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, 6 insertions, 0 deletions

diff --git a/src/runtime/stack.go b/src/runtime/stack.go
index 648603db35..c7bfc0434b 100644
--- a/src/runtime/stack.go
+++ b/src/runtime/stack.go
@@ -186,6 +186,7 @@ func stackpoolalloc(order uint8) gclinkptr {
 		if s.manualFreeList.ptr() != nil {
 			throw("bad manualFreeList")
 		}
+		osStackAlloc(s)
 		s.elemsize = _FixedStack << order
 		for i := uintptr(0); i < _StackCacheSize; i += s.elemsize {
 			x := gclinkptr(s.base() + i)
@@ -238,6 +239,7 @@ func stackpoolfree(x gclinkptr, order uint8) {
 		// By not freeing, we prevent step #4 until GC is done.
 		stackpool[order].remove(s)
 		s.manualFreeList = 0
+		osStackFree(s)
 		mheap_.freeManual(s, &memstats.stacks_inuse)
 	}
 }
@@ -385,6 +387,7 @@ func stackalloc(n uint32) stack {
 			if s == nil {
 				throw("out of memory")
 			}
+			osStackAlloc(s)
 			s.elemsize = uintptr(n)
 		}
 		v = unsafe.Pointer(s.base())
@@ -463,6 +466,7 @@ func stackfree(stk stack) {
 		if gcphase == _GCoff {
 			// Free the stack immediately if we're
 			// sweeping.
+			osStackFree(s)
 			mheap_.freeManual(s, &memstats.stacks_inuse)
 		} else {
 			// If the GC is running, we can't return a
@@ -1112,6 +1116,7 @@ func freeStackSpans() {
 			if s.allocCount == 0 {
 				list.remove(s)
 				s.manualFreeList = 0
+				osStackFree(s)
 				mheap_.freeManual(s, &memstats.stacks_inuse)
 			}
 			s = next
@@ -1126,6 +1131,7 @@ func freeStackSpans() {
 		for s := stackLarge.free[i].first; s != nil; {
 			next := s.next
 			stackLarge.free[i].remove(s)
+			osStackFree(s)
 			mheap_.freeManual(s, &memstats.stacks_inuse)
 			s = next
 		}