This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. Unix system calls (part 1) history and usage of Python basic data types and the type hierarchy syntax modules and variable scopes

This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. Unix system calls (part 1)

This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. This is one part of a larger series. You may need to view previous parts to understand this material.

It’s a Unix system!

System V BSD 1980’s

Linux Mac OS X FreeBSD, OpenBSD today

POSIX (Portable Operating System Interface for Unix) SUS (Single Unix Specification)

Process C Process B kernel Process A jump to system call code via special instruction RAM

0x system call 0 0x system call 1 0x system call 2 0xA system call 3 0xFF system call 4 0xFF system call 5 0xFF system call 6 0xFF system call 7 ……

stack code heap kernel code  pages only accessible in system calls jump to system call code via special instruction

frame of main frame of cat frame of dog frame of fish stack space frame of syscall

created waitingrunning blocked terminated

processes files networking sockets signals inter-process communication terminals threads I/O devices

ssize_t read(int fd, void *buf, size_t count);

read(fd)

process: address space user ids file descriptors environment current and root directory stack heap code heap

stack code initialized data heap kernel code uninitialized data  global variables with initial values  global variables without initial values

stack code initialized data heap kernel code uninitialized data  a.k.a. the “text”  global variables with initial values  global variables without initial values  starts empty, grows automatically  explicitly allocated during execution

mmap (‘memory map’ pages to the process address space) munmap (‘memory unmap’ pages from the process address space)

mmap (‘memory map’ pages to the process address space) munmap (‘memory unmap’ pages from the process address space) address = mmap(5000) … # do stuff with memory at address munmap(address)

stack code initialized data heap kernel code uninitialized data heap mmap fails when not enough space

if fork() == 0: … // new (child) process else: … // original (parent) process

RAM byte 0 byte n HD stack heap code heap fork

stack heap code heap RAM byte 0 byte n HD stack heap code heap fork

stack heap code heap RAM byte 0 byte n HD stack heap code heap fork

stack heap code heap RAM byte 0 byte n HD stack heap code heap write  fork

stack heap code heap RAM byte 0 byte n HD stack heap code heap copy write  fork

stack heap code heap exec

code exec (executable)

if fork() == 0: // new (child) process exec(‘/games/pong’) else: … // original (parent) process

pid 1 (init) pid 85 pid 17 pid 24pid 230 pid 104 pid 34 pid 50

_exit (terminate the process) _exit(0)

wait (block the process until child process terminates) pid = fork() if pid == 0: // new (child) process exec(‘/games/pong’) else: // original (parent) process code = wait(pid)

TERM=xterm SHELL=/bin/bash USER=greys MAIL=/var/mail/ted PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin PWD=/home/ted EDITOR=vim name=value

pid 1 (init), user 0 pid 85, user 8 pid 17, user 4 pid 24, user 33pid 230, user 8 pid 104, user 33 pid 34, user 4 pid 50, user 4

user accounts: /etc/passwd

user accounts: /etc/passwd superuser/root = user id 0 privileged to do anything it wants

each process has three user ids: each file and directory is owned by a single user “real” id: the owning user “effective” id: determines privileges “saved” id: set by exec to match the effective id

exec (sets effective and saved ids when binary file has setuid bit)

seteuid (sets effective user id) setuid (sets real, effective, and saved user ids)

exec (sets effective and saved ids when binary file has setuid bit) seteuid (sets effective user id) setuid (sets real, effective, and saved user ids) non-superuser can only directly set effective id to match the real or saved id

pid 1 (init), user 0 pid 3 (shell), user 1780 pid 2 (login), user 0

pid 1 (init), user 0 pid 3 (shell), user 1780 pid 2 (login), user 0 fork, exec

pid 1 (init), user 0 pid 3 (shell), user 1780 pid 2 (login), user 0 fork, exec fork, setuid, exec

user groups: /etc/group user may belong to multiple groups but has one “primary” group each file and directory is owned by one group each process has a real, effective, and saved group id binary files have setgid bit setegid and setgid

rwx rwx rwx usergroupother

rwx rwx rwx usergroupother if file_user_id == effective_user_id: user class else if file_group_id == effective_group_id: group class else: other

file permissions: read: can read bytes of file write: can modify bytes of file execute: can exec file

directory permissions: read: can get names of files write: can add/remove/rename files execute: can use in file paths

directory permissions: read: can get names of files write: can add/remove/rename files execute: can use in file paths /adams/taft/garfield/eisenhower

directory permissions: read: can get names of files write: can add/remove/rename files execute: can use in file paths /adams/taft/garfield/eisenhower /adams/taft/ (OK)

r-xr-xr-x rw-r----- r-x--x--x rwx /adams/lincoln /adams/cleveland /roosevelt /fillmore rwx rwx rwx usergroupother

dr-xrw-r-x /adams/ rwx rwx rwx usergroupother

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