System Calls & Libraries

1. System Calls & Libraries Vivek Pai Lecture 4, COS318 Sep 25, 2001

2. Gedankundmathematics Recall the pointer verification case for fread( ) • Can you speed up the checking process? • What’s the best you could achieve? O(n)? O(logn)? O(1)? • What happens if you have >32 bits? • Aside: # atoms in universe = 1080, or 2256 • Does this provide any other benefits? System Calls & Libraries

3. Mechanics • Is the project workable? • Has everyone started? • Barring major problems, due Tuesday midnight • Readings updated System Calls & Libraries

4. Protection Issues • I/O protection • Prevent users from performing illegal I/Os • Memory protection • Prevent users from modifying kernel code and data structures • CPU protection • Prevent a user from using the CPU for too long System Calls & Libraries

5. Protection Is Not Safety/Security • Protection is a prerequisite • Safety can be separation of concerns • Security related to overall design Examples? • Bad pointer access causing seg fault • Sniffing cleartext passwords on the wire System Calls & Libraries

6. Support in Modern Processors:User  Kernel An interrupt or exception (INT) User mode • Regular instructions • Access user-mode memory Kernel (privileged) mode • Regular instructions • Access user-mode memory A special instruction (IRET) System Calls & Libraries

7. Why a Privileged Mode? • Special Instructions • Mapping, TLB, etc • Device registers • I/O channels, etc. • Mode Bits • Processor features • Device access System Calls & Libraries

8. x86 Protection Rings Privileged instructions Can be executed only When current privileged Level (CPR) is 0 Level 3 Level 2 Level 1 Operating system kernel Level 0 Operating system services Applications System Calls & Libraries

9. Other Design Approaches • “Capabilities” • Fine-grained access control • Crypto-like tokens • Microkernels • OS services in user space • Small core “hypervisor” System Calls & Libraries

10. Monolithic User program User program • All kernel routines are together • A system call interface • Examples: • Linux • Most Unix OS • NT return call entry Kernel many many things System Calls & Libraries

11. Monolithic Pros and Cons Pros • Relatively few crossings • Shared kernel address space • Performance Cons • Flexibility • Stability • Experimentation System Calls & Libraries

12. Layered Structure • Hiding information at each layer • Develop a layer at a time • Examples • THE (6 layers) • MS-DOS (4 layers) Level N . . . Level 2 Level 1 Hardware System Calls & Libraries

13. Layering Pros and Cons Pros • Separation of concerns • Simplicity / elegance Cons • Boundary crossings • Performance? System Calls & Libraries

14. Microkernel User program Services • Micro-kernel is “micro” • Services are implemented as regular process • Micro-kernel get services on behalf of users by messaging with the service processes • Examples: Taos, Mach, L4 return call entry m-kernel System Calls & Libraries

15. Microkernel Pros and Cons Pros • Easier to develop services • Fault isolation • Customization • Smaller kernel => easier to optimize Cons • Lots of boundary crossings • Really poor performance System Calls & Libraries

16. Virtual Machine • Virtual machine monitor • provide multiple virtual “real” hardware • run different OS codes • Example • IBM VM/370 • virtual 8086 mode • Java • VMWare user user OS1 OSn . . . VM1 VMn Small kernel Bare hardware System Calls & Libraries

17. Hardware Support • What is the minimal support? • Can virtual machine be protected without such support? • Hint: what is a Turing machine? System Calls & Libraries

18. User code can be arbitrary User code cannot modify kernel memory Makes a system call with parameters The call mechanism switches code to kernel mode Execute system call Return with results System Call Mechanism User program User program return call entry Kernel in protected memory System Calls & Libraries

19. Interrupt and Exceptions • Interrupt Sources • Hardware (by external devices) • Software: INTn • Exceptions • Program error: faults, traps, and aborts • Software generated: INT 3 • Machine-check exceptions • See Intel document chapter 5, volume 3 for details System Calls & Libraries

20. Interrupt and Exceptions (1) System Calls & Libraries

21. Interrupt and Exceptions (2) System Calls & Libraries

22. System Calls • Interface between a process and the operating system kernel • Categories • Process management • Memory management • File management • Device management • Communication System Calls & Libraries

23. Interrupt service routines System services Exception handlers Exception dispatcher VM manager’s pager OS Kernel: Trap Handler HW Device Interrupt Sys_call_table System service dispatcher System Service Call HW exceptions SW exceptions Virtual address exceptions HW implementation of the boundary System Calls & Libraries

24. Passing Parameters • Affects and depends on • Architecture • Compiler • OS • Different choices for different purposes System Calls & Libraries

25. Passing Parameters - Registers Place parameters in registers • # of registers • # of usable registers • # of parameters in system call • Spill/fill code in compiler Really fast System Calls & Libraries

26. Passing Parameters - Vector Register holds vector address • Single register • Vector in user’s memory • Nothing horrible, just not common System Calls & Libraries

27. Passing Parameters - Stack Place parameters on stack • Similar to vector approach • Stack already exists • Gets copied anyway Top frame frame System Calls & Libraries

28. Use read( fd, buf, size) as an example: int read( int fd, char * buf, int size) { move fd, buf, size to R1, R2, R3 move READ to R0 int $0x80 move result to Rresult } Library Stubs for System Calls User stack User memory Registers Kernel stack Registers Linux: 80 NT: 2E Kernel memory System Calls & Libraries

29. Assume passing parameters in registers EntryPoint: switch to kernel stack save context check R0 call the real code pointed by R0 restore context switch to user stack iret (change to user mode and return) System Call Entry Point User stack User memory Registers Kernel stack Registers Kernel memory System Calls & Libraries

30. Design & Performance Issues • Can user code lie? • One result register – large results? • Parameters in user memory • Multiprocessors System Calls & Libraries

31. General Design Aesthetics • Simplicity, obviousness • Generality – same call handles many cases • Composition / decomposition But: • Expressiveness • Performance System Calls & Libraries

32. Separation Of Concerns Memory management • Kernel allocates “pages” – hw protection • Programs use malloc( ) – fine grained • Kernel doesn’t care about small allocs • Allocates pages to library • Library handles malloc/free System Calls & Libraries

33. Library Benefits • Call overhead • Chains of alloc/free don’t go to kernel • Flexibility – easy to change policy • Fragmentation • Coalescing, free list management • Easier to program System Calls & Libraries

34. Feedback To The Program • System calls, libraries are program to OS • What about other direction? • Various exceptional conditions • General information, like screen resize • When would this occur? Answer: signals System Calls & Libraries