lib: Use new mount api for overlay to mount overlayfs

This is required on kernel >= 6.5 to handle escaped commas in upper or
lowerdirs due to an overlayfs change in escape handling behaviour
caused by the switch to the new mount api internally.

For anything post 6.5 we fallback to the old legacy implementation
which supports escaping.

See containers#213 for details.

Signed-off-by: Alexander Larsson <[email protected]>

libcomposefs/lcfs-mount.c

@@ -403,6 +403,117 @@ static int lcfs_mount_ovl_legacy(struct lcfs_mount_state_s *state, char *imagemo
 	return res;
 }
 
+static int lcfs_mount_ovl(struct lcfs_mount_state_s *state, char *imagemount)
+{
+#ifdef HAVE_NEW_MOUNT_API
+	struct lcfs_mount_options_s *options = state->options;
+
+	bool require_verity =
+		(options->flags & LCFS_MOUNT_FLAGS_REQUIRE_VERITY) != 0;
+	bool readonly = (options->flags & LCFS_MOUNT_FLAGS_READONLY) != 0;
+
+	cleanup_fd int fd_fs = syscall_fsopen("overlay", FSOPEN_CLOEXEC);
+	if (fd_fs < 0)
+		return -errno;
+
+	int res = syscall_fsconfig(fd_fs, FSCONFIG_SET_STRING, "metacopy", "on", 0);
+	if (res < 0)
+		return -errno;
+
+	res = syscall_fsconfig(fd_fs, FSCONFIG_SET_STRING, "redirect_dir", "on", 0);
+	if (res < 0)
+		return -errno;
+
+	if (require_verity) {
+		res = syscall_fsconfig(fd_fs, FSCONFIG_SET_STRING, "verity",
+				       "require", 0);
+		if (res < 0)
+			return -errno;
+	}
+
+	/* Here we're using the new mechanism to append to lowerdir that was added in
+	 * 6.5 (commit b36a5780cb44), because that is the only way to handle escaping
+	 * of commas (i.e. we don't need to in this case) with the new mount api.
+	 * Also, since 6.5 has data-only lowerdir support we can just always use it.
+	 *
+	 * For older kernels a lack of append support will make the mount fail with EINVAL
+	 * and print an "empty lowerdir" error, and a lack of comma in the options will
+	 * cause the fsconfig to fail with EINVAL. If any of these happen we fall back to
+	 * the legacy implementation (via ENOSYS).
+	 */
+	res = syscall_fsconfig(fd_fs, FSCONFIG_SET_STRING, "lowerdir", imagemount, 0);
+	/* EINVAL probably the lack of support for commas in options as per above, fallback */
+	if (errno == EINVAL)
+		return -ENOSYS;
+	if (res < 0)
+		return -errno;
+
+	for (size_t i = 0; i < state->options->n_objdirs; i++) {
+		const char *objdir = state->options->objdirs[i];
+		cleanup_free char *opt = malloc(strlen(objdir) + 2 + 1);
+		if (opt == NULL)
+			return -ENOMEM;
+		strcpy(opt, "::"); /* starting with : means we append a dataonly lowerdir */
+		strcat(opt, objdir);
+
+		res = syscall_fsconfig(fd_fs, FSCONFIG_SET_STRING, "lowerdir",
+				       opt, 0);
+		if (res < 0) {
+			/* EINVAL probably the lack of support for commas in options as per above, fallback */
+			if (errno == EINVAL)
+				return -ENOSYS;
+			return -errno;
+		}
+	}
+
+	if (options->upperdir) {
+		res = fsconfig(fd_fs, FSCONFIG_SET_STRING, "upperdir",
+			       options->upperdir, 0);
+		if (res < 0) {
+			/* EINVAL probably the lack of support for commas in options as per above, fallback */
+			if (errno == EINVAL)
+				return -ENOSYS;
+			return -errno;
+		}
+	}
+	if (options->workdir) {
+		res = fsconfig(fd_fs, FSCONFIG_SET_STRING, "workdir",
+			       options->workdir, 0);
+		if (res < 0) {
+			/* EINVAL probably the lack of support for commas in options as per above, fallback */
+			if (errno == EINVAL)
+				return -ENOSYS;
+			return -errno;
+		}
+	}
+
+	res = syscall_fsconfig(fd_fs, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+	if (res < 0) {
+		/* EINVAL probably the lack of support for dataonly dirs as per above, fallback */
+		if (errno == EINVAL)
+			return -ENOSYS;
+		return -errno;
+	}
+
+	int mount_flags = 0;
+	if (readonly)
+		mount_flags |= MS_RDONLY;
+
+	cleanup_fd int fd_mnt = syscall_fsmount(fd_fs, FSMOUNT_CLOEXEC, mount_flags);
+	if (fd_mnt < 0)
+		return -errno;
+
+	res = syscall_move_mount(fd_mnt, "", AT_FDCWD, state->mountpoint,
+				 MOVE_MOUNT_F_EMPTY_PATH);
+	if (res < 0)
+		return -errno;
+
+	return 0;
+#else
+	return -ENOSYS;
+#endif
+}
+
 static int lcfs_mount_erofs(const char *source, const char *target,
 			    uint32_t image_flags, struct lcfs_mount_state_s *state)
 {
@@ -520,7 +631,9 @@ static int lcfs_mount_erofs_ovl(struct lcfs_mount_state_s *state,
 	/* We use the legacy API to mount overlayfs, because the new API doesn't allow use
 	 * to pass in escaped directory names
 	 */
-	res = lcfs_mount_ovl_legacy(state, imagemount);
+	res = lcfs_mount_ovl(state, imagemount);
+	if (res == -ENOSYS)
+		res = lcfs_mount_ovl_legacy(state, imagemount);
 
 	umount2(imagemount, MNT_DETACH);
 	if (created_tmpdir) {