Shared mappings not being inherited by children processes in GNU/Hurd

[Agustina Arzille]

Hello, everyone.

It appears that in GNU/Hurd, 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.");
     }
   else
     {
       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.

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'.

An easy workaround is to simply remove the 'vm_allocate' optimization, and
always use 'vm_map'. There's a patch attached that does just this.

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