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Index: head/lib/libc/sys/minherit.2
===================================================================
--- head/lib/libc/sys/minherit.2
+++ head/lib/libc/sys/minherit.2
@@ -29,7 +29,7 @@
.\"
.\" @(#)minherit.2 8.1 (Berkeley) 6/9/93
.\"
-.Dd October 30, 2007
+.Dd March 15, 2017
.Dt MINHERIT 2
.Os
.Sh NAME
@@ -91,6 +91,11 @@
after the parent forks and there is no way to get the previous
shared-backing-store mapping without unmapping and remapping the address
space in the parent.
+.It Dv INHERIT_ZERO
+This option causes the address space in question to be mapped as new
+anonymous pages,
+which would be initialized to all zero bytes,
+in the child process.
.El
.Sh RETURN VALUES
.Rv -std minherit
@@ -130,6 +135,13 @@
.Ox
and then in
.Fx 2.2 .
+.Pp
+The
+.Dv INHERIT_ZERO
+support first appeared in
+.Ox 5.6
+and then in
+.Fx 12.0 .
.Sh BUGS
Once you set inheritance to
.Dv MAP_PRIVATE
Index: head/sys/sys/mman.h
===================================================================
--- head/sys/sys/mman.h
+++ head/sys/sys/mman.h
@@ -43,6 +43,7 @@
#define INHERIT_SHARE 0
#define INHERIT_COPY 1
#define INHERIT_NONE 2
+#define INHERIT_ZERO 3
#endif
/*
Index: head/sys/vm/vm.h
===================================================================
--- head/sys/vm/vm.h
+++ head/sys/vm/vm.h
@@ -68,6 +68,7 @@
#define VM_INHERIT_SHARE ((vm_inherit_t) 0)
#define VM_INHERIT_COPY ((vm_inherit_t) 1)
#define VM_INHERIT_NONE ((vm_inherit_t) 2)
+#define VM_INHERIT_ZERO ((vm_inherit_t) 3)
#define VM_INHERIT_DEFAULT VM_INHERIT_COPY
typedef u_char vm_prot_t; /* protection codes */
Index: head/sys/vm/vm_map.c
===================================================================
--- head/sys/vm/vm_map.c
+++ head/sys/vm/vm_map.c
@@ -2285,6 +2285,7 @@
case VM_INHERIT_NONE:
case VM_INHERIT_COPY:
case VM_INHERIT_SHARE:
+ case VM_INHERIT_ZERO:
break;
default:
return (KERN_INVALID_ARGUMENT);
@@ -3443,6 +3444,36 @@
vm_map_copy_entry(old_map, new_map, old_entry,
new_entry, fork_charge);
break;
+
+ case VM_INHERIT_ZERO:
+ /*
+ * Create a new anonymous mapping entry modelled from
+ * the old one.
+ */
+ new_entry = vm_map_entry_create(new_map);
+ memset(new_entry, 0, sizeof(*new_entry));
+
+ new_entry->start = old_entry->start;
+ new_entry->end = old_entry->end;
+ new_entry->avail_ssize = old_entry->avail_ssize;
+ new_entry->adj_free = old_entry->adj_free;
+ new_entry->max_free = old_entry->max_free;
+ new_entry->eflags = old_entry->eflags &
+ ~(MAP_ENTRY_USER_WIRED | MAP_ENTRY_IN_TRANSITION |
+ MAP_ENTRY_VN_WRITECNT);
+ new_entry->protection = old_entry->protection;
+ new_entry->max_protection = old_entry->max_protection;
+ new_entry->inheritance = VM_INHERIT_ZERO;
+
+ vm_map_entry_link(new_map, new_map->header.prev,
+ new_entry);
+ vmspace_map_entry_forked(vm1, vm2, new_entry);
+
+ new_entry->cred = curthread->td_ucred;
+ crhold(new_entry->cred);
+ *fork_charge += (new_entry->end - new_entry->start);
+
+ break;
}
old_entry = old_entry->next;
}

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