Add MAP_EXCL flag for mmap(2). It should be combined with MAP_FIXED,

and prevents the request from deleting existing mappings in the
region, failing instead.

Reviewed by:	alc
Discussed with:	jhb
Tested by:	markj, pho (previous version, as part of the bigger patch)
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

lib/libc/sys/mmap.2

@@ -28,7 +28,7 @@
 .\"	@(#)mmap.2	8.4 (Berkeley) 5/11/95
 .\" $FreeBSD$
 .\"
-.Dd September 9, 2013
+.Dd June 19, 2014
 .Dt MMAP 2
 .Os
 .Sh NAME
@@ -141,6 +141,12 @@ argument must be 0.
 This flag is identical to
 .Dv MAP_ANON
 and is provided for compatibility.
+.It Dv MAP_EXCL
+This flag can only be used in combination with
+.Dv MAP_FIXED .
+Please see the definition of
+.Dv MAP_FIXED
+for the description of its effect.
 .It Dv MAP_FIXED
 Do not permit the system to select a different address than the one
 specified.
@@ -152,17 +158,21 @@ If
 is specified,
 .Fa addr
 must be a multiple of the pagesize.
-If a
+If
+.Dv MAP_EXCL
+is not specified, a successfull
 .Dv MAP_FIXED
-request is successful, the mapping established by
+request replaces any previous mappings for the process'
-.Fn mmap
+pages in the range from
-replaces any previous mappings for the process' pages in the range from
 .Fa addr
 to
 .Fa addr
 +
 .Fa len .
-Use of this option is discouraged.
+In contrast, if
+.Dv MAP_EXCL
+is specified, the request will fail if a mapping
+already exists within the range.
 .It Dv MAP_HASSEMAPHORE
 Notify the kernel that the region may contain semaphores and that special
 handling may be necessary.
@@ -393,6 +403,17 @@ argument was not -1.
 was specified and the
 .Fa offset
 argument was not 0.
+.It Bq Er EINVAL
+Both
+.Dv MAP_FIXED
+and
+.Dv MAP_EXCL
+were specified, but the requested region is already used by a mapping.
+.It Bq Er EINVAL
+.Dv MAP_EXCL
+was specified, but
+.Dv MAP_FIXED
+was not.
 .It Bq Er ENODEV
 .Dv MAP_ANON
 has not been specified and

sys/sys/mman.h

@@ -89,6 +89,7 @@
 /*
  * Extended flags
  */
+#define	MAP_EXCL	 0x00004000 /* for MAP_FIXED, fail if address is used */
 #define	MAP_NOCORE	 0x00020000 /* dont include these pages in a coredump */
 #define	MAP_PREFAULT_READ 0x00040000 /* prefault mapping for reading */
 #ifdef __LP64__

sys/vm/vm_map.c

@@ -1393,7 +1393,8 @@ vm_map_fixed(vm_map_t map, vm_object_t object, vm_ooffset_t offset,
 	    ("vm_map_fixed: non-NULL backing object for stack"));
 	vm_map_lock(map);
 	VM_MAP_RANGE_CHECK(map, start, end);
-	(void) vm_map_delete(map, start, end);
+	if ((cow & MAP_CHECK_EXCL) == 0)
+		vm_map_delete(map, start, end);
 	if ((cow & (MAP_STACK_GROWS_DOWN | MAP_STACK_GROWS_UP)) != 0) {
 		result = vm_map_stack_locked(map, start, length, sgrowsiz,
 		    prot, max, cow);

sys/vm/vm_map.h

@@ -315,6 +315,7 @@ long vmspace_resident_count(struct vmspace *vmspace);
 #define MAP_PREFAULT		0x0008
 #define MAP_PREFAULT_PARTIAL	0x0010
 #define MAP_DISABLE_SYNCER	0x0020
+#define	MAP_CHECK_EXCL		0x0040
 #define MAP_DISABLE_COREDUMP	0x0100
 #define MAP_PREFAULT_MADVISE	0x0200	/* from (user) madvise request */
 #define	MAP_VN_WRITECOUNT	0x0400

sys/vm/vm_mmap.c

@@ -245,6 +245,8 @@ sys_mmap(td, uap)
 		flags |= MAP_ANON;
 		pos = 0;
 	}
+	if ((flags & (MAP_EXCL | MAP_FIXED)) == MAP_EXCL)
+		return (EINVAL);
 
 	/*
 	 * Align the file position to a page boundary,
@@ -1626,6 +1628,8 @@ vm_mmap(vm_map_t map, vm_offset_t *addr, vm_size_t size, vm_prot_t prot,
 			return (EINVAL);
 		docow |= MAP_STACK_GROWS_DOWN;
 	}
+	if ((flags & MAP_EXCL) != 0)
+		docow |= MAP_CHECK_EXCL;
 
 	if (fitit) {
 		if ((flags & MAP_ALIGNMENT_MASK) == MAP_ALIGNED_SUPER)