So you need to restore it, let the instruction be processed by CPU, and then put INT3 back - so the next time this instruction is executed, you get break again – Xeor Oct 12 '10 at 20:24 Using a software breakpoint as we did when executed db 0x40066d is adding CC (INT3) to the instruction and thus it changed the result of the calculation that is done by the loop at 0x400661. The term interrupt, fault, and exception are used. Types of Exceptions The terminology used to describe exceptional situations where the normal execution order of instruction is changed varies among machines. Breakpoint Interrupt-Type 3: The type 3 interrupt is produced by execution of the INT3 instruction. Learn vocabulary, terms, and more with flashcards, games, and other study tools. This allows us to verify that the exploit was successful. After executing an instruction, the cs and eip pair of registers contain the logical address of the next instruction to be executed.

of a push esp / ret or some other type of instruction that transfers control to where the series of int3’s should reside. The main use of the type 3 interrupt is to implement a breakpoint function in a system. This interrupt vector is initialized during system startup, along with other important vectors like the system … For example, this … For example, INT 13H will generate the software interrupt 0x13 (19 in decimal), causing the function pointed to by the 20th vector in the interrupt table to be executed, which is typically a DOS API call. Once debugger starts to handle the int3 (#BP), if you want single-stepping, tell the debugger to set the Trap Flag (TF) in EFLAGS. Let us look at a very simple example that inserts a breakpoint in a program at compile time and not through a debugger. When the INT3 is executed, if the program is not debugged, control will be given to the exception handler of the protection and execution will continue. The type 3 interrupt is produced by execution of the INT3 instruction. For example, the CPU issues an exception if the current instruction tries to divide by 0.

This special instruction on the Intel platform is “int 3”. xyz are for the 2nd, 3rd, and 4th instructions after IT. Then CPU will generate a int 1 (#DB) after each single instruction. The breakpoint exception is commonly used in debuggers: When the user sets a breakpoint, the debugger overwrites the corresponding instruction with the int3 instruction so that the CPU throws the breakpoint exception when it reaches that line. It will then get the CS value for the start of the type 2 interrupt service procedure from address 0000AH and the IP value for the start of the procedure from address 00008H. When executed it calls the debugger’s exception handler. Start studying Chapter 12 Exception Types. This concludes our … Example. We use the term exception to cover all these mechanisms, including the following: I/O device request When this instruction is executed, a specific

We use vector 0x80 to transfer control to the kernel. IT (If-then)'s syntax: ITxyz cc.

Force the exception If the page contains RETN instruction, replace it with INT3, RETN Like above, pass INT3 exception with Shift+F6 Let the handler run and update the magic values Then RETN will proceed as normal More work, if exception handler checks for the exception vector Patch it manually