propagate argv0 when re-execing from /proc/self/exe

This allows runc to be used as a target for docker's reexec module that
depends on a correct argv0 to select which process entrypoint to invoke.
Without this patch, when runc re-execs argv0 is set to "/proc/self/exe"
and the reexec module doesn't know what to do with it.

Signed-off-by: Petros Angelatos <petrosagg@gmail.com>

libcontainer/container_linux.go

@@ -40,6 +40,7 @@ type linuxContainer struct {
 	config               *configs.Config
 	cgroupManager        cgroups.Manager
 	intelRdtManager      intelrdt.Manager
+	initPath             string
 	initArgs             []string
 	initProcess          parentProcess
 	initProcessStartTime uint64
@@ -413,7 +414,8 @@ func (c *linuxContainer) newParentProcess(p *Process, doInit bool) (parentProces
 }
 
 func (c *linuxContainer) commandTemplate(p *Process, childPipe *os.File) (*exec.Cmd, error) {
-	cmd := exec.Command(c.initArgs[0], c.initArgs[1:]...)
+	cmd := exec.Command(c.initPath, c.initArgs[1:]...)
+	cmd.Args[0] = c.initArgs[0]
 	cmd.Stdin = p.Stdin
 	cmd.Stdout = p.Stdout
 	cmd.Stderr = p.Stderr

libcontainer/factory_linux.go

@@ -134,7 +134,8 @@ func New(root string, options ...func(*LinuxFactory) error) (Factory, error) {
 	}
 	l := &LinuxFactory{
 		Root:      root,
-		InitArgs:  []string{"/proc/self/exe", "init"},
+		InitPath:  "/proc/self/exe",
+		InitArgs:  []string{os.Args[0], "init"},
 		Validator: validate.New(),
 		CriuPath:  "criu",
 	}
@@ -155,6 +156,10 @@ type LinuxFactory struct {
 	// Root directory for the factory to store state.
 	Root string
 
+	// InitPath is the path for calling the init responsibilities for spawning
+	// a container.
+	InitPath string
+
 	// InitArgs are arguments for calling the init responsibilities for spawning
 	// a container.
 	InitArgs []string
@@ -207,6 +212,7 @@ func (l *LinuxFactory) Create(id string, config *configs.Config) (Container, err
 		id:            id,
 		root:          containerRoot,
 		config:        config,
+		initPath:      l.InitPath,
 		initArgs:      l.InitArgs,
 		criuPath:      l.CriuPath,
 		newuidmapPath: l.NewuidmapPath,
@@ -243,6 +249,7 @@ func (l *LinuxFactory) Load(id string) (Container, error) {
 		initProcessStartTime: state.InitProcessStartTime,
 		id:                   id,
 		config:               &state.Config,
+		initPath:             l.InitPath,
 		initArgs:             l.InitArgs,
 		criuPath:             l.CriuPath,
 		newuidmapPath:        l.NewuidmapPath,