runtime/debug: avoid overflow in SetMaxThreads

Fixes #16076

Change-Id: I91fa87b642592ee4604537dd8c3197cd61ec8b31
Reviewed-on: https://go-review.googlesource.com/31516
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/debug/garbage_test.go

@@ -114,3 +114,16 @@ func TestSetGCPercent(t *testing.T) {
 		t.Errorf("SetGCPercent(123); SetGCPercent(x) = %d, want 123", new)
 	}
 }
+
+func TestSetMaxThreadsOvf(t *testing.T) {
+	// Verify that a big threads count will not overflow the int32
+	// maxmcount variable, causing a panic (see Issue 16076).
+	//
+	// This can only happen when ints are 64 bits, since on platforms
+	// with 32 bit ints SetMaxThreads (which takes an int parameter)
+	// cannot be given anything that will overflow an int32.
+	//
+	// Call SetMaxThreads with 1<<31, but only on 64 bit systems.
+	nt := SetMaxThreads(1 << (30 + ^uint(0)>>63))
+	SetMaxThreads(nt) // restore previous value
+}

src/runtime/proc.go

@@ -4294,7 +4294,11 @@ func runqsteal(_p_, p2 *p, stealRunNextG bool) *g {
 func setMaxThreads(in int) (out int) {
 	lock(&sched.lock)
 	out = int(sched.maxmcount)
-	sched.maxmcount = int32(in)
+	if in > 0x7fffffff { // MaxInt32
+		sched.maxmcount = 0x7fffffff
+	} else {
+		sched.maxmcount = int32(in)
+	}
 	checkmcount()
 	unlock(&sched.lock)
 	return