image-hd: add support for overridding optional fields of the MBR

Makefile.am

@@ -100,6 +100,7 @@ EXTRA_DIST += \
 	test/hdimage7.config \
 	test/hdimage7.fdisk \
 	test/hole.config \
+	test/hole-skip-optionals.config \
 	test/hdimage-hybrid.config \
 	test/hdimage-hybrid.fdisk \
 	test/hdimage-fail1.config \

README.rst

@@ -425,6 +425,9 @@ Options:
 			up to the end of the last partition. This might make the file
 			bigger. This is necessary if the image will be processed by
 			such tools as libvirt, libguestfs or parted.
+:mbr-skip-optionals:	Boolean. If true, then the 6 bytes of optional fields in the MBR
+			are skipped, allowing an image occupying the first 446 bytes
+			instead of just the first 440 bytes to be written.
 
 iso
 ***

image-hd.c

@@ -47,6 +47,7 @@ struct hdimage {
 	unsigned long long gpt_location;
 	cfg_bool_t gpt_no_backup;
 	cfg_bool_t fill;
+	cfg_bool_t mbr_skip_optionals;
 	unsigned long long file_size;
 };
 
@@ -64,9 +65,14 @@ struct mbr_partition_entry {
 } __attribute__((packed));
 ct_assert(sizeof(struct mbr_partition_entry) == 16);
 
-struct mbr_tail {
+struct mbr_tail_optionals {
 	uint32_t disk_signature;
 	uint16_t copy_protect;
+} __attribute__((packed));
+ct_assert(sizeof(struct mbr_tail_optionals) == 6);
+
+struct mbr_tail {
+	struct mbr_tail_optionals optionals;
 	struct mbr_partition_entry part_entry[4];
 	uint16_t boot_signature;
 } __attribute__((packed));
@@ -141,6 +147,9 @@ static int hdimage_insert_mbr(struct image *image, struct list_head *partitions)
 	struct hdimage *hd = image->handler_priv;
 	struct mbr_tail mbr;
 	struct partition *part;
+	unsigned long long mbr_offset;
+	const void *mbr_data;
+	size_t mbr_size;
 	int ret, i = 0;
 
 	if (hd->table_type == TYPE_HYBRID) {
@@ -150,7 +159,7 @@ static int hdimage_insert_mbr(struct image *image, struct list_head *partitions)
 	}
 
 	memset(&mbr, 0, sizeof(mbr));
-	memcpy(&mbr.disk_signature, &hd->disksig, sizeof(hd->disksig));
+	memcpy(&mbr.optionals.disk_signature, &hd->disksig, sizeof(hd->disksig));
 
 	list_for_each_entry(part, partitions, list) {
 		struct mbr_partition_entry *entry;
@@ -192,7 +201,16 @@ static int hdimage_insert_mbr(struct image *image, struct list_head *partitions)
 
 	mbr.boot_signature = htole16(0xaa55);
 
-	ret = insert_data(image, &mbr, imageoutfile(image), sizeof(mbr), 440);
+	mbr_offset = 440;
+	mbr_size = sizeof(mbr);
+	mbr_data = &mbr;
+	if (hd->mbr_skip_optionals) {
+		mbr_offset += sizeof(struct mbr_tail_optionals);
+		mbr_size -= sizeof(struct mbr_tail_optionals);
+		mbr_data = &mbr.part_entry[0];
+	}
+
+	ret = insert_data(image, mbr_data, imageoutfile(image), mbr_size, mbr_offset);
 	if (ret) {
 		if (hd->table_type == TYPE_HYBRID) {
 			image_error(image, "failed to write hybrid MBR\n");
@@ -968,6 +986,7 @@ static int hdimage_setup(struct image *image, cfg_t *cfg)
 	hd->gpt_location = cfg_getint_suffix(cfg, "gpt-location");
 	hd->gpt_no_backup = cfg_getbool(cfg, "gpt-no-backup");
 	hd->fill = cfg_getbool(cfg, "fill");
+	hd->mbr_skip_optionals = cfg_getbool(cfg, "mbr-skip-optionals");
 
 	if (is_block_device(imageoutfile(image))) {
 		if (image->size) {
@@ -1029,8 +1048,19 @@ static int hdimage_setup(struct image *image, cfg_t *cfg)
 	}
 
 	if (hd->table_type != TYPE_NONE) {
-		struct partition *mbr = fake_partition("[MBR]", 512 - sizeof(struct mbr_tail),
-						       sizeof(struct mbr_tail));
+		struct partition *mbr;
+		unsigned long long mbr_offset;
+		unsigned long long mbr_size;
+
+		mbr_offset = 512 - sizeof(struct mbr_tail);
+		mbr_size = sizeof(struct mbr_tail);
+
+		if (hd->mbr_skip_optionals) {
+			mbr_offset += sizeof(struct mbr_tail_optionals);
+			mbr_size -= sizeof(struct mbr_tail_optionals);
+		}
+
+		mbr = fake_partition("[MBR]", mbr_offset, mbr_size);
 
 		list_add_tail(&mbr->list, &image->partitions);
 		now = partition_end(mbr);
@@ -1215,6 +1245,7 @@ static cfg_opt_t hdimage_opts[] = {
 	CFG_STR("gpt-location", NULL, CFGF_NONE),
 	CFG_BOOL("gpt-no-backup", cfg_false, CFGF_NONE),
 	CFG_BOOL("fill", cfg_false, CFGF_NONE),
+	CFG_BOOL("mbr-skip-optionals", cfg_false, CFGF_NONE),
 	CFG_END()
 };
 

test/hdimage.test

@@ -159,6 +159,20 @@ test_expect_success "bootloader-hole5" "
 	setup_gpt_files &&
 	OFFSET=128K run_genimage hole.config"
 
+# Test the mbr-skip-optionals
+test_expect_success "bootloader-hole6" "
+	rm -rf input &&
+	mkdir input &&
+	truncate -s 100K input/bootloader.img &&
+	MBR_SKIP_OPTIONALS=true run_genimage hole-skip-optionals.config"
+
+# Test the mbr-skip-optionals
+test_expect_success "bootloader-hole7" "
+	rm -rf input &&
+	mkdir input &&
+	truncate -s 100K input/bootloader.img &&
+	MBR_SKIP_OPTIONALS=false test_must_fail run_genimage hole-skip-optionals.config"
+
 test_expect_success hexdump "hdimage no-partition" "
 	dd if=/dev/zero bs=1 count=100 | tr '\000' '\377' > input/block1.img &&
 	dd if=/dev/zero bs=1 count=50  | tr '\000' '\252' > input/block2.img &&

test/hole-skip-optionals.config

@@ -0,0 +1,17 @@
+image test.hole {
+	hdimage {
+		mbr-skip-optionals = "${MBR_SKIP_OPTIONALS}"
+	}
+
+	partition bootloader {
+		in-partition-table = false
+		offset = 0
+		image = "bootloader.img"
+	}
+}
+
+image bootloader.img {
+	file {
+		holes = "(444; 512)"
+	}
+}