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

2. Unix system calls (part 1) http://codeschool.org/ This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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

4. It’s a Unix system!

5. 1980’s System V BSD

6. today Linux Mac OS X FreeBSD, OpenBSD

7. POSIX (Portable Operating System Interface for Unix)SUS(Single UnixSpecification)

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

9. … … system call 7 0xFF 31 01 11 system call 6 0xFF 90 44 44 system call 5 0xFF 31 01 11 system call 4 0xFF 31 21 14 system call 3 0xA2 22 00 10 system call 2 0x82 87 95 94 system call 1 0x20 15 10 00 system call 0 0x76 00 00 00

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

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

12. created terminated waiting running blocked

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

14. ssize_t read(intfd, void *buf, size_t count);

15. ssize_t read(intfd, void *buf, size_t count); read(fd)

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

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

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

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

20. 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)

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

22. garbage collection

23. iffork() == 0: …// new (child) process else: …// original (parent) process

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

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

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

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

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

29. exec stack heap heap code

30. exec code (executable)

31. iffork() == 0: // new (child) process exec(‘/games/pong’) else: …// original (parent) process

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

33. _exit (terminate the process) _exit(0)

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

35. 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

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

37. user accounts: /etc/passwd

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

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

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

41. 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)

42. 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

43. pid2 (login), user 0 pid 1 (init), user 0 • pid3 (shell), user 1780

44. fork, exec • pid2 (login), user 0 pid 1 (init), user 0 • pid3 (shell), user 1780

45. fork, exec • pid2 (login), user 0 pid 1 (init), user 0 fork, setuid, exec • pid3 (shell), user 1780

46. 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

47. rwxrwxrwx user group other

48. rwxrwxrwx user group other if file_user_id == effective_user_id: user class else iffile_group_id == effective_group_id: group class else: other

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

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