runtime: improve error messages after allocating a stack that is too big

In the current implementation, we can observe crashes after calling
debug.SetMaxStack and allocating a stack larger than 4GB since
stackalloc works with 32-bit sizes. To avoid this, we define an upper
limit as the largest feasible point we can grow a stack to and provide a
better error message when we get a stack overflow.

Fixes #41228

Change-Id: I55fb0a824f47ed9fb1fcc2445a4dfd57da9ef8d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/255997
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Giovanni Bajo <rasky@develer.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/debug/garbage.go

@@ -106,6 +106,8 @@ func FreeOSMemory() {
 // the program crashes.
 // SetMaxStack returns the previous setting.
 // The initial setting is 1 GB on 64-bit systems, 250 MB on 32-bit systems.
+// There may be a system-imposed maximum stack limit regardless
+// of the value provided to SetMaxStack.
 //
 // SetMaxStack is useful mainly for limiting the damage done by
 // goroutines that enter an infinite recursion. It only limits future

src/runtime/proc.go

@@ -128,6 +128,11 @@ func main() {
 		maxstacksize = 250000000
 	}
 
+	// An upper limit for max stack size. Used to avoid random crashes
+	// after calling SetMaxStack and trying to allocate a stack that is too big,
+	// since stackalloc works with 32-bit sizes.
+	maxstackceiling = 2 * maxstacksize
+
 	// Allow newproc to start new Ms.
 	mainStarted = true
 

src/runtime/stack.go

@@ -497,6 +497,8 @@ func stackfree(stk stack) {
 
 var maxstacksize uintptr = 1 << 20 // enough until runtime.main sets it for real
 
+var maxstackceiling = maxstacksize
+
 var ptrnames = []string{
 	0: "scalar",
 	1: "ptr",
@@ -1050,8 +1052,12 @@ func newstack() {
 		}
 	}
 
-	if newsize > maxstacksize {
+	if newsize > maxstacksize || newsize > maxstackceiling {
+		if maxstacksize < maxstackceiling {
 			print("runtime: goroutine stack exceeds ", maxstacksize, "-byte limit\n")
+		} else {
+			print("runtime: goroutine stack exceeds ", maxstackceiling, "-byte limit\n")
+		}
 		print("runtime: sp=", hex(sp), " stack=[", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n")
 		throw("stack overflow")
 	}