bootctrl: fall back to active slot if current slot not found in cmdline

While postmarketOS makes use of a kernel cmdline argument, this isn't
the case for all distros. Furthermore, this argument isn't present in
those added automatically by the Android bootloader.

In order to still be able to use `qbootctl` on other distros, this
change ensures we use the currently active slot as the current slot when
the cmdline argument is missing. This ensure `qbootctl -m` picks up the
correct slot more often than not, instead of always defaulting to `_a`.

bootctrl_impl.c

@@ -331,7 +331,34 @@ int get_boot_attr(struct gpt_disk *disk, unsigned slot, enum part_attr_type attr
 	return get_partition_attribute(disk, bootPartition, attr);
 }
 
-static unsigned int get_current_slot_from_kernel_cmdline()
+unsigned get_active_boot_slot()
+{
+	struct gpt_disk disk = { 0 };
+	uint32_t num_slots = get_number_slots();
+
+	if (num_slots <= 1) {
+		// Slot 0 is the only slot around.
+		return 0;
+	}
+
+	for (uint32_t i = 0; i < num_slots; i++) {
+		if (get_boot_attr(&disk, i, ATTR_SLOT_ACTIVE)) {
+			gpt_disk_free(&disk);
+			return i;
+		}
+	}
+
+	fprintf(stderr, "%s: Failed to find the active boot slot\n", __func__);
+	gpt_disk_free(&disk);
+	return 0;
+}
+
+/*
+ * The current slot is usually made available via the kernel cmdline. If it isn't for some reason
+ * (e.g. because we booted via a secondary bootloader that removes Android cmdline args) then we
+ * assume that the active slot is the current slot
+ */
+static unsigned int get_current_or_active_slot()
 {
 	uint32_t num_slots = 0;
 	char bootSlotProp[MAX_CMDLINE_SIZE] = { '\0' };
@@ -342,10 +369,10 @@ static unsigned int get_current_slot_from_kernel_cmdline()
 		return 0;
 	}
 
-	get_kernel_cmdline_arg(BOOT_SLOT_PROP, bootSlotProp, "_a");
+	get_kernel_cmdline_arg(BOOT_SLOT_PROP, bootSlotProp, "N/A");
 	if (!strncmp(bootSlotProp, "N/A\n", strlen("N/A"))) {
 		fprintf(stderr, "%s: Unable to read boot slot property\n", __func__);
-		return 0;
+		return get_active_boot_slot();
 	}
 
 	// Iterate through a list of partitons named as boot+suffix
@@ -538,28 +565,6 @@ static int boot_ctl_set_active_slot_for_partitions(struct gpt_disk *disk,
 	return 0;
 }
 
-unsigned get_active_boot_slot()
-{
-	struct gpt_disk disk = { 0 };
-	uint32_t num_slots = get_number_slots();
-
-	if (num_slots <= 1) {
-		// Slot 0 is the only slot around.
-		return 0;
-	}
-
-	for (uint32_t i = 0; i < num_slots; i++) {
-		if (get_boot_attr(&disk, i, ATTR_SLOT_ACTIVE)) {
-			gpt_disk_free(&disk);
-			return i;
-		}
-	}
-
-	fprintf(stderr, "%s: Failed to find the active boot slot\n", __func__);
-	gpt_disk_free(&disk);
-	return 0;
-}
-
 int set_active_boot_slot(unsigned slot, bool ignore_missing_bsg)
 {
 	enum boot_chain chain = (enum boot_chain)slot;
@@ -638,7 +643,7 @@ int is_slot_marked_successful(unsigned slot)
 }
 
 const struct boot_control_module bootctl = {
-	.getCurrentSlot = get_current_slot_from_kernel_cmdline,
+	.getCurrentSlot = get_current_or_active_slot,
 	.markBootSuccessful = mark_boot_successful,
 	.setActiveBootSlot = set_active_boot_slot,
 	.setSlotAsUnbootable = set_slot_as_unbootable,

gpt-utils.c

@@ -403,7 +403,7 @@ static int gpt_get_headers(const char *partname, uint8_t **primary, uint8_t **ba
 		goto error;
 	}
 
-	fd = open(devpath, O_RDWR);
+	fd = open(devpath, O_RDONLY);
 	if (fd < 0) {
 		fprintf(stderr, "%s: Failed to open %s : %s\n", __func__, devpath, strerror(errno));
 		return -1;
@@ -638,7 +638,7 @@ int gpt_disk_get_disk_info(const char *dev, struct gpt_disk *disk)
 
 	disk->hdr_bak_crc = efi_crc32(disk->hdr_bak, gpt_header_size);
 
-	fd = open(disk->devpath, O_RDWR);
+	fd = open(disk->devpath, O_RDONLY);
 	if (fd < 0) {
 		fprintf(stderr, "%s: Failed to open %s: %s\n", __func__, disk->devpath,
 			strerror(errno));