[PATCH 6.18.y 3/4] dm-verity-fec: replace {MAX,MIN}_RSN with {MIN,MAX}_ROOTS

[Sasha Levin <sashal@kernel.org>]

From: Eric Biggers <ebiggers@kernel.org>

[ Upstream commit 82fbd6a3e29a329d439690cd7ccc4162c9cd8db6 ]

Every time DM_VERITY_FEC_{MAX,MIN}_RSN are used, they are subtracted
from DM_VERITY_FEC_RSM to get the bounds on the number of roots.
Therefore, replace these with {MIN,MAX}_ROOTS constants which are more
directly useful.  (Note the inversion, where MAX_RSN maps to MIN_ROOTS
and MIN_RSN maps to MAX_ROOTS.)  No functional change.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Stable-dep-of: 31d6e6c0ba8d ("dm-verity: fix buffer overflow in FEC calculation")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/md/dm-verity-fec.c | 6 +++---
 drivers/md/dm-verity-fec.h | 5 ++---
 2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
index a8fbf97b97517f..cebcc8fd25d700 100644
--- a/drivers/md/dm-verity-fec.c
+++ b/drivers/md/dm-verity-fec.c
@@ -87,7 +87,7 @@ static int fec_decode_bufs(struct dm_verity *v, struct dm_verity_io *io,
 	int r, corrected = 0, res;
 	struct dm_buffer *buf;
 	unsigned int n, i, j, parity_pos, to_copy;
-	uint16_t par_buf[DM_VERITY_FEC_RSM - DM_VERITY_FEC_MIN_RSN];
+	uint16_t par_buf[DM_VERITY_FEC_MAX_ROOTS];
 	u8 *par, *block;
 	u64 parity_block;
 	struct bio *bio = dm_bio_from_per_bio_data(io, v->ti->per_io_data_size);
@@ -605,8 +605,8 @@ int verity_fec_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v,
 
 	} else if (!strcasecmp(arg_name, DM_VERITY_OPT_FEC_ROOTS)) {
 		if (sscanf(arg_value, "%hhu%c", &num_c, &dummy) != 1 || !num_c ||
-		    num_c < (DM_VERITY_FEC_RSM - DM_VERITY_FEC_MAX_RSN) ||
-		    num_c > (DM_VERITY_FEC_RSM - DM_VERITY_FEC_MIN_RSN)) {
+		    num_c < DM_VERITY_FEC_MIN_ROOTS ||
+		    num_c > DM_VERITY_FEC_MAX_ROOTS) {
 			ti->error = "Invalid " DM_VERITY_OPT_FEC_ROOTS;
 			return -EINVAL;
 		}
diff --git a/drivers/md/dm-verity-fec.h b/drivers/md/dm-verity-fec.h
index 6f11abb3c7d972..178ecdddbbd310 100644
--- a/drivers/md/dm-verity-fec.h
+++ b/drivers/md/dm-verity-fec.h
@@ -13,9 +13,8 @@
 
 /* Reed-Solomon(M, N) parameters */
 #define DM_VERITY_FEC_RSM		255
-#define DM_VERITY_FEC_MAX_RSN		253
-#define DM_VERITY_FEC_MIN_RSN		231	/* ~10% space overhead */
-#define DM_VERITY_FEC_MAX_ROOTS	(DM_VERITY_FEC_RSM - DM_VERITY_FEC_MIN_RSN)
+#define DM_VERITY_FEC_MIN_ROOTS	2	/* RS(255, 253): ~0.8% space overhead */
+#define DM_VERITY_FEC_MAX_ROOTS	24	/* RS(255, 231): ~10% space overhead */
 
 /* buffers for deinterleaving and decoding */
 #define DM_VERITY_FEC_BUF_PREALLOC	1	/* buffers to preallocate */
-- 
2.53.0