ccon: Recursively mkdir for missing mount targets

This makes it possible to mount content inside a tmpfs.  For example,
systemd wants a tmpfs at /dev [1] and you may want to also mount
/dev/pts, /dev/shm, etc.  Or you may want to put a tmpfs at
/sys/fs/cgroup and a cgroup filesystem at /sys/fs/cgroup/systemd.
There's also some background discussion in [2].

[1]: https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface/
[2]: opencontainers/runtime-spec#591
     Subject: Require the mount destination to be created before hand

Author: wking

README.md

@@ -264,7 +264,9 @@ processes inside a user namespace.
 
 If they don't start with a slash, **`source`** and **`target`** are
 interpreted as paths relative to ccon's [current working
-directory][getcwd.3].
+directory][getcwd.3].  If **`target`** does not exist, ccon will
+attempt to create it by calling [`mkdir`][mkdir.3p], making multiple
+calls if necessary.
 
 In addition to the usual types supported by [`mount`][mount.2], ccon
 supports a `pivot-root` **`type`** that invokes the
@@ -837,6 +839,7 @@ be distributed under the GPLv3+.
 [exec.3]: http://man7.org/linux/man-pages/man3/exec.3.html
 [getcwd.3]: http://man7.org/linux/man-pages/man3/getcwd.3.html
 [grantpt.3p]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/grantpt.html
+[mkdir.3p]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/mkdir.html
 [posix_openpt.3p]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_openpt.html
 [stdin.3]: http://man7.org/linux/man-pages/man3/stdin.3.html
 [pts.4]: http://man7.org/linux/man-pages/man4/pty.4.html

ccon.c

@@ -124,6 +124,7 @@ static int close_pipe(int pipe_fd[]);
 static int sendfd(int socket, int *fd);
 static int recvfd(int socket, int *fd);
 static int splice_pseudoterminal_master(int *master);
+static int mkdir_all(const char *path, mode_t mode);
 
 int main(int argc, char **argv)
 {
@@ -1889,6 +1890,10 @@ static int handle_mounts(json_t * config)
 				return 1;
 			}
 		} else {
+			if (mkdir_all(target, 0777) == -1) {
+				return 1;
+			}
+
 			LOG("mount %lu: %s to %s (type: %s, flags: %lu, data %s)\n", (unsigned long int)i, source, target, type, flags, data);
 			if (mount(source, target, type, flags, data) == -1) {
 				PERROR("mount");
@@ -2366,3 +2371,37 @@ static int splice_pseudoterminal_master(int *master)
 
 	return err;
 }
+
+static int mkdir_all(const char *path, mode_t mode)
+{
+	struct stat buf;
+	char *path_copy = NULL, *dir = NULL;
+	int err = 0;
+
+	if (stat(path, &buf) == -1) {
+		if (errno == ENOENT) {
+			path_copy = strdup(path);
+			dir = dirname(path_copy);
+			if (mkdir_all(dir, mode) == -1) {
+				err = -1;
+				goto cleanup;
+			}
+			LOG("create directory %s\n", path);;
+			if (mkdir(path, mode) == -1) {
+				PERROR("mkdir");
+				err = -1;
+				goto cleanup;
+			}
+		} else {
+			PERROR("stat");
+			err = -1;
+			goto cleanup;
+		}
+	}
+
+ cleanup:
+	if (path_copy != NULL) {
+		free(path_copy);
+	}
+	return err;
+}

test/t2002-mount-namespace.t

@@ -110,4 +110,50 @@ test_expect_success BUSYBOX,ID 'Test mount namespace pivot root' "
 	test_cmp expected actual
 "
 
+test_expect_success BUSYBOX,ID 'Test mount namespace creates destination directories' "
+	mkdir -p rootfs &&
+	ccon --verbose --config-string '{
+		  \"version\": \"0.4.0\",
+		  \"namespaces\": {
+		    \"user\": {
+		      \"setgroups\": false,
+		      \"uidMappings\": [
+		        {
+		          \"containerID\": 0,
+		          \"hostID\": $(id -u),
+		          \"size\": 1
+		        }
+		      ],
+		      \"gidMappings\": [
+		        {
+		          \"containerID\": 0,
+		          \"hostID\": $(id -u),
+		          \"size\": 1
+		        }
+		      ]
+		    },
+		    \"mount\": {
+		      \"mounts\": [
+		        {
+		          \"target\": \"/tmp/\",
+		          \"type\": \"tmpfs\"
+		        },
+		        {
+		          \"target\": \"/tmp/foo/bar\",
+		          \"type\": \"tmpfs\"
+		        }
+		      ]
+		    }
+		  },
+		  \"process\": {
+		    \"args\": [\"/bin/busybox\", \"ls\", \"/tmp/foo\"],
+		    \"host\": true
+		  }
+		}' >actual &&
+	cat <<-EOF >expected &&
+		bar
+	EOF
+	test_cmp expected actual
+"
+
 test_done