runtime: allocate crash stack via stackalloc

On some platforms (notably OpenBSD), stacks must be specifically allocated
and marked as being stack memory. Allocate the crash stack using stackalloc,
which ensures these requirements are met, rather than using a global Go
variable.

Fixes #63794

Change-Id: I6513575797dd69ff0a36f3bfd4e5fc3bd95cbf50
Reviewed-on: https://go-review.googlesource.com/c/go/+/538457
Run-TryBot: Joel Sing <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Reviewed-by: Mauri de Souza Meneguzzo <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: 4a6f656c

src/runtime/proc.go

@@ -543,16 +543,9 @@ func badctxt() {
 	throw("ctxt != 0")
 }
 
-// crashstack is a space that can be used as the stack when it is
-// crashing on bad stack conditions, e.g. morestack on g0.
-// gcrash is the corresponding (fake) g.
-var crashstack [16384]byte
-
-var gcrash = g{
-	stack:       stack{uintptr(unsafe.Pointer(&crashstack)), uintptr(unsafe.Pointer(&crashstack)) + unsafe.Sizeof(crashstack)},
-	stackguard0: uintptr(unsafe.Pointer(&crashstack)) + 1000,
-	stackguard1: uintptr(unsafe.Pointer(&crashstack)) + 1000,
-}
+// gcrash is a fake g that can be used when crashing due to bad
+// stack conditions.
+var gcrash g
 
 var crashingG atomic.Pointer[g]
 
@@ -803,6 +796,12 @@ func schedinit() {
 	parsedebugvars()
 	gcinit()
 
+	// Allocate stack space that can be used when crashing due to bad stack
+	// conditions, e.g. morestack on g0.
+	gcrash.stack = stackalloc(16384)
+	gcrash.stackguard0 = gcrash.stack.lo + 1000
+	gcrash.stackguard1 = gcrash.stack.lo + 1000
+
 	// if disableMemoryProfiling is set, update MemProfileRate to 0 to turn off memprofile.
 	// Note: parsedebugvars may update MemProfileRate, but when disableMemoryProfiling is
 	// set to true by the linker, it means that nothing is consuming the profile, it is