Operating Systems 2017F Lecture 12

Video

Video from the lecture given on October 17, 2017 is now available.

Notes

In-Class

Lecture 12
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* 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
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 - 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
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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
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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

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