CSCE 548 - Student presentation LARRY PARKER CATCHING EXCEPTIONS CSCE 548 - Student presentation LARRY PARKER

Technical Overview - Exception handling is an often misused feature - What is Exception Handling? - When something goes wrong & the program or system executes immediate actions - Try-Catch blocks are used in programming languages - Windows operating systems/Objective C++ have structured exception handling - 3 types of blocks (try, except, & finally) - UNIX-based operating systems/Linux/Mac Os utilize signal handling

Example #1 Sinful Structured Exception Handling (SEH) - Microsoft-Windows operating system - Uses Structured Exception Handling - Includes keywords (try, except, & finally) int Filter ( DWORD dwExceptionCode ) { if ( dwExceptionCode == EXCEPTION_INTEGER_OVERFLOW ) return EXCEPTION_EXECUTE_HANDLER; else

Example #1 (cont’d) return EXCEPTION_CONTINUE_SEARCH; } void Foo() { __try //invokes the exception handler in the first _except block; operating system creates pop-ups { DoSomethingScary(); } __except( Filter( GetExceptionCode() ) ) //the filter expression is called & makes decisions based on exception code { printf ("Integer overflow!\n"); return E_FAIL; } __finally //If the _try block is exited normally or through an exception, then the _finally block is executed { // Clean up from __try block } }

Example #2 Sinful Signal Handling - UNIX-based operating systems - Signal handlers process various signals - Passed into a process - Errors that happen internal to a process - User-defined signals - Problems - Unstable application being repaired through recovery or performance clean-up tasks may cause additional problems - Common to UNIX-based operating systems (BSD, System V, & Linux)

Example #2 (cont’d) - Signal handlers will resume at the instruction that raised the signal - Ex. A SH for numerical error, such as divide by zero, can easily get into an infinite loop

Detection Methods - Pattern - Using catch(…) - Using catch(Exception) - Using _except(EXCEPTION_EXECUTE_HANDLER) - Code Review - Look for catch blocks that catch all exceptions - Examine whether exception-safe techniques are in use - Search for uninitialized variables - Testing Techniques - Attach a debugger & cause SEH exception handler to break on all first-chance exceptions

Avoidance of Error - Examine code for code that catches exceptions or handles signals - Ensure clean-up tasks occur on properly initialized objects - Review try-except blocks - Ensure no __except blocks handle exceptions - Audit signal handles - Only safe functions are called - Do not ever attempt to handle segmentation faults

Conclusion - Do catch only specific exceptions. - Do handle only structured exceptions that your code can handle. - Do handle signals with safe functions. - Do not catch(…). - Do not catch (Exception). - Do not __except(EXCEPTION_EXECUTE_HANDLER). - Do not handle SIG_SEGV signals, except to log.

References Howard, Michael, David LeBlanc and John Viega. 24 Deadly Sins of Software Security. New York: McGraw-Hill Co., 2010. Kindle Edition