fork fails after nmap with hint address in an unmapped memory region

[Stéphane Mbape via cygwin]

Hello,

While embeding luajit in a c  program, I found myself unable to fork 
processes.
Investigations prove that it was related to nmap.
To be accurate, calling nmap with hint address in a unmapped memory 
region will cause all forks to fail with
"fixup_mmaps_after_fork: ReadProcessMemory failed for MAP_PRIVATE 
address 0x6FFFFFE0000, Win32 error 299"

There is a sample code below.

Thank you for reading.


#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdint.h>

#define MMAP_PROTÂ Â Â Â Â Â Â Â Â Â  (PROT_READ|PROT_WRITE)
#define MMAP_FLAGS_PROBEÂ Â Â  (MAP_PRIVATE|MAP_ANONYMOUS)

int main() {
 Â Â Â  printf("I am master %d\n", (int) getpid());

 Â Â Â  size_t size = ((size_t)128U * (size_t)1024U);
 Â Â Â  uintptr_t hint_addr = 0;

 Â Â Â  void *p = mmap((void *)hint_addr, size, MMAP_PROT, 
MMAP_FLAGS_PROBE, -1, 0);
 Â Â Â  printf ("nmap() = %p, hint_addr = %p\n", p, (void *) hint_addr);

 Â Â Â  uintptr_t addr = (uintptr_t) p;

 Â Â Â  munmap(p, size); // make sure there is an unmapped memory

 Â Â Â  // hint_addr = addr; // produces no error
 Â Â Â  hint_addr = addr + 1;
 Â Â Â  p = mmap((void *)hint_addr, size, MMAP_PROT, MMAP_FLAGS_PROBE, -1, 0);
 Â Â Â  printf ("nmap() = %p, hint_addr = %p\n", p, (void *) hint_addr);

 Â Â Â  pid_t child_pid = fork();

 Â Â Â  if (child_pid < 0) {
 Â Â Â Â Â Â Â  perror("fork failed");
 Â Â Â  } else if (child_pid == 0) {
 Â Â Â Â Â Â Â  printf("I am worker %d\n", (int) getpid());
 Â Â Â Â Â Â Â  sleep(2);
 Â Â Â Â Â Â Â  printf("worker exiting\n");
 Â Â Â Â Â Â Â  exit(0);
 Â Â Â  }

 Â Â Â  wait(NULL);

 Â Â Â  printf("master exiting\n");
 Â Â Â  return 0;
}


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