CS 480 M01 Lecture Notes 7 - Sep 14, 2022

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Ch 4. Process Control Part 2

Process Monitoring
/proc Filesystemd, strace, Runaway Processes
Periodic Processes
cron and crontab (command and files)
common uses

Ch 5. File System

• "everything" is mapped through the file system: regular files, directories, processes, devices, kernel data, IPC channels, ...
• basic purpose : to represent and organize the system’s storage resources
• other objects mapped into the filesystem namespace too and managed through it
• 4 main components:
  1. namespace - naming and organizing
  2. API - navigating & manipulating objects
  3. security model - scheme for protecting, hiding, sharing
  4. implementation - sw tying the model to actual hw
• Pathnames - single unified hierarchy that starts at the root directory ( "/ "- in each file system)
  - absolute (/tmp/foo) or relative ( book4/filesystem - same as ./book4/filesystem)
  
• Mounting / Unmounting
  overall filesystem / file tree is composed of individual pieces also called filesystems: each of those starts with / and is created on a single partition or logical volume normally (also network server, kernel component, "loop" for individual files, ...)
  mount, umount, fsck, fuser, lsof , /etc/fstab

  	fuser /dev/tty7
  	echo $?
  	fuser /dev/tty1
  	echo $?
  	if fuser -s /dev/tty1; then :; else echo "NOT USED"; fi
  	lsof /dev/sda2
  	lsof -i 4 -a -p 1458
  	

• FS Layout / File Tree
• File Types
  Related commands & system calls: cp, mv , mkdir, rmdir, mknod, ln -s, socket, link, unlink, rm
  (rm -- -f ...     rm -i ...)
  
  1. Regular Files no structure, both sequential and random access allowed
  2. Directories
     • contain named references to other files (i-node number + name)
       file names stored within its parent directory (more than one entry can refer to the same file)
     • special :   . ,   ..
     • links (i-node num) are within the FS - 'hard links'
     • cp vs. ln
  3. Character Device Files
  4. Block Device Files
     • Allow kernel to comunicate with systems HW and peripherals through modules - device drivers ( requests passed to them from kernel)
     • just a rendezvous point to comunicate with drivers
     • char - drivers allowed to do their buffering
     • block - data accessed through large chunks, kernel does buffering
     • characterized by major (which driver) and minor (which physical unit - usually - up to the driver cwto interpret it ) dev number:
       ex. /dev/lp0 is 6,0 - major 6 for parallel port driver, minor 0 for the first of those
     • in past ... mknod, rm and script /dev/MAKEDEV
     • now udev & udevd - Linux dynamic device management & event managing daemon
  5. Unix (Local) Domain Sockets
     Local (not network) communication connections between processes socket(), rm, unlink
  6. Named Pipes (FIFO files) mknod, rm
     Also for communication between processes on a single host - mknod, rm
  7. Symbolic (Soft) Links
     • reference by name (absolute or relative path)
     • can cross FSs or reference non-existing file, loops can be created
     • create with ln -s
     • ! when working with them (chmod, chown, rm -r , ...)
• File Attributes
  • Traditionally 12 bits for each file: the file's mode (plus 4 more bits : file's type)
  • 9 permission bits - read, write, execute for owner, group, others
  • setuid & setgid bits (4000 , 2000)
    • setgid on directory - newly created file has group ownership of the directory (not group ownership of a user creating it)
  • sticky bit (1000)
    • on regular files ignored (original meaning: keep program text on swap device)
    • on directories - only the owner of the file and the owner of that directory may remove the file from that directory

    ---

    to finish on 9/14 below ...
    
    
  • ls -l (ls -ld), chmod
  • chmod:
    • encoding
      
    • mnemonic syntax
      
    • and reference (chmod --reference= ....)
  • chown user.group file (-R)
  • umask encoding
    
  • Ext2 and Ext3 bonus flags plus lsattr and chattr:
    • A - never update access time
    • a - allow writing only (append)
    • D - force dir updates to be written synchronously
    • d - no backup ( make dump ignore this file)
    • i - immutable and undeletable ( only root)
    • j - keep journal data changes as well as metadata
    • S - force changes to be written synchronously ( no buffering)
• Access Control Lists
  • supported for ext2, ext3, ext4, reiserfs, XFS, JFS
  • mount -o [no]acl
  • allows rwx to be set independantly for any user.group combination
  • getfacl, setfacl ( plus man acl)
  • NFSv4 - superset of POSIX ACLs plus all permission bits and most semantics from Windows

Ch 8. User Management

traditional account model only (not PAM nor LDAP) for now ...
• passwd file
  • login names: ≤ 32 chars, case sensitive and (in some cases) even special chars ( ☹, ...) but Bob x bob (standardize on lower case only, some mail systems may treat them as the same user) , ..., ( legacy / NIS - only 8)
  • encrypted password ( or *) - do NOT leave empty
    • DES, MD5($1$..), Blowfish($2y$), SHA-256 ($5$), SHA-512 ($6$),
      check /etc/login.defs or (was) /etc/default/passwd plus PAM ( and on RHEL/CentoS authconfig)..
  • UID (32-bit integer)
    • more 0 ?
    • do not recycle them (or as late as possible) - ? why
    • should be unique in the whole organization ( else NFS problems, ..)
  • GID
  • GECOS (finger's interpretation)
  • home dir
  • login shell
• shadow file (x in passwd file)
  • Login name
  • Encrypted password
  • Days since Jan 1, 1970 that password was last changed
  • Days before password may be changed
  • Days after which password must be changed
  • Days before password is to expire that user is warned
  • Days after password expires that account is disabled
  • Days since Jan 1, 1970 that account is disabled
  • A reserved field

  • useradd, userdel, usermod, pwconv, pwunconv
    /etc/default/useradd ..
• group & gshadow file
  • group name, password, GID, user list
  • groupadd, groupdel, grpconv, grpunconv
  • newgrp ( different from groupadd)
• Adding Users
  • Required minimum (for each user):
    1. entry in passwd & shadow files,
    2. set initial password,
    3. create home directory (owned by user and users default/primary group)
       (should also copy the default startup files from /etc/skel and change ownership)
  • Plus
    • mail, groups, disk quotas, ...
    • (roles and admin privileges)
  • Editing passwd and shadow : useradd, vipw
  • Setting a password: passwd
  • Creating home directory: mkdir + cp or straight cp -pr plus chmod, chown, ..
  • adding users in bulk: newusers
• Removing Users
  • removing ALL references to login name
  • userdel
  • by hand : checklist in the book
  • orphaned files : find with -nouser (+ -xdev or -mount)
  • backup before delete
• Disabling Logins
  • invalid shell + message
  • is it enough ? (ftp ...)
• PAM and LDAP ...