Nsenter checks for argv 0 before running

Signed-off-by: Michael Crosby <michael@docker.com>

namespaces/execin.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"strconv"
 	"syscall"
 
@@ -15,19 +16,27 @@ import (
 	"github.com/docker/libcontainer/system"
 )
 
+// ExecIn reexec's the initPath with the argv 0 rewrite to "nsenter" so that it is able to run the
+// setns code in a single threaded environment joining the existing containers' namespaces.
 func ExecIn(container *libcontainer.Config, state *libcontainer.State, userArgs []string, initPath string,
 	stdin io.Reader, stdout, stderr io.Writer, console string, startCallback func(*exec.Cmd)) (int, error) {
 
-	args := []string{"--nspid", strconv.Itoa(state.InitPid)}
+	args := []string{"nsenter", "--nspid", strconv.Itoa(state.InitPid)}
 
 	if console != "" {
 		args = append(args, "--console", console)
 	}
 
-	args = append(args, "nsenter", "--")
-	args = append(args, userArgs...)
+	cmd := &exec.Cmd{
+		Path: initPath,
+		Args: append(args, append([]string{"--"}, userArgs...)...),
+	}
 
-	cmd := exec.Command(initPath, args...)
+	if filepath.Base(initPath) == initPath {
+		if lp, err := exec.LookPath(initPath); err == nil {
+			cmd.Path = lp
+		}
+	}
 
 	pipe, err := syncpipe.NewSyncPipe()
 	if err != nil {

namespaces/nsenter/nsenter.c

@@ -71,7 +71,7 @@ int setns(int fd, int nstype)
 void print_usage()
 {
 	fprintf(stderr,
-		"<binary> nsenter --nspid <pid> -- cmd1 arg1 arg2...\n");
+		"nsenter --nspid <pid> --console <console> -- cmd1 arg1 arg2...\n");
 }
 
 void nsenter()
@@ -80,22 +80,10 @@ void nsenter()
 	char **argv;
 	get_args(&argc, &argv);
 
-	// Ignore if this is not for us.
-	if (argc < 4) {
-		return;
-	}
-
-	int found_nsenter = 0;
-	for (c = 0; c < argc; ++c) {
-		if (strcmp(argv[c], kNsEnter) == 0) {
-			found_nsenter = 1;
-			break;
-		}
-	}
-
-	if (!found_nsenter) {
-		return;
-	}
+    // check argv 0 to ensure that we are supposed to setns
+    if (strcmp(argv[0], kNsEnter) != 0) {
+        return;
+    }
 
 	static const struct option longopts[] = {
 		{"nspid", required_argument, NULL, 'n'},
@@ -117,10 +105,6 @@ void nsenter()
 		}
 	}
 
-    if (strcmp(argv[optind], kNsEnter) != 0) {
-        return;
-    }
-
 	if (init_pid_str == NULL) {
 		print_usage();
 		exit(1);