rootless: add new function to join existing namespace

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

Closes: #1371
Approved by: rhatdan

pkg/rootless/rootless_linux.c

@@ -83,7 +83,37 @@ get_cmd_line_args (pid_t pid)
 }
 
 int
-reexec_in_user_namespace(int ready)
+reexec_userns_join (int userns)
+{
+  pid_t ppid = getpid ();
+  char uid[16];
+  char **argv;
+  int pid;
+
+  sprintf (uid, "%d", geteuid ());
+
+  argv = get_cmd_line_args (ppid);
+  if (argv == NULL)
+    _exit (EXIT_FAILURE);
+
+  pid = fork ();
+  if (pid)
+    return pid;
+
+  setenv ("_LIBPOD_USERNS_CONFIGURED", "init", 1);
+  setenv ("_LIBPOD_ROOTLESS_UID", uid, 1);
+
+  if (setns (userns, 0) < 0)
+    _exit (EXIT_FAILURE);
+  close (userns);
+
+  execvp (argv[0], argv);
+
+  _exit (EXIT_FAILURE);
+}
+
+int
+reexec_in_user_namespace (int ready)
 {
   int ret;
   pid_t pid;

pkg/rootless/rootless_linux.go

@@ -23,6 +23,7 @@ import (
 /*
 extern int reexec_in_user_namespace(int ready);
 extern int reexec_in_user_namespace_wait(int pid);
+extern int reexec_userns_join(int userns);
 */
 import "C"
 
@@ -84,6 +85,32 @@ func tryMappingTool(tool string, pid int, hostID int, mappings []idtools.IDMap)
 	return cmd.Run()
 }
 
+// JoinNS re-exec podman in a new userNS and join the user namespace of the specified
+// PID.
+func JoinNS(pid uint) (bool, int, error) {
+	if os.Getuid() == 0 || os.Getenv("_LIBPOD_USERNS_CONFIGURED") != "" {
+		return false, -1, nil
+	}
+
+	userNS, err := GetUserNSForPid(pid)
+	if err != nil {
+		return false, -1, err
+	}
+	defer userNS.Close()
+
+	pidC := C.reexec_userns_join(C.int(userNS.Fd()))
+	if int(pidC) < 0 {
+		return false, -1, errors.Errorf("cannot re-exec process")
+	}
+
+	ret := C.reexec_in_user_namespace_wait(pidC)
+	if ret < 0 {
+		return false, -1, errors.New("error waiting for the re-exec process")
+	}
+
+	return true, int(ret), nil
+}
+
 // BecomeRootInUserNS re-exec podman in a new userNS.  It returns whether podman was re-executed
 // into a new user namespace and the return code from the re-executed podman process.
 // If podman was re-executed the caller needs to propagate the error code returned by the child
@@ -183,7 +210,7 @@ func BecomeRootInUserNS() (bool, int, error) {
 
 	ret := C.reexec_in_user_namespace_wait(pidC)
 	if ret < 0 {
-		return false, -1, errors.Wrapf(err, "error waiting for the re-exec process")
+		return false, -1, errors.New("error waiting for the re-exec process")
 	}
 
 	return true, int(ret), nil

pkg/rootless/rootless_unsupported.go

@@ -33,6 +33,12 @@ func SkipStorageSetup() bool {
 	return false
 }
 
+// JoinNS re-exec podman in a new userNS and join the user namespace of the specified
+// PID.
+func JoinNS(pid uint) (bool, int, error) {
+	return false, -1, errors.New("this function is not supported on this os")
+}
+
 // GetUserNSForPid returns an open FD for the first direct child user namespace that created the process
 func GetUserNSForPid(pid uint) (*os.File, error) {
 	return nil, errors.New("this function is not supported on this os")