control system such as seLinux)
- VERY DANGEROUS

(Equivalent to an administrator account on Windows)

Page table entries
------------------

page tables are "tables" (really very wide trees) of page table entries

page table entries contain the top bits of the physical address along with
metadata:
- readable?
- writable?
- executable?

most pages are either r+x (code) or r+w (data)

- valid?
- dirty?
- is data in page different from data on disk?

other bits to help with deciding what pages to keep in RAM or not
- e.g., when was it last accessed
</pre>

← Older revision		Revision as of 15:00, 21 October 2017
Line 111:		Line 111:
	Scheduling		Scheduling

			----
	Paging - fixed sized memory allocations of usually 4k or 8k blocks (cab have some larger ones)		Paging - fixed sized memory allocations of usually 4k or 8k blocks (cab have some larger ones)

			----

	Paging only works well if you have hardware to help you - specifically Memory Management Unit		Paging only works well if you have hardware to help you - specifically Memory Management Unit
	Need page tables, TLB (Translation Lookaside Buffer: memory cache that speeds up access time to a user memory lcoation)		Need page tables, TLB (Translation Lookaside Buffer: memory cache that speeds up access time to a user memory lcoation)
	Do "table lookup" on every memory access		Do "table lookup" on every memory access
			----

	If you do not have full paging support how would you manage memory?		If you do not have full paging support how would you manage memory?
	Basic approach: put everything at a fixed place in memory		Basic approach: put everything at a fixed place in memory
	- Useful if you have limited RAM		Useful if you have limited RAM
			----

	Classic Way: Put code, data into "segments" instead of pages		Classic Way: Put code, data into "segments" instead of pages
	- Semantic units of memory (stack, heap, "text")		Semantic units of memory (stack, heap, "text")
	- Execve loads segments into memory		Execve loads segments into memory
	- Still use this terminology when referring to parts of a binary program		Still use this terminology when referring to parts of a binary program
			----

	Segments:		Segments:
Line 131:		Line 137:
	Bound: how long is it		Bound: how long is it
	Key idea is segments can be placed in different parts of the address space:		Key idea is segments can be placed in different parts of the address space:
	- to move, you just change the base		to move, you just change the base
	- Code inside a segment refers to rest of segment via offsets		Code inside a segment refers to rest of segment via offsets
	- On each memory access, CPU adds segment register to offset to get physical address		On each memory access, CPU adds segment register to offset to get physical address
	- Having a base address with offsets is still used in userspace		Having a base address with offsets is still used in userspace
			----

	16-bit x86 (8086) is a segmented architecture		16-bit x86 (8086) is a segmented architecture
	- Memory access is msotly relative toa segment base address		Memory access is msotly relative toa segment base address
	Standard modes for 32-bit x86 (80386+) and 64-bit x86 (amd64) is "flat" things can be wherever		Standard modes for 32-bit x86 (80386+) and 64-bit x86 (amd64) is "flat" things can be wherever
	- Segmented memory can be a pain because it can "wrap around" - going past the last address can put you in the first, or it just gives error		Segmented memory can be a pain because it can "wrap around" - going past the last address can put you in the first, or it just gives error

			----

	Inode fields:		Inode fields:
	Stat, lstat gives you details on an inode		Stat, lstat gives you details on an inode
	Inodes allows access to contents of files and have metadata such as indoe number, user id, stuff about time		Inodes allows access to contents of files and have metadata such as indoe number, user id, stuff about time
	- Size		Size
	- "Mode" - rwx (he shoes us how to change read write permissions the normal way and also using chmod number to set individual bits and also through GUI), others		"Mode" - rwx (he shoes us how to change read write permissions the normal way and also using chmod number to set individual bits and also through GUI), others
	- Owner, group		Owner, group

	Xdg-open foo.js same as double clicking knows what preferred program to open a file		Xdg-open foo.js same as double clicking knows what preferred program to open a file

			----

	Users, groups		Users, groups
Line 164:		Line 175:
	Kernel does not know where username is stored		Kernel does not know where username is stored
	Hash function returns fixed size input over variable input and are used to securely store passwords. Even the hashes are stored in shadow files because you can password breach by guessing the password and hashing it and comparing it to the hashed password values. Lot of work but can be successful if you have a weak common password. There are rainbow tables that store some of the hashed passwords. Hashed password stored in etc\shadow		Hash function returns fixed size input over variable input and are used to securely store passwords. Even the hashes are stored in shadow files because you can password breach by guessing the password and hashing it and comparing it to the hashed password values. Lot of work but can be successful if you have a weak common password. There are rainbow tables that store some of the hashed passwords. Hashed password stored in etc\shadow

			----

	Page table entries		Page table entries

← Older revision		Revision as of 14:58, 21 October 2017
Line 152:		Line 152:
	Users, groups		Users, groups
	Users and group are used to restrict access		Users and group are used to restrict access
	- each file has a user and group associated with it, permissions		each file has a user and group associated with it, permissions
	- Each process has a user and group		Each process has a user and group
	What if file ownership/permissions are "wrong"		What if file ownership/permissions are "wrong"
	- Use "whoami" command to find user class		Use "whoami" command to find user class

	In UNIX root user has all power		In UNIX root user has all power
	- Can bypass all permission checks (Equivalent to Administrator on Windows)		Can bypass all permission checks (Equivalent to Administrator on Windows)
	- Dangerous because you can become another user and do anything		Dangerous because you can become another user and do anything
	- You can view permission model in System Monitor and see processes owned by root, anilclass, etc. Regular users users cannot usually send kill commands to other processes. If you are the root no one else can send you a signal but you can have access to everything so very dangerous if attacker takes over root		You can view permission model in System Monitor and see processes owned by root, anilclass, etc. Regular users users cannot usually send kill commands to other processes. If you are the root no one else can send you a signal but you can have access to everything so very dangerous if attacker takes over root
	- Id command calls getuid		Id command calls getuid
	- Kernel does not know where username is stored		Kernel does not know where username is stored
	- Hash function returns fixed size input over variable input and are used to securely store passwords. Even the hashes are stored in shadow files because you can password breach by guessing the password and hashing it and comparing it to the hashed password values. Lot of work but can be successful if you have a weak common password. There are rainbow tables that store some of the hashed passwords. Hashed password stored in etc\shadow		Hash function returns fixed size input over variable input and are used to securely store passwords. Even the hashes are stored in shadow files because you can password breach by guessing the password and hashing it and comparing it to the hashed password values. Lot of work but can be successful if you have a weak common password. There are rainbow tables that store some of the hashed passwords. Hashed password stored in etc\shadow

	Page table entries		Page table entries
	Page tables are like tables (actually more like very wide trees) used to map virtual adrdress to phsyical address		Page tables are like tables (actually more like very wide trees) used to map virtual adrdress to phsyical address
	Contain top bits of physical address along with metadata		Contain top bits of physical address along with metadata
	- Readable?, writable? - executable (contains binary code)?		Readable?, writable? - executable (contains binary code)?
	Most pages are either r+x(code) or r+w(data)		Most pages are either r+x(code) or r+w(data)