[tip:core/rslib] rslib: Fix decoding of shortened codes

[tip-bot for Ferdinand Blomqvist]

Commit-ID:  2034a42d1747fc1e1eeef2c6f1789c4d0762cb9c
Gitweb:     https://git.kernel.org/tip/2034a42d1747fc1e1eeef2c6f1789c4d0762cb9c
Author:     Ferdinand Blomqvist <ferdinand.blomqvist@xxxxxxxxx>
AuthorDate: Thu, 20 Jun 2019 17:10:34 +0300
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Wed, 26 Jun 2019 14:55:45 +0200

rslib: Fix decoding of shortened codes

The decoding of shortenend codes is broken. It only works as expected if
there are no erasures.

When decoding with erasures, Lambda (the error and erasure locator
polynomial) is initialized from the given erasure positions. The pad
parameter is not accounted for by the initialisation code, and hence
Lambda is initialized from incorrect erasure positions.

The fix is to adjust the erasure positions by the supplied pad.

Signed-off-by: Ferdinand Blomqvist <ferdinand.blomqvist@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190620141039.9874-3-ferdinand.blomqvist@xxxxxxxxx

---
 lib/reed_solomon/decode_rs.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/reed_solomon/decode_rs.c b/lib/reed_solomon/decode_rs.c
index 1db74eb098d0..3313bf944ff1 100644
--- a/lib/reed_solomon/decode_rs.c
+++ b/lib/reed_solomon/decode_rs.c
@@ -99,9 +99,9 @@
 	if (no_eras > 0) {
 		/* Init lambda to be the erasure locator polynomial */
 		lambda[1] = alpha_to[rs_modnn(rs,
-					      prim * (nn - 1 - eras_pos[0]))];
+					prim * (nn - 1 - (eras_pos[0] + pad)))];
 		for (i = 1; i < no_eras; i++) {
-			u = rs_modnn(rs, prim * (nn - 1 - eras_pos[i]));
+			u = rs_modnn(rs, prim * (nn - 1 - (eras_pos[i] + pad)));
 			for (j = i + 1; j > 0; j--) {
 				tmp = index_of[lambda[j - 1]];
 				if (tmp != nn) {