IIRC you have to probe every page of the stack on Windows. You cannot just subtract a value from ESP/RSP. If you don't probe every page in order, you get a page fault or some other exception (I don't remember which one).
IIRC you have to probe every page of the stack on Windows. You cannot just subtract a value from ESP/RSP. If you don't probe every page in order, you get a page fault or some other exception (I don't remember which one).
The reason for this is to ensure stack overflows are detected. The OS places a guard page above the top of the stack, which will cause a segfault if accessed. That way stack overflows are guaranteed to crash rather than stomping on valid memory that belongs to something else. However, if a stack frame is larger than a page (say, because it includes a large buffer), then it is possible for the program to "jump over" the guard page and access memory beyond.
In order to protect against this, if a stack frame is larger than a page, then the compiler inserts some dummy reads or writes to ensure every page is touched in order from bottom to top. This ensures the guard page is hit before the application has a chance to write to memory beyond it.
Here's an example: https://godbolt.org/z/oTbzTczM6
How else would the OS know your read/write 16 pages away from the current stack pointer is in fact an attempt to increase the stack and not just really bad pointer arithmetic and a bug? How many pages should the runtime let you skip before its just a segfault?