Index: lib/libc/sys/mmap.2
===================================================================
--- lib/libc/sys/mmap.2
+++ lib/libc/sys/mmap.2
@@ -351,6 +351,8 @@
 automatically grow.
 Guards prevent inadvertent use of the regions into which those
 stacks can grow without requiring mapping the whole stack in advance.
+Size of the guard, in pages, is specified by sysctl
+.Va security.bsd.stack_guard_page .
 .El
 .Pp
 The
@@ -452,6 +454,11 @@
 was specified.
 At least one of these flags must be included.
 .It Bq Er EINVAL
+.Dv MAP_STACK
+was specified and
+.Va len
+is less or equal to the guard size.
+.It Bq Er EINVAL
 .Dv MAP_FIXED
 was specified and the
 .Fa addr