rslib: Fix decoding of shortened codes [Linux 4.9.187]

This Linux kernel change "rslib: Fix decoding of shortened codes" is included in the Linux 4.9.187 release. This change is authored by Ferdinand Blomqvist <ferdinand.blomqvist [at] gmail.com> on Thu Jun 20 17:10:34 2019 +0300. The commit for this change in Linux stable tree is 5c345e2 (patch) which is from upstream commit 2034a42. The same Linux upstream change may have been applied to various maintained Linux releases and you can find all Linux releases containing changes from upstream 2034a42.

rslib: Fix decoding of shortened codes

[ 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>

There are 4 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 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 0ec3f25..8eed0f9 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) {

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