rootless: fix regression when using exec on old containers

fallback to the previous behavior of joining only the user namespace,
when we cannot join the conmon userns+mount namespaces.

Closes: https://github.com/containers/libpod/issues/2673

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

1 files changed, 12 insertions, 7 deletions

diff --git a/cmd/podman/exec.go b/cmd/podman/exec.go
index a6afbf75a..fc1c76e9f 100644
--- a/cmd/podman/exec.go
+++ b/cmd/podman/exec.go
@@ -112,14 +112,19 @@ func execCmd(c *cliconfig.ExecValues) error {
 		var ret int
 
 		data, err := ioutil.ReadFile(ctr.Config().ConmonPidFile)
-		if err != nil {
-			return errors.Wrapf(err, "cannot read conmon PID file %q", ctr.Config().ConmonPidFile)
-		}
-		conmonPid, err := strconv.Atoi(string(data))
-		if err != nil {
-			return errors.Wrapf(err, "cannot parse PID %q", data)
+		if err == nil {
+			conmonPid, err := strconv.Atoi(string(data))
+			if err != nil {
+				return errors.Wrapf(err, "cannot parse PID %q", data)
+			}
+			became, ret, err = rootless.JoinDirectUserAndMountNS(uint(conmonPid))
+		} else {
+			pid, err := ctr.PID()
+			if err != nil {
+				return err
+			}
+			became, ret, err = rootless.JoinNS(uint(pid), c.PreserveFDs)
 		}
-		became, ret, err = rootless.JoinDirectUserAndMountNS(uint(conmonPid))
 		if err != nil {
 			return err
 		}