1. Mid Review of Class Argument Validation and Synchronization Guidelines April 26, 2000 Instructor: Gary Kimura

2. 26/26/2015 Today’s Lecture Quick class outline of what we have and will be covering Two loose ends –Argument validation –Choosing the right synchronization tool Review of material so far But before I forget, start reading Chapter 8 for Friday

3. 36/26/2015 Class Material Breakdown OS Kernel, processes, threads, scheduling, synchronization, and deadlocks (1 st third of class) Memory management, I/O, File Systems, Secondary storage (2 nd third of class) Special topics such as distributed systems, accounting, security, RPC, etc. (remaining time)

4. 46/26/2015 Argument Validation User supplied parameters are never to be trusted The kernel must validate all user supplied parameters –The range of the input values must be verified –Output buffers must be verified as writeable –Input buffers must be readable and contain correct values –Privileges must be checked if applicable Remember that kernel mode can access data that user mode cannot. So the kernel must not blindly assume pointers are good just because the kernel can access the memory

5. 56/26/2015 Probe and Capture One method used is to probe and capture user parameters –Probing is verifying that a pointer supplied by the user is valid –Capturing is making a copy of any user supplied input data buffer –All copying of data to and from user buffers must be “bullet- proof” Open file example –OpenFile( IN PSTRING FileName, … ); –The kernel must probe and capture the file name into a private buffer. Then it can work through trying to actually open the file. –What can happen if the kernel keeps using the user buffer?

6. 66/26/2015 Some Argument Validation Gotchas Range checking of input values is not too hard but what happens if… –The user supplied a good kernel address for either an input or output buffer? –The user unmaps or remaps a buffer while the kernel is using it? –System calls that take a pointer to a structure that itself contains pointers are a validations worse nightmare.

7. 76/26/2015 Choosing the Right Synchronization Tool Often times it is helpful to view the necessary synchronization method as either code centric or data centric –Code Centric meaning that we want to structure the program such that executing sections of code are mutually exclusive –Data Centric meaning that we want controlled access to a data item. The latter one can be shared and/or exclusive access. All the synchronization methods can be used for code or data centric access but keeping the correct paradigm in minds helps in the overall design.

8. 86/26/2015 Synchronization Tools Spinlocks - for exclusive access, cheap and fast, but serializes a lot, and wastes coprocessor cycles, other usage limitations Great for code centric exclusion Interlocked Instructions - cheap and fast, somewhat difficult to use. Great for small data centric synchronization Mutexes - for exclusive access, sort of cheap, can lead to context switches. Great for code centric exclusion Semaphores - sort of cheap, can lead to context switches. In the raw form somewhat difficult to use. Events - sort of cheap, can lead to context switches. Great for synchronizing threads. Full blown reader/writer resources - Most expensive, but a great programming paradigm to use especially for data centric access

9. 96/26/2015 Review of Material so Far Major components and functions of an OS Hardware support Processes/threads and scheduling Synchronization and deadlocks (just talked about)

10. 106/26/2015 Major Components Process management Memory management I/O management Secondary storage management File management Protection system Accounting Etc.

11. 116/26/2015 Hardware Support Timers Synchronization (atomic instructions) Memory protection I/O control and operation Interrupts and exceptions Dual execution modes Protected instructions System calls

12. 126/26/2015 Processes/Threads and Scheduling What is a process/thread Process/threads states Scheduling algorithms –First-Come First-Served –Shortest Job First –Round Robin –Priority based scheduling What is a context switch