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.


diff --git a/sysdeps/mach/hurd/mmap.c b/sysdeps/mach/hurd/mmap.c
index c6ddb39..a4e240e 100644
--- a/sysdeps/mach/hurd/mmap.c
+++ b/sysdeps/mach/hurd/mmap.c
@@ -44,30 +44,6 @@ __mmap (__ptr_t addr, size_t len, int prot, int flags, int fd, off_t offset)
   if ((mapaddr & (vm_page_size - 1)) || (offset & (vm_page_size - 1)))
     return (__ptr_t) (long int) __hurd_fail (EINVAL);
 
-  if ((flags & (MAP_TYPE|MAP_INHERIT)) == MAP_ANON
-      && prot == (PROT_READ|PROT_WRITE)) /* cf VM_PROT_DEFAULT */
-    {
-      /* vm_allocate has (a little) less overhead in the kernel too.  */
-      err = __vm_allocate (__mach_task_self (), &mapaddr, len,
-			   mapaddr == NULL);
-
-      if (err == KERN_NO_SPACE)
-	{
-	  if (flags & MAP_FIXED)
-	    {
-	      /* XXX this is not atomic as it is in unix! */
-	      /* The region is already allocated; deallocate it first.  */
-	      err = __vm_deallocate (__mach_task_self (), mapaddr, len);
-	      if (!err)
-		err = __vm_allocate (__mach_task_self (), &mapaddr, len, 0);
-	    }
-	  else if (mapaddr != NULL)
-	    err = __vm_allocate (__mach_task_self (), &mapaddr, len, 1);
-	}
-
-      return err ? (__ptr_t) (long int) __hurd_fail (err) : (__ptr_t) mapaddr;
-    }
-
   vmprot = VM_PROT_NONE;
   if (prot & PROT_READ)
     vmprot |= VM_PROT_READ;