[PATCH v2] android_ab: Fixes: Fix backup offset calculation

[Joshua Watt <jpewhacker@gmail.com>]

The backup offset is in bytes, but was incorrectly be interpreted as
blocks, leading to it being written to the wrong location. Fix the
calculation, clarify that ANDROID_AB_BACKUP_OFFSET is in bytes and must
be a multiple of the block size, and add a runtime check to validate the
offset.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
---
 boot/android_ab.c | 9 +++++++--
 common/Kconfig    | 3 ++-
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/boot/android_ab.c b/boot/android_ab.c
index 143f373aae..1196a189ed 100644
--- a/boot/android_ab.c
+++ b/boot/android_ab.c
@@ -139,8 +139,13 @@ static int ab_control_store(struct blk_desc *dev_desc,
 {
 	ulong abc_offset, abc_blocks, ret;
 
-	abc_offset = offset +
-		     offsetof(struct bootloader_message_ab, slot_suffix) /
+	if (offset % part_info->blksz) {
+		log_err("ANDROID: offset not block aligned\n");
+		return -EINVAL;
+	}
+
+	abc_offset = (offset +
+		      offsetof(struct bootloader_message_ab, slot_suffix)) /
 		     part_info->blksz;
 	abc_blocks = DIV_ROUND_UP(sizeof(struct bootloader_control),
 				  part_info->blksz);
diff --git a/common/Kconfig b/common/Kconfig
index 83c81edac2..e1b8557e0c 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -986,7 +986,8 @@ config ANDROID_AB_BACKUP_OFFSET
 	help
 	  If non-zero, a backup bootloader message starting at this offset in
 	  the partition will tried in the event that the primary one (starting
-	  at offset 0) fails its checksum.
+	  at offset 0) fails its checksum. The offset is in bytes and must be
+	  multiple of the block size.
 
 endmenu
 
-- 
2.45.2