What is Structured Exception Handling and how it works?
Microsoft's Structured Exception Handling is a mechanism for handling hardware and software exceptions (both system and user defined), which allows recovering from errors and perform cleanup if necessary instead of terminating a program immediately.
The SEH represented as a linked list, whose records are stored on the stack. To ease access to this SEH chain, its head pointer maintained in Win32 Thread Information Block (TIB) structure. The TIB structure stores information about currently running thread. On x86 systems, the FS segment register points on TIB structure. SEH chain head located at offset 0x00, and therefore, you can refer to SEH chain head as FS:.
Each entry (_EXCEPTION_REGISTRATION_RECORD structure) consists from two 4-byte pointers:
- Pointer to the next exception registration record in the chain
- Pointer to the exception handling routine
The chain's last record always contains 0xFFFFFFFF value as the "next entry" and pointer to OS default exception handler routine (located in ntdll.dll!FinalExceptionHandler)