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Shared mappings not being inherited by children processes in GNU/Hurd
From: |
Agustina Arzille |
Subject: |
Shared mappings not being inherited by children processes in GNU/Hurd |
Date: |
Mon, 04 Apr 2016 12:57:17 -0300 |
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.
mmap.diff
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