Make overlayfs mounts work directly

When users wanted to mount overlay directories with lxc.mount.entry they had to
create upperdirs and workdirs beforehand in order to mount them. To create it
for them we add the functions mount_entry_create_overlay_dirs() and
mount_entry_create_aufs_dirs() which do this for them. User can now simply
specify e.g.:

        lxc.mount.entry = /lower merged overlay lowerdir=/lower,upper=/upper,workdir=/workdir,create=dir

and /upper and /workdir will be created for them. /upper and /workdir need to
be absolute paths to directories which are created under the containerdir (e.g.
under $lxcpath/CONTAINERNAME/). Relative mountpoints, mountpoints outside the
containerdir, and mountpoints within the container's rootfs are ignored. (The
latter *might* change in the future should it be considered safe/useful.)

Specifying

        lxc.mount.entry = /lower merged overlay lowerdir=/lower:/lower2,create=dir

will lead to a read-only overlay mount in accordance with the
kernel-documentation.

Specifying

        lxc.mount.entry = /lower merged overlay lowerdir=/lower,create=dir

will fail when no upperdir and workdir options are given.

Signed-off-by: Christian Brauner <christianvanbrauner@gmail.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>

src/lxc/conf.c

@@ -1815,13 +1815,151 @@ static void cull_mntent_opt(struct mntent *mntent)
 	}
 }
 
+static int mount_entry_create_overlay_dirs(const struct mntent *mntent,
+					   const struct lxc_rootfs *rootfs)
+{
+	char *del = NULL;
+	char *lxcpath = NULL;
+	char *upperdir = NULL;
+	char *workdir = NULL;
+	char **opts = NULL;
+	size_t arrlen = 0;
+	size_t dirlen = 0;
+	size_t i;
+	size_t len = 0;
+	size_t rootfslen = 0;
+
+	if (!rootfs->path)
+		return -1;
+
+	opts = lxc_string_split(mntent->mnt_opts, ',');
+	if (opts)
+		arrlen = lxc_array_len((void **)opts);
+	else
+		return -1;
+
+	for (i = 0; i < arrlen; i++) {
+		if (strstr(opts[i], "upperdir=") && (strlen(opts[i]) > (len = strlen("upperdir="))))
+			upperdir = opts[i] + len;
+		else if (strstr(opts[i], "workdir=") && (strlen(opts[i]) > (len = strlen("workdir="))))
+			workdir = opts[i] + len;
+	}
+
+	lxcpath = strdup(rootfs->path);
+	if (!lxcpath) {
+		lxc_free_array((void **)opts, free);
+		return -1;
+	}
+
+	del = strstr(lxcpath, "/rootfs");
+	if (!del) {
+		free(lxcpath);
+		lxc_free_array((void **)opts, free);
+		return -1;
+	}
+	*del = '\0';
+
+	dirlen = strlen(lxcpath);
+	rootfslen = strlen(rootfs->path);
+
+	/* We neither allow users to create upperdirs and workdirs outside the
+	 * containerdir nor inside the rootfs. The latter might be debatable. */
+	if (upperdir)
+		if ((strncmp(upperdir, lxcpath, dirlen) == 0) && (strncmp(upperdir, rootfs->path, rootfslen) != 0))
+			if (mkdir_p(upperdir, 0755) < 0) {
+				WARN("Failed to create upperdir");
+			}
+
+
+	if (workdir)
+		if ((strncmp(workdir, lxcpath, dirlen) == 0) && (strncmp(workdir, rootfs->path, rootfslen) != 0))
+			if (mkdir_p(workdir, 0755) < 0) {
+				WARN("Failed to create workdir");
+			}
+
+	free(lxcpath);
+	lxc_free_array((void **)opts, free);
+	return 0;
+}
+
+static int mount_entry_create_aufs_dirs(const struct mntent *mntent,
+					const struct lxc_rootfs *rootfs)
+{
+	char *del = NULL;
+	char *lxcpath = NULL;
+	char *scratch = NULL;
+	char *tmp = NULL;
+	char *upperdir = NULL;
+	char **opts = NULL;
+	size_t arrlen = 0;
+	size_t i;
+	size_t len = 0;
+
+	if (!rootfs->path)
+		return -1;
+
+	opts = lxc_string_split(mntent->mnt_opts, ',');
+	if (opts)
+		arrlen = lxc_array_len((void **)opts);
+	else
+		return -1;
+
+	for (i = 0; i < arrlen; i++) {
+		if (strstr(opts[i], "br=") && (strlen(opts[i]) > (len = strlen("br="))))
+			tmp = opts[i] + len;
+	}
+	if (!tmp) {
+		lxc_free_array((void **)opts, free);
+		return -1;
+	}
+
+	upperdir = strtok_r(tmp, ":=", &scratch);
+	if (!upperdir) {
+		lxc_free_array((void **)opts, free);
+		return -1;
+	}
+
+	lxcpath = strdup(rootfs->path);
+	if (!lxcpath) {
+		lxc_free_array((void **)opts, free);
+		return -1;
+	}
+
+	del = strstr(lxcpath, "/rootfs");
+	if (!del) {
+		free(lxcpath);
+		lxc_free_array((void **)opts, free);
+		return -1;
+	}
+	*del = '\0';
+
+	/* We neither allow users to create upperdirs outside the containerdir
+	 * nor inside the rootfs. The latter might be debatable. */
+	if ((strncmp(upperdir, lxcpath, strlen(lxcpath)) == 0) && (strncmp(upperdir, rootfs->path, strlen(rootfs->path)) != 0))
+		if (mkdir_p(upperdir, 0755) < 0) {
+			WARN("Failed to create upperdir");
+		}
+
+	free(lxcpath);
+	lxc_free_array((void **)opts, free);
+	return 0;
+}
+
 static int mount_entry_create_dir_file(const struct mntent *mntent,
-				       const char* path)
+				       const char* path, const struct lxc_rootfs *rootfs)
 {
 	char *pathdirname = NULL;
 	int ret = 0;
 	FILE *pathfile = NULL;
 
+	if (strncmp(mntent->mnt_type, "overlay", 7) == 0) {
+		if (mount_entry_create_overlay_dirs(mntent, rootfs) < 0)
+			return -1;
+	} else if (strncmp(mntent->mnt_type, "aufs", 4) == 0) {
+		if (mount_entry_create_aufs_dirs(mntent, rootfs) < 0)
+			return -1;
+	}
+
 	if (hasmntopt(mntent, "create=dir")) {
 		if (mkdir_p(path, 0755) < 0) {
 			WARN("Failed to create mount target '%s'", path);
@@ -1839,23 +1977,23 @@ static int mount_entry_create_dir_file(const struct mntent *mntent,
 		if (!pathfile) {
 			WARN("Failed to create mount target '%s'", path);
 			ret = -1;
-		}
-		else
+		} else {
 			fclose(pathfile);
+		}
 	}
 	free(pathdirname);
 	return ret;
 }
 
 static inline int mount_entry_on_generic(struct mntent *mntent,
-                 const char* path, const char *rootfs)
+                 const char* path, const struct lxc_rootfs *rootfs)
 {
 	unsigned long mntflags;
 	char *mntdata;
 	int ret;
 	bool optional = hasmntopt(mntent, "optional") != NULL;
 
-	ret = mount_entry_create_dir_file(mntent, path);
+	ret = mount_entry_create_dir_file(mntent, path, rootfs);
 
 	if (ret < 0)
 		return optional ? 0 : -1;
@@ -1867,11 +2005,11 @@ static inline int mount_entry_on_generic(struct mntent *mntent,
 		return -1;
 	}
 
-	ret = mount_entry(mntent->mnt_fsname, path, mntent->mnt_type,
-			  mntflags, mntdata, optional, rootfs);
+	ret = mount_entry(mntent->mnt_fsname, path, mntent->mnt_type, mntflags,
+			  mntdata, optional,
+			  rootfs->path ? rootfs->mount : NULL);
 
 	free(mntdata);
-
 	return ret;
 }
 
@@ -1924,17 +2062,17 @@ static int mount_entry_on_absolute_rootfs(struct mntent *mntent,
 		return -1;
 	}
 
-	return mount_entry_on_generic(mntent, path, rootfs->mount);
+	return mount_entry_on_generic(mntent, path, rootfs);
 }
 
 static int mount_entry_on_relative_rootfs(struct mntent *mntent,
-					  const char *rootfs)
+					  const struct lxc_rootfs *rootfs)
 {
 	char path[MAXPATHLEN];
 	int ret;
 
 	/* relative to root mount point */
-	ret = snprintf(path, sizeof(path), "%s/%s", rootfs, mntent->mnt_dir);
+	ret = snprintf(path, sizeof(path), "%s/%s", rootfs->mount, mntent->mnt_dir);
 	if (ret >= sizeof(path)) {
 		ERROR("path name too long");
 		return -1;
@@ -1961,7 +2099,7 @@ static int mount_file_entries(const struct lxc_rootfs *rootfs, FILE *file,
 		/* We have a separate root, mounts are relative to it */
 		if (mntent.mnt_dir[0] != '/') {
 			if (mount_entry_on_relative_rootfs(&mntent,
-							   rootfs->mount))
+							   rootfs))
 				goto out;
 			continue;
 		}