Question

Let’s say that — for some strange reason — the stack pointer, ESP, for some function gets decremented momentarily and then incremented again:

;; ... we're saving registers ...
push EAX
push EBX
push ECX
push EDX

add ESP, 4        ;; Whoops!
sub ESP, 4        ;; Ah, we're fine; we restored it... or are we?

Now, it’s perfectly possible for an interrupt to get triggered immediately before your sub instruction.

If I understand correctly, an interrupt will cause the CPU to push a few values onto the stack.

Does that mean your stack will now be corrupted? Or does the OS somehow (how?) use a different stack/memory to store the context of the program? Or does it depend on the privilege level of the CPU? (If so, how?)

Answer 1

This post seems to contradict the answers above, stating that it this is in fact safe:

Aren’t you screwed if an interrupt occurs?

Those of you who have programmed in DOS are likely squirming at this point about the possibility of interrupts. Ordinarily, reusing the stack pointer like this is a really bad idea because you have no idea when an interrupt might strike, and when one does, the CPU dutifully pushes the current program counter and flags onto the stack. If you have reused ESP, this would cause random data structures to be trashed. In this kind of environment, ESP must always point to valid and sufficient stack space to service an interrupt, and whenever this does not hold, interrupts must be disabled. Running with interrupts disabled for a long time lowers system responsiveness (lost interrupts and bad latency), and isn’t practical for a big routine.

However, we’re running in protected mode here.

When running in user space in Win32, interrupts do not push onto the user stack, but onto a kernel stack instead. If you think about it, it isn’t possible for the user stack to be used. If the thread were out of stack space, or even just had an invalid stack, when the CPU tried to push EIP and EFLAGS, it would page fault, and you can’t page fault in an interrupt handler. Thus, the scheduler can do any number of context switches while a no-stack routine is running, and any data structures that are being pointed to be ESP will not be affected.