gistfile1.txt

#include <Windows.h>
#include <stdio.h>
#include <stdint.h>
// #pf detection from usermode, by waryas

void wgs(uint16_t value) {
    __asm__ volatile("mov %0, %%gs" : : "r"(value));
}

uint16_t rgs() {
    uint16_t gs;
    __asm__ volatile("mov %%gs, %0" : "=r"(gs));
    return gs;
}

int main() {
    auto x = VirtualAlloc(0, 0x1000, MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);

    *(uint64_t*)x = 5; // #pf for first write
    VirtualUnlock(x, 0x1000); // if u comment this out, next write won't #pf
    wgs(1);
    unsigned int count = 0;

    *(uint64_t*)x = 5; // if virtualunlock this will #pf and count will be 0

    _asm {
        mov rax, 0x0
        syscall //syscall that don't generate interrupt won't reset gs segment
    }

    while (rgs() == 1)
        ++count;

    // interrupt happened, only way for gs segment to be reset.

    printf("Took %d loop\n", count);

    return 0;
}