#include <fstream>
#include <iostream>
#include <sys/mman.h>
/* Write @len bytes at @ptr to @addr in this address space using
* /proc/self/mem.
*/
void memwrite(void *addr, char *ptr, size_t len) {
  std::ofstream ff("/proc/self/mem");
  ff.seekp(reinterpret_cast<size_t>(addr));
  ff.write(ptr, len);
  ff.flush();
}
int main(int argc, char **argv) {
  // Map an unwritable page. (read-only)
  auto mymap =
      (int *)mmap(NULL, 0x9000,
                  PROT_READ, // <<<<<<<<<<<<<<<<<<<<< READ ONLY <<<<<<<<
                  MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
  if (mymap == MAP_FAILED) {
    std::cout << "FAILED\n";
    return 1;
  }
  std::cout << "Allocated PROT_READ only memory: " << mymap << "\n";
  getchar();
  // Try to write to the unwritable page.
  memwrite(mymap, "\x40\x41\x41\x41", 4);
  std::cout << "did mymap[0] = 0x41414140 via proc self mem..";
  getchar();
  std::cout << "mymap[0] = 0x" << std::hex << mymap[0] << "\n";
  getchar();
  // Try to writ to the text segment (executable code) of libc.
  auto getchar_ptr = (char *)getchar;
  memwrite(getchar_ptr, "\xcc", 1);
  // Run the libc function whose code we modified. If the write worked,
  // we will get a SIGTRAP when the 0xcc executes.
  getchar();
}

static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
{
        [...]
        if (write) { // If performing a write..
                if (!(vm_flags & VM_WRITE)) { // And the page is unwritable..
                        if (!(gup_flags & FOLL_FORCE)) // *Unless* FOLL_FORCE..
                                return -EFAULT; // Return an error
        [...]
        return 0; // Otherwise, proceed with lookup
}