Web lists-archives.com

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




Sorry,

Cygwin version: 2.9.0
OS: Windows 10
Arch: 64bit

The sample was also attached.

Le 09/12/2017 à 16:48, Andrey Repin a écrit :
Greetings, Stéphane Mbape!

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.
You forgot to mention Cygwin version you're using, and please provide the
sample as an attach to save people the copy-pasting issues.



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

#define MMAP_PROT           (PROT_READ|PROT_WRITE)
#define MMAP_FLAGS_PROBE    (MAP_PRIVATE|MAP_ANONYMOUS)

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

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

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

    uintptr_t addr = (uintptr_t) p;

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

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

    pid_t child_pid = fork();

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

    wait(NULL);

    printf("master exiting\n");
    return 0;
}
--
Problem reports:       http://cygwin.com/problems.html
FAQ:                   http://cygwin.com/faq/
Documentation:         http://cygwin.com/docs.html
Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple