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Shared mappings not being inherited by children

From: Agustina Arzille
Subject: Shared mappings not being inherited by children
Date: Sat, 02 Apr 2016 13:35:49 -0300

Hello, everyone.

It appears that memory mappings obtained by 'mmap' with MAP_SHARED
and MAP_ANON as its flags are not being inherited by children processes.

Here's a simple program that illustrates the issue:


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

int main (void)
  void *p = mmap (0, 4096, PROT_READ | PROT_WRITE,
    MAP_SHARED | MAP_ANON, -1, 0);

  if (p == MAP_FAILED)
      puts ("mmap failed.");
      return (1);

  int pid = fork ();
  if (pid < 0)
      puts ("fork failed.");
      return (1);
  else if (pid == 0)
      *(int *)p = 69;
      puts ("value was set.");
      int r;
      wait (&r);
      printf ("done waiting for the child"
              "\nvalue is: %d\n", *(int *)p);

  return (0);


The parent process ends up printing zero, which is wrong, of course.

Strangely enough, setting the protection to RWX seems to make it work.
Another alternative is to call 'vm_inherit' to specifiy a shared mapping
prior to doing the fork.

A quick inspection at the source code tells me that this code ends up calling
'vm_allocate' as an optimization when it sees that the user requested an
anonymous mapping with protection RW. However, it's not taking into account
that 'vm_allocate' has a default inheritance value of 'COPY'.

As a workaround, we could always use 'vm_map', no matter what, since the
idea that 'vm_allocate' has a little less overhead is somewhat bogus to me, or
keep using 'vm_allocate', but do an additional 'vm_inherit' if the user 
a shared mapping.

What do you guys think?

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