rslib: Fix decoding of shortened codes

BugLink: https://bugs.launchpad.net/bugs/1839036
[ Upstream commit 2034a42d1747fc1e1eeef2c6f1789c4d0762cb9c ]

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@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190620141039.9874-3-ferdinand.blomqvist@gmail.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/lib/reed_solomon/decode_rs.c b/lib/reed_solomon/decode_rs.c
index 0ec3f257ffdf9d38b96dad16276e7b3164f13e63..8eed0f9ac49503c72b39b79b257d5309a18b075a 100644 (file)
--- 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) {