runtime: unskip TestG0StackOverflow

The stack bounds from pthread are not always accurate, and could
cause seg fault if we run out of the actual stack space before
reaching the bounds. Here we use an artificially small stack bounds
to check overflow without actually running out of the system stack.

Change-Id: I8067c5e1297307103b315d9d0c60120293b57aab
Reviewed-on: https://go-review.googlesource.com/c/go/+/523695
Reviewed-by: Michael Pratt <[email protected]>
Run-TryBot: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: cherrymui

src/runtime/crash_test.go

@@ -792,11 +792,6 @@ func TestRuntimePanic(t *testing.T) {
 func TestG0StackOverflow(t *testing.T) {
 	testenv.MustHaveExec(t)
 
-	switch runtime.GOOS {
-	case "android", "darwin", "dragonfly", "freebsd", "ios", "linux", "netbsd", "openbsd":
-		t.Skipf("g0 stack is wrong on pthread platforms (see golang.org/issue/26061)")
-	}
-
 	if os.Getenv("TEST_G0_STACK_OVERFLOW") != "1" {
 		cmd := testenv.CleanCmdEnv(exec.Command(os.Args[0], "-test.run=^TestG0StackOverflow$", "-test.v"))
 		cmd.Env = append(cmd.Env, "TEST_G0_STACK_OVERFLOW=1")

src/runtime/export_test.go

@@ -682,6 +682,15 @@ func unexportedPanicForTesting(b []byte, i int) byte {
 
 func G0StackOverflow() {
 	systemstack(func() {
+		g0 := getg()
+		sp := getcallersp()
+		// The stack bounds for g0 stack is not always precise.
+		// Use an artificially small stack, to trigger a stack overflow
+		// without actually run out of the system stack (which may seg fault).
+		g0.stack.lo = sp - 4096
+		g0.stackguard0 = g0.stack.lo + stackGuard
+		g0.stackguard1 = g0.stackguard0
+
 		stackOverflow(nil)
 	})
 }