Use different Ctty FDs based on the go version.

An upcoming change in Go 1.13 [1] changes the semantics of the SysProcAttr.Ctty
field. Prior to the change, the FD must be an FD in the child process's FD
table (aka "post-shuffle"). After the change, the FD must be an FD in the
current process's FD table (aka "pre-shuffle").

To be compatible with both versions this CL introduces a new boolean
"CttyFdIsPostShuffle" which indicates whether a pre- or post-shuffle FD should
be provided. We use build tags to chose the correct one.

1: https://go-review.googlesource.com/c/go/+/178919/
PiperOrigin-RevId: 255015303

runsc/cmd/exec.go

@@ -237,6 +237,16 @@ func (ex *Exec) execChildAndWait(waitStatus *syscall.WaitStatus) subcommands.Exi
 			Setctty: true,
 			Ctty:    int(tty.Fd()),
 		}
+		// TODO(b/133868570): Delete this check once Go 1.12 is no
+		// longer supported.
+		if console.CttyFdIsPostShuffle {
+			// In go1.12 and before, the Ctty FD must be the FD in
+			// the child process's FD table. Since we set
+			// cmd.Stdin/Stdout/Stderr to the tty FD, we can use
+			// any of 0, 1, or 2 here.
+			cmd.SysProcAttr.Ctty = 0
+		}
+
 	}
 
 	if err := cmd.Start(); err != nil {

runsc/console/BUILD

@@ -4,7 +4,11 @@ package(licenses = ["notice"])
 
 go_library(
     name = "console",
-    srcs = ["console.go"],
+    srcs = [
+        "console.go",
+        "ctty_1_12.go",
+        "ctty_1_13.go",
+    ],
     importpath = "gvisor.dev/gvisor/runsc/console",
     visibility = [
         "//runsc:__subpackages__",

runsc/console/ctty_1_12.go

@@ -0,0 +1,24 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build go1.12
+// +build !go1.13
+
+// TODO(b/133868570): Delete once Go 1.12 is no longer supported.
+
+package console
+
+// CttyFdIsPostShuffle indicates that in go1.12 and earlier, the
+// SysProcAttr.Ctty FD is determined "pre-shuffle" (not "post-shuffle").
+var CttyFdIsPostShuffle = false

runsc/console/ctty_1_13.go

@@ -0,0 +1,24 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build go1.13
+// +build !go1.14
+
+// TODO(b/133868570): Delete once Go 1.12 is no longer supported.
+
+package console
+
+// CttyFdIsPostShuffle indicates that in go1.13 and later, the SysProcAttr.Ctty
+// FD is determined "post-shuffle".
+var CttyFdIsPostShuffle = true

runsc/sandbox/sandbox.go

@@ -437,11 +437,16 @@ func (s *Sandbox) createSandboxProcess(conf *boot.Config, args *Args, startSyncF
 		defer tty.Close()
 
 		// Set the TTY as a controlling TTY on the sandbox process.
-		// Note that the Ctty field must be the FD of the TTY in the
-		// *new* process, not this process. Since we are about to
-		// assign the TTY to nextFD, we can use that value here.
 		cmd.SysProcAttr.Setctty = true
-		cmd.SysProcAttr.Ctty = nextFD
+		cmd.SysProcAttr.Ctty = int(tty.Fd())
+		// TODO(b/133868570): Delete this check once Go 1.12 is no
+		// longer supported.
+		if console.CttyFdIsPostShuffle {
+			// In go1.12 and before, the Ctty FD must be the FD in
+			// the child process's FD table, which will be "nextFD"
+			// in this case.
+			cmd.SysProcAttr.Ctty = nextFD
+		}
 
 		// Pass the tty as all stdio fds to sandbox.
 		for i := 0; i < 3; i++ {