Implement INHERIT_ZERO for minherit(2).

INHERIT_ZERO is an OpenBSD feature.

When a page is marked as such, it would be zeroed
upon fork().

This would be used in new arc4random(3) functions.

PR:	182610
Reviewed by:	kib (earlier version)
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D427

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 @@ it will no longer be shared in the parent
 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 @@ system call first appeared in
 .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

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
 
 /*

sys/vm/vm.h

@@ -68,6 +68,7 @@ typedef char vm_inherit_t;	/* inheritance codes */
 #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 */

sys/vm/vm_map.c

@@ -2285,6 +2285,7 @@ vm_map_inherit(vm_map_t map, vm_offset_t start, vm_offset_t end,
 	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 @@ vmspace_fork(struct vmspace *vm1, vm_ooffset_t *fork_charge)
 			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;
 	}