ifwitool: Calculate checksum for subpart_dir

Checksum is calculated by using 2s complement method. 8-bit sum of the
entire subpart directory from first byte of header to last byte of last
partition directory entry.

BUG=chrome-os-partner:53508

Change-Id: I991d79dfdb5331ab732bf0d71cf8223d63426fa8
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/15200
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

util/cbfstool/ifwitool.c

@@ -101,8 +101,8 @@ struct subpart_dir_header {
 	/* Length of directory header in bytes. */
 	uint8_t header_length;
 	/*
-	 * TODO(furquan): Add checksum calculation once more details are
+	 * 2s complement of 8-bit sum from first byte of header to last byte of
-	 * available.
+	 * last directory entry.
 	 */
 	uint8_t checksum;
 	/* ASCII short name of sub-partition. */
@@ -750,7 +750,27 @@ static void parse_sbpdt(void *data, size_t size)
 			  "S-BPDT");
 }
 
-static void validate_subpart_dir(struct subpart_dir *s, const char *name)
+static uint8_t calc_checksum(struct subpart_dir *s)
+{
+	size_t size = subpart_dir_size(&s->h);
+	uint8_t *data = (uint8_t *)s;
+	uint8_t checksum = 0;
+	size_t i;
+
+	uint8_t old_checksum = s->h.checksum;
+	s->h.checksum = 0;
+
+	for (i = 0; i < size; i++)
+		checksum += data[i];
+
+	s->h.checksum = old_checksum;
+
+	/* 2s complement */
+	return -checksum;
+}
+
+static void validate_subpart_dir(struct subpart_dir *s, const char *name,
+				 bool checksum_check)
 {
 	if ((s->h.marker != SUBPART_DIR_MARKER) ||
 	    (s->h.header_version != SUBPART_DIR_HEADER_VERSION_SUPPORTED) ||
@@ -759,6 +779,27 @@ static void validate_subpart_dir(struct subpart_dir *s, const char *name)
 		ERROR("Invalid subpart_dir for %s.\n", name);
 		exit(-1);
 	}
+
+	if (checksum_check == false)
+		return;
+
+	uint8_t checksum = calc_checksum(s);
+
+	if (checksum != s->h.checksum)
+		ERROR("Invalid checksum for %s (Expected=0x%x, Actual=0x%x).\n",
+		      name, checksum, s->h.checksum);
+}
+
+static void validate_subpart_dir_without_checksum(struct subpart_dir *s,
+						  const char *name)
+{
+	validate_subpart_dir(s, name, 0);
+}
+
+static void validate_subpart_dir_with_checksum(struct subpart_dir *s,
+					       const char *name)
+{
+	validate_subpart_dir(s, name, 1);
 }
 
 static void parse_subpart_dir(struct buffer *subpart_dir_buf,
@@ -784,7 +825,7 @@ static void parse_subpart_dir(struct buffer *subpart_dir_buf,
 	memcpy(hdr.name, data + offset, sizeof(hdr.name));
 	offset += sizeof(hdr.name);
 
-	validate_subpart_dir((struct subpart_dir *)&hdr, name);
+	validate_subpart_dir_without_checksum((struct subpart_dir *)&hdr, name);
 
 	assert(size > subpart_dir_size(&hdr));
 	alloc_buffer(subpart_dir_buf, subpart_dir_size(&hdr), "Subpart Dir");
@@ -805,6 +846,8 @@ static void parse_subpart_dir(struct buffer *subpart_dir_buf,
 				     &e[i].rsvd);
 	}
 
+	validate_subpart_dir_with_checksum(subpart_dir, name);
+
 	print_subpart_dir(subpart_dir);
 }
 
@@ -1353,7 +1396,7 @@ static void create_subpart(struct buffer *dst, struct buffer *info[],
 		       buffer_size(info[i]));
 	}
 
-	h->checksum = 0;
+	h->checksum = calc_checksum(buffer_get(&subpart_dir_buff));
 
 	struct subpart_dir *dir = buffer_get(&subpart_dir_buff);
 
@@ -1717,7 +1760,8 @@ static enum ifwi_ret ifwi_dir_replace(int type)
 		s->e[i].offset += offset;
 	}
 
-	s->h.checksum = 0;
+	/* Re-calculate checksum. */
+	s->h.checksum = calc_checksum(s);
 
 	/* Convert members to litte-endian. */
 	subpart_dir_fixup_write_buffer(&subpart_dir_buf);