systemd socker activation: check listener to prevent panic

Commit 5fa6f686db added a regression which was fixed in eb71712626f9.
Apply the same fix again to prevent a panic and return a proper error
instead.

To not regress again I added a e2e test which makes sure we do not panic.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

cmd/podman/system/service_abi.go

@@ -46,6 +46,10 @@ func restService(flags *pflag.FlagSet, cfg *entities.PodmanConfig, opts entities
 			return fmt.Errorf("wrong number of file descriptors for socket activation protocol (%d != 1)", len(listeners))
 		}
 		listener = listeners[0]
+		// note that activation.Listeners() returns nil when it cannot listen on the fd (i.e. udp connection)
+		if listener == nil {
+			return fmt.Errorf("unexpected fd received from systemd: cannot listen on it")
+		}
 		libpodRuntime.SetRemoteURI(listeners[0].Addr().String())
 	} else {
 		uri, err := url.Parse(opts.URI)

test/e2e/systemd_activate_test.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"io/fs"
+	"net"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -21,6 +22,7 @@ var _ = Describe("Systemd activate", func() {
 	var tempDir string
 	var err error
 	var podmanTest *PodmanTestIntegration
+	var activate string
 
 	BeforeEach(func() {
 		tempDir, err = testUtils.CreateTempDirInTempDir()
@@ -31,17 +33,10 @@ var _ = Describe("Systemd activate", func() {
 
 		podmanTest = PodmanTestCreate(tempDir)
 		podmanTest.Setup()
-	})
 
-	AfterEach(func() {
-		podmanTest.Cleanup()
-		processTestResult(CurrentGinkgoTestDescription())
-	})
-
-	It("stop podman.service", func() {
 		SkipIfRemote("Testing stopped service requires both podman and podman-remote binaries")
 
-		activate, err := exec.LookPath("systemd-socket-activate")
+		activate, err = exec.LookPath("systemd-socket-activate")
 		if err != nil {
 			activate = "/usr/bin/systemd-socket-activate"
 		}
@@ -54,7 +49,14 @@ var _ = Describe("Systemd activate", func() {
 		case err != nil:
 			Skip(err.Error())
 		}
+	})
 
+	AfterEach(func() {
+		podmanTest.Cleanup()
+		processTestResult(CurrentGinkgoTestDescription())
+	})
+
+	It("stop podman.service", func() {
 		// systemd-socket-activate does not support DNS lookups
 		host := "127.0.0.1"
 		port, err := podmanUtils.GetRandomPort()
@@ -103,4 +105,37 @@ var _ = Describe("Systemd activate", func() {
 		Expect(abiSession).To(Exit(0))
 		Expect(abiSession.OutputToString()).To(Equal("true"))
 	})
+
+	It("invalid systemd file descriptor", func() {
+		host := "127.0.0.1"
+		port, err := podmanUtils.GetRandomPort()
+		Expect(err).ToNot(HaveOccurred())
+
+		addr := fmt.Sprintf("%s:%d", host, port)
+
+		// start systemd activation with datagram socket
+		activateSession := testUtils.StartSystemExec(activate, []string{
+			"--datagram", "--listen", addr,
+			podmanTest.PodmanBinary,
+			"--root=" + filepath.Join(tempDir, "server_root"),
+			"system", "service",
+			"--time=0",
+		})
+		Expect(activateSession.Exited).ShouldNot(Receive(), "Failed to start podman service")
+
+		// we have to wait for systemd-socket-activate to become ready
+		time.Sleep(1 * time.Second)
+
+		// now dial the socket to start podman
+		conn, err := net.Dial("udp", addr)
+		Expect(err).ToNot(HaveOccurred())
+		defer conn.Close()
+		_, err = conn.Write([]byte("test"))
+		Expect(err).ToNot(HaveOccurred())
+
+		// wait for podman to exit
+		activateSession.Wait(10)
+		Expect(activateSession).To(Exit(125))
+		Expect(activateSession.ErrorToString()).To(ContainSubstring("Error: unexpected fd received from systemd: cannot listen on it"))
+	})
 })