dm verity fec: fix misaligned RS roots IO

commit df7b59b ("dm verity: fix FEC for RS roots unaligned to
block size") introduced the possibility for misaligned roots IO
relative to the underlying device's logical block size. E.g. Android's
default RS roots=2 results in dm_bufio->block_size=1024, which causes
the following EIO if the logical block size of the device is 4096,
given v->data_dev_block_bits=12:

E sd 0    : 0:0:0: [sda] tag#30 request not aligned to the logical block size
E blk_update_request: I/O error, dev sda, sector 10368424 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
E device-mapper: verity-fec: 254:8: FEC 9244672: parity read failed (block 18056): -5

Fix this by onlu using f->roots for dm_bufio blocksize IFF it is
aligned to v->data_dev_block_bits.

Fixes: df7b59b ("dm verity: fix FEC for RS roots unaligned to block size")
Cc: stable@vger.kernel.org
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-verity-fec.c

@@ -65,7 +65,7 @@ static u8 *fec_read_parity(struct dm_verity *v, u64 rsb, int index,
 	u8 *res;
 
 	position = (index + rsb) * v->fec->roots;
-	block = div64_u64_rem(position, v->fec->roots << SECTOR_SHIFT, &rem);
+	block = div64_u64_rem(position, v->fec->io_size, &rem);
 	*offset = (unsigned)rem;
 
 	res = dm_bufio_read(v->fec->bufio, block, buf);
@@ -154,7 +154,7 @@ static int fec_decode_bufs(struct dm_verity *v, struct dm_verity_fec_io *fio,
 
 		/* read the next block when we run out of parity bytes */
 		offset += v->fec->roots;
-		if (offset >= v->fec->roots << SECTOR_SHIFT) {
+		if (offset >= v->fec->io_size) {
 			dm_bufio_release(buf);
 
 			par = fec_read_parity(v, rsb, block_offset, &offset, &buf);
@@ -742,8 +742,13 @@ int verity_fec_ctr(struct dm_verity *v)
 		return -E2BIG;
 	}
 
+	if ((f->roots << SECTOR_SHIFT) & ((1 << v->data_dev_block_bits) - 1))
+		f->io_size = 1 << v->data_dev_block_bits;
+	else
+		f->io_size = v->fec->roots << SECTOR_SHIFT;
+
 	f->bufio = dm_bufio_client_create(f->dev->bdev,
-					  f->roots << SECTOR_SHIFT,
+					  f->io_size,
 					  1, 0, NULL, NULL);
 	if (IS_ERR(f->bufio)) {
 		ti->error = "Cannot initialize FEC bufio client";

drivers/md/dm-verity-fec.h

@@ -36,6 +36,7 @@ struct dm_verity_fec {
 	struct dm_dev *dev;	/* parity data device */
 	struct dm_bufio_client *data_bufio;	/* for data dev access */
 	struct dm_bufio_client *bufio;		/* for parity data access */
+	size_t io_size;		/* IO size for roots */
 	sector_t start;		/* parity data start in blocks */
 	sector_t blocks;	/* number of blocks covered */
 	sector_t rounds;	/* number of interleaving rounds */