util/cbfstool: Add "expand" command to make CBFS span an fmap region

vboot images come with multiple regions carrying CBFS file systems. To
expedite hashing (from slow flash memory), the FW_MAIN_* regions are
truncated since they typically have pretty large unused space at the
end that is of no interest.
For test purposes it can be useful to re-engage that space, so add a
command that creates a new empty file entry covering that area (except
for the last 4 bytes for the master header pointer, as usual).

BUG=b:65853903
BRANCH=none
TEST=`cbfstool test.bin expand -r FW_MAIN_A` creates a new empty file of
the expected size on a Chrome OS firmware image.

Change-Id: I160c8529ce4bfcc28685166b6d9035ade4f6f1d1
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/21598
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/cbfstool/cbfs_image.c

@@ -442,6 +442,44 @@ int cbfs_copy_instance(struct cbfs_image *image, struct buffer *dst)
 	return 0;
 }
 
+int cbfs_expand_to_region(struct buffer *region)
+{
+	if (buffer_get(region) == NULL)
+		return 1;
+
+	struct cbfs_image image;
+	memset(&image, 0, sizeof(image));
+	if (cbfs_image_from_buffer(&image, region, 0)) {
+		ERROR("reading CBFS failed!\n");
+		return 1;
+	}
+
+	uint32_t region_sz = buffer_size(region);
+
+	struct cbfs_file *entry;
+	for (entry = buffer_get(region);
+	     cbfs_is_valid_entry(&image, entry);
+	     entry = cbfs_find_next_entry(&image, entry)) {
+	     /* just iterate through */
+	}
+
+	/* entry now points to the first aligned address after the last valid
+	 * file header. That's either outside the image or exactly the place
+	 * where we need to create a new file.
+	 */
+	int last_entry_size = region_sz - ((void *)entry - buffer_get(region))
+		- cbfs_calculate_file_header_size("") - sizeof(int32_t);
+
+	if (last_entry_size > 0) {
+		cbfs_create_empty_entry(entry, CBFS_COMPONENT_NULL,
+			last_entry_size, "");
+		/* If the last entry was an empty file, merge them. */
+		cbfs_walk(&image, cbfs_merge_empty_entry, NULL);
+	}
+
+	return 0;
+}
+
 static size_t cbfs_file_entry_metadata_size(const struct cbfs_file *f)
 {
 	return ntohl(f->offset);