runtime: remap stack spans with MAP_STACK on OpenBSD

OpenBSD 6.4 is going to start requiring that the SP points to memory
that was mapped with MAP_STACK on system call entry, traps, and when
switching to the alternate signal stack [1]. Currently, Go doesn't map
any memory MAP_STACK, so the kernel quickly kills Go processes.

Fix this by remapping the memory that backs stack spans with
MAP_STACK, and re-remapping it without MAP_STACK when it's returned to
the heap.

[1] http://openbsd-archive.7691.n7.nabble.com/stack-register-checking-td338238.html

Fixes #26142.

Change-Id: I656eb84385a22833445d49328bb304f8cdd0e225
Reviewed-on: https://go-review.googlesource.com/121657
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/runtime/os_nonopenbsd.go b/src/runtime/os_nonopenbsd.go
new file mode 100644
index 0000000000..e65697bdb3
--- /dev/null
+++ b/src/runtime/os_nonopenbsd.go
@@ -0,0 +1,17 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !openbsd
+
+package runtime
+
+// osStackAlloc performs OS-specific initialization before s is used
+// as stack memory.
+func osStackAlloc(s *mspan) {
+}
+
+// osStackFree undoes the effect of osStackAlloc before s is returned
+// to the heap.
+func osStackFree(s *mspan) {
+}