Shared mappings not being inherited by children processes in GNU/Hurd
Commit Message
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.
@@ -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;