lxccontainer: add the umount API function

src/lxc/lxccontainer.c

@@ -5025,7 +5025,66 @@ out:
 }
 
 WRAP_API_6(int, lxcapi_mount, const char *, const char *, const char *,
-	   unsigned long, const void *, struct lxc_mount*)
+	   unsigned long, const void *, struct lxc_mount *)
+
+static int do_lxcapi_umount(struct lxc_container *c, const char *target,
+			    unsigned long mountflags, struct lxc_mount *mnt)
+{
+	pid_t pid, init_pid;
+	int ret = -1;
+
+	if (!c || !c->lxc_conf) {
+		ERROR("Container or configuration is NULL");
+		return -EINVAL;
+	}
+
+	/* Do the fork */
+	pid = fork();
+	if (pid < 0) {
+		SYSERROR("Could not fork");
+		return -1;
+	}
+
+	if (pid == 0) {
+		init_pid = do_lxcapi_init_pid(c);
+		if (init_pid < 0) {
+			ERROR("Failed to obtain container's init pid");
+			_exit(EXIT_FAILURE);
+		}
+
+		/* Enter the container namespaces */
+		if (!lxc_list_empty(&c->lxc_conf->id_map)) {
+			if (!switch_to_ns(init_pid, "user")) {
+				ERROR("Failed to enter user namespace");
+				_exit(EXIT_FAILURE);
+			}
+		}
+
+		if (!switch_to_ns(init_pid, "mnt")) {
+			ERROR("Failed to enter mount namespace");
+			_exit(EXIT_FAILURE);
+		}
+
+		/* Do the unmount */
+		ret = umount2(target, mountflags);
+		if (ret < 0) {
+			SYSERROR("Failed to umount \"%s\"", target);
+			_exit(EXIT_FAILURE);
+		}
+
+		_exit(EXIT_SUCCESS);
+	}
+
+	ret = wait_for_pid(pid);
+	if (ret < 0) {
+		SYSERROR("Wait for the child with pid %ld failed", (long)pid);
+		return -ret;
+	}
+
+	return 0;
+}
+
+WRAP_API_3(int, lxcapi_umount, const char *, unsigned long, struct lxc_mount*)
 
 static int lxcapi_attach_run_waitl(struct lxc_container *c, lxc_attach_options_t *options, const char *program, const char *arg, ...)
 {
@@ -5180,6 +5239,7 @@ struct lxc_container *lxc_container_new(const char *name, const char *configpath
 	c->migrate = lxcapi_migrate;
 	c->console_log = lxcapi_console_log;
 	c->mount = lxcapi_mount;
+	c->umount = lxcapi_umount;
 
 	return c;
 

src/lxc/lxccontainer.h

@@ -859,6 +859,12 @@ struct lxc_container {
 				 const char *source, const char *target,
 				 const char *filesystemtype, unsigned long mountflags,
 				 const void *data, struct lxc_mount *mnt);
+
+	/*!
+	 * \brief Unmount the container's path `target`.
+	 */
+	int (*umount)(struct lxc_container *c, const char *target,
+				  unsigned long mountflags, struct lxc_mount *mnt);
 };
 
 /*!