int instruction from user space

Question

I was under the impression that \"int\" instruction on x86 is not privileged. So, I thought we should be able to execute this instruction from the user space application. But do

Answer 1

Generally the old dispatcher mechanism for user mode code to transition to kernel mode in order to invoke kernel services was implemented by int 2Eh (now replaced by sysenter). Also int 3 is still reserved for breakpoints to this very day.

Basically the kernel sets up traps for certain interrupts (don't remember whether for all, though) and depending the trap code they will perform some service for the user mode invoker or if that is not possible your application would get killed, because it attempts a privileged operation.

Details would anyway depend on the exact interrupt you were trying to invoke. The functions DbgBreakPoint (ntdll.dll) and DebugBreak (kernel32.dll) do nothing other than invoking int 3 (or actually the specific opcode int3), for example.

Edit 1: On newer Windows versions (XP SP2 and newer, IIRC) sysenter replaces int 2Eh as I wrote in my answer. One possible reason why it gets terminated - although you should be able to catch this via exception handling - is because you don't pass the parameters that it expects on the stack. Basically the usermode part of the native API places the parameters for the system service you call onto the stack, then places the number of the service (an index into the system service dispatcher table - SSDT, sometimes SDT) into a specific register and then calls on newer systems sysenter and on older systems int 2Eh.