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Message-ID: <alpine.DEB.2.21.1906172249580.1963@nanos.tec.linutronix.de>
Date: Mon, 17 Jun 2019 23:00:27 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: Ferdinand Blomqvist <ferdinand.blomqvist@...il.com>
cc: linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 7/7] rslib: Fix remaining decoder flaws
On Sat, 30 Mar 2019, Ferdinand Blomqvist wrote:
> The decoder is flawed in the following ways:
...
> - Also fix errors in parity when correcting in-place. Another option
> would be to completely disregard errors in the parity, but then the
> interface makes it impossible to write tests that test for silent
> failures.
>
> Other changes:
>
> - Only fill the correction buffer and error position buffer if both of
> them are provided. Otherwise correct in place. Previously the error
> position buffer was always populated with the positions of the
> corrected errors, irrespective of whether a correction buffer was
> supplied or not. The rationale for this change is that there seems to
> be two use cases for the decoder; correct in-place or use the
> correction buffers. The caller does not need the positions of the
> corrected errors when in-place correction is used. If in-place
> correction is not used, then both the correction buffer and error
> position buffer need to be populated.
Again. A perfect changelog! Very nice to read, informative and technically
on the point.
> diff --git a/lib/reed_solomon/decode_rs.c b/lib/reed_solomon/decode_rs.c
> index 7ecc449e57e9..33621ea67f67 100644
> --- a/lib/reed_solomon/decode_rs.c
> +++ b/lib/reed_solomon/decode_rs.c
> @@ -22,6 +22,7 @@
> uint16_t *index_of = rs->index_of;
> uint16_t u, q, tmp, num1, num2, den, discr_r, syn_error;
> int count = 0;
> + int num_corrected;
> uint16_t msk = (uint16_t) rs->nn;
>
> /*
> @@ -182,6 +183,15 @@
> if (lambda[i] != nn)
> deg_lambda = i;
> }
> +
> + if (deg_lambda == 0) {
> + /*
> + * deg(lambda) is zero even though the syndrome is non-zero
> + * => uncorrectable error detected
> + */
> + return -EBADMSG;
Good catch. Now that I paged all that stuff back in it's obvious.
> + }
> +
> /* Find roots of error+erasure locator polynomial by Chien search */
> memcpy(®[1], &lambda[1], nroots * sizeof(reg[0]));
> count = 0; /* Number of roots of lambda(x) */
> @@ -195,6 +205,12 @@
> }
> if (q != 0)
> continue; /* Not a root */
> +
> + if (k < pad) {
> + /* Impossible error location. Uncorrectable error. */
> + return -EBADMSG;
True.
> + }
> +
> /* store root (index-form) and error location number */
> root[count] = i;
> loc[count] = k;
> @@ -229,7 +245,9 @@
> /*
> * Compute error values in poly-form. num1 = omega(inv(X(l))), num2 =
> * inv(X(l))**(fcr-1) and den = lambda_pr(inv(X(l))) all in poly-form
> + * Note: we reuse the buffer for b to store the correction pattern
> */
> + num_corrected = 0;
> for (j = count - 1; j >= 0; j--) {
> num1 = 0;
> for (i = deg_omega; i >= 0; i--) {
> @@ -237,6 +255,12 @@
> num1 ^= alpha_to[rs_modnn(rs, omega[i] +
> i * root[j])];
> }
> +
> + if (num1 == 0) {
> + b[j] = 0;
> + continue;
A comment for this would be appreciated.
> + }
> +
> num2 = alpha_to[rs_modnn(rs, root[j] * (fcr - 1) + nn)];
> den = 0;
>
> @@ -248,29 +272,52 @@
> i * root[j])];
> }
> }
> - /* Apply error to data */
> - if (num1 != 0 && loc[j] >= pad) {
> - uint16_t cor = alpha_to[rs_modnn(rs,index_of[num1] +
> - index_of[num2] +
> - nn - index_of[den])];
> - /* Store the error correction pattern, if a
> - * correction buffer is available */
> - if (corr) {
> - corr[j] = cor;
> - } else {
> - /* If a data buffer is given and the
> - * error is inside the message,
> - * correct it */
> - if (data && (loc[j] < (nn - nroots)))
> - data[loc[j] - pad] ^= cor;
> - }
> +
> + b[j] = alpha_to[rs_modnn(rs, index_of[num1] +
> + index_of[num2] +
> + nn - index_of[den])];
Way simpler indeed.
> + num_corrected++;
> + }
> +
> + /*
> + * We compute the syndrome of the 'error' to and check that it matches
s/to// or s/to/too ?
> + * the syndrome of the received word
> + */
> + for (i = 0; i < nroots; i++) {
> + tmp = 0;
> + for (j = 0; j < count; j++) {
> + if (b[j] == 0)
> + continue;
> +
> + k = (fcr + i) * prim * (nn-loc[j]-1);
> + tmp ^= alpha_to[rs_modnn(rs, index_of[b[j]] + k)];
> }
> +
> + if (tmp != alpha_to[s[i]])
> + return -EBADMSG;
Interesting it never occured to me that this can actually happen.
> }
>
> - if (eras_pos != NULL) {
> - for (i = 0; i < count; i++)
> - eras_pos[i] = loc[i] - pad;
> + /*
> + * Store the error correction pattern, if a
> + * correction buffer is available
> + */
> + if (corr && eras_pos) {
> + j = 0;
> + for (i = 0; i < count; i++) {
> + if (b[i]) {
> + corr[j] = b[i];
> + eras_pos[j++] = loc[i] - pad;
> + }
> + }
> + } else if (data && par) {
> + /* Apply error to data and parity */
> + for (i = 0; i < count; i++) {
> + if (loc[i] < (nn - nroots))
> + data[loc[i] - pad] ^= b[i];
> + else
> + par[loc[i] - pad - len] ^= b[i];
> + }
Aside of the fact that I had to wrap my brain around this crime I committed
more than a decade ago, all of this was really a pleasure to review.
Thanks a lot for putting that effort in! I'm looking forward to V2 of that.
Thanks,
tglx
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