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![Thomas Gleixner](/assets/img/avatar_default.png)
To get rid of the variable length arrays on stack in the RS decoder it's necessary to allocate the decoder buffers per control structure instance. All usage sites have been checked for potential parallel decoder usage and fixed where necessary. Kees confirmed that the pstore decoding is strictly single threaded so there should be no surprises. Allocate them in the rs control structure sized depending on the number of roots for the chosen codec and adapt the decoder code to make use of them. Document the fact that decode operations based on a particular rs control instance cannot run in parallel and the caller has to ensure that as it's not possible to provide a proper locking construct which fits all use cases. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: Kees Cook <keescook@chromium.org> Cc: Boris Brezillon <boris.brezillon@free-electrons.com> Cc: Tony Luck <tony.luck@intel.com> Cc: Segher Boessenkool <segher@kernel.crashing.org> Cc: Kernel Hardening <kernel-hardening@lists.openwall.com> Cc: Richard Weinberger <richard@nod.at> Cc: Mike Snitzer <snitzer@redhat.com> Cc: Anton Vorontsov <anton@enomsg.org> Cc: Colin Cross <ccross@android.com> Cc: Andrew Morton <akpm@linuxfoundation.org> Cc: David Woodhouse <dwmw2@infradead.org> Cc: Alasdair Kergon <agk@redhat.com> Signed-off-by: Kees Cook <keescook@chromium.org>
271 lines
7 KiB
C
271 lines
7 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* Generic Reed Solomon encoder / decoder library
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*
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* Copyright 2002, Phil Karn, KA9Q
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* May be used under the terms of the GNU General Public License (GPL)
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*
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* Adaption to the kernel by Thomas Gleixner (tglx@linutronix.de)
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*
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* Generic data width independent code which is included by the wrappers.
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*/
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{
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struct rs_codec *rs = rsc->codec;
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int deg_lambda, el, deg_omega;
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int i, j, r, k, pad;
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int nn = rs->nn;
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int nroots = rs->nroots;
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int fcr = rs->fcr;
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int prim = rs->prim;
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int iprim = rs->iprim;
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uint16_t *alpha_to = rs->alpha_to;
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uint16_t *index_of = rs->index_of;
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uint16_t u, q, tmp, num1, num2, den, discr_r, syn_error;
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int count = 0;
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uint16_t msk = (uint16_t) rs->nn;
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/*
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* The decoder buffers are in the rs control struct. They are
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* arrays sized [nroots + 1]
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*/
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uint16_t *lambda = rsc->buffers + RS_DECODE_LAMBDA * (nroots + 1);
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uint16_t *syn = rsc->buffers + RS_DECODE_SYN * (nroots + 1);
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uint16_t *b = rsc->buffers + RS_DECODE_B * (nroots + 1);
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uint16_t *t = rsc->buffers + RS_DECODE_T * (nroots + 1);
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uint16_t *omega = rsc->buffers + RS_DECODE_OMEGA * (nroots + 1);
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uint16_t *root = rsc->buffers + RS_DECODE_ROOT * (nroots + 1);
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uint16_t *reg = rsc->buffers + RS_DECODE_REG * (nroots + 1);
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uint16_t *loc = rsc->buffers + RS_DECODE_LOC * (nroots + 1);
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/* Check length parameter for validity */
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pad = nn - nroots - len;
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BUG_ON(pad < 0 || pad >= nn);
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/* Does the caller provide the syndrome ? */
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if (s != NULL)
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goto decode;
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/* form the syndromes; i.e., evaluate data(x) at roots of
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* g(x) */
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for (i = 0; i < nroots; i++)
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syn[i] = (((uint16_t) data[0]) ^ invmsk) & msk;
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for (j = 1; j < len; j++) {
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for (i = 0; i < nroots; i++) {
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if (syn[i] == 0) {
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syn[i] = (((uint16_t) data[j]) ^
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invmsk) & msk;
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} else {
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syn[i] = ((((uint16_t) data[j]) ^
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invmsk) & msk) ^
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alpha_to[rs_modnn(rs, index_of[syn[i]] +
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(fcr + i) * prim)];
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}
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}
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}
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for (j = 0; j < nroots; j++) {
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for (i = 0; i < nroots; i++) {
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if (syn[i] == 0) {
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syn[i] = ((uint16_t) par[j]) & msk;
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} else {
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syn[i] = (((uint16_t) par[j]) & msk) ^
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alpha_to[rs_modnn(rs, index_of[syn[i]] +
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(fcr+i)*prim)];
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}
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}
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}
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s = syn;
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/* Convert syndromes to index form, checking for nonzero condition */
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syn_error = 0;
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for (i = 0; i < nroots; i++) {
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syn_error |= s[i];
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s[i] = index_of[s[i]];
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}
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if (!syn_error) {
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/* if syndrome is zero, data[] is a codeword and there are no
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* errors to correct. So return data[] unmodified
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*/
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count = 0;
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goto finish;
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}
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decode:
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memset(&lambda[1], 0, nroots * sizeof(lambda[0]));
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lambda[0] = 1;
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if (no_eras > 0) {
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/* Init lambda to be the erasure locator polynomial */
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lambda[1] = alpha_to[rs_modnn(rs,
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prim * (nn - 1 - eras_pos[0]))];
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for (i = 1; i < no_eras; i++) {
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u = rs_modnn(rs, prim * (nn - 1 - eras_pos[i]));
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for (j = i + 1; j > 0; j--) {
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tmp = index_of[lambda[j - 1]];
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if (tmp != nn) {
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lambda[j] ^=
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alpha_to[rs_modnn(rs, u + tmp)];
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}
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}
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}
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}
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for (i = 0; i < nroots + 1; i++)
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b[i] = index_of[lambda[i]];
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/*
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* Begin Berlekamp-Massey algorithm to determine error+erasure
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* locator polynomial
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*/
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r = no_eras;
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el = no_eras;
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while (++r <= nroots) { /* r is the step number */
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/* Compute discrepancy at the r-th step in poly-form */
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discr_r = 0;
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for (i = 0; i < r; i++) {
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if ((lambda[i] != 0) && (s[r - i - 1] != nn)) {
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discr_r ^=
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alpha_to[rs_modnn(rs,
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index_of[lambda[i]] +
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s[r - i - 1])];
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}
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}
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discr_r = index_of[discr_r]; /* Index form */
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if (discr_r == nn) {
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/* 2 lines below: B(x) <-- x*B(x) */
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memmove (&b[1], b, nroots * sizeof (b[0]));
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b[0] = nn;
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} else {
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/* 7 lines below: T(x) <-- lambda(x)-discr_r*x*b(x) */
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t[0] = lambda[0];
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for (i = 0; i < nroots; i++) {
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if (b[i] != nn) {
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t[i + 1] = lambda[i + 1] ^
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alpha_to[rs_modnn(rs, discr_r +
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b[i])];
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} else
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t[i + 1] = lambda[i + 1];
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}
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if (2 * el <= r + no_eras - 1) {
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el = r + no_eras - el;
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/*
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* 2 lines below: B(x) <-- inv(discr_r) *
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* lambda(x)
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*/
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for (i = 0; i <= nroots; i++) {
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b[i] = (lambda[i] == 0) ? nn :
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rs_modnn(rs, index_of[lambda[i]]
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- discr_r + nn);
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}
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} else {
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/* 2 lines below: B(x) <-- x*B(x) */
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memmove(&b[1], b, nroots * sizeof(b[0]));
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b[0] = nn;
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}
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memcpy(lambda, t, (nroots + 1) * sizeof(t[0]));
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}
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}
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/* Convert lambda to index form and compute deg(lambda(x)) */
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deg_lambda = 0;
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for (i = 0; i < nroots + 1; i++) {
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lambda[i] = index_of[lambda[i]];
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if (lambda[i] != nn)
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deg_lambda = i;
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}
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/* Find roots of error+erasure locator polynomial by Chien search */
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memcpy(®[1], &lambda[1], nroots * sizeof(reg[0]));
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count = 0; /* Number of roots of lambda(x) */
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for (i = 1, k = iprim - 1; i <= nn; i++, k = rs_modnn(rs, k + iprim)) {
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q = 1; /* lambda[0] is always 0 */
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for (j = deg_lambda; j > 0; j--) {
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if (reg[j] != nn) {
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reg[j] = rs_modnn(rs, reg[j] + j);
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q ^= alpha_to[reg[j]];
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}
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}
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if (q != 0)
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continue; /* Not a root */
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/* store root (index-form) and error location number */
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root[count] = i;
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loc[count] = k;
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/* If we've already found max possible roots,
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* abort the search to save time
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*/
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if (++count == deg_lambda)
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break;
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}
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if (deg_lambda != count) {
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/*
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* deg(lambda) unequal to number of roots => uncorrectable
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* error detected
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*/
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count = -EBADMSG;
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goto finish;
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}
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/*
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* Compute err+eras evaluator poly omega(x) = s(x)*lambda(x) (modulo
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* x**nroots). in index form. Also find deg(omega).
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*/
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deg_omega = deg_lambda - 1;
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for (i = 0; i <= deg_omega; i++) {
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tmp = 0;
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for (j = i; j >= 0; j--) {
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if ((s[i - j] != nn) && (lambda[j] != nn))
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tmp ^=
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alpha_to[rs_modnn(rs, s[i - j] + lambda[j])];
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}
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omega[i] = index_of[tmp];
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}
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/*
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* Compute error values in poly-form. num1 = omega(inv(X(l))), num2 =
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* inv(X(l))**(fcr-1) and den = lambda_pr(inv(X(l))) all in poly-form
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*/
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for (j = count - 1; j >= 0; j--) {
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num1 = 0;
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for (i = deg_omega; i >= 0; i--) {
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if (omega[i] != nn)
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num1 ^= alpha_to[rs_modnn(rs, omega[i] +
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i * root[j])];
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}
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num2 = alpha_to[rs_modnn(rs, root[j] * (fcr - 1) + nn)];
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den = 0;
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/* lambda[i+1] for i even is the formal derivative
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* lambda_pr of lambda[i] */
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for (i = min(deg_lambda, nroots - 1) & ~1; i >= 0; i -= 2) {
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if (lambda[i + 1] != nn) {
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den ^= alpha_to[rs_modnn(rs, lambda[i + 1] +
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i * root[j])];
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}
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}
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/* Apply error to data */
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if (num1 != 0 && loc[j] >= pad) {
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uint16_t cor = alpha_to[rs_modnn(rs,index_of[num1] +
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index_of[num2] +
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nn - index_of[den])];
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/* Store the error correction pattern, if a
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* correction buffer is available */
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if (corr) {
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corr[j] = cor;
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} else {
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/* If a data buffer is given and the
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* error is inside the message,
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* correct it */
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if (data && (loc[j] < (nn - nroots)))
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data[loc[j] - pad] ^= cor;
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}
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}
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}
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finish:
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if (eras_pos != NULL) {
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for (i = 0; i < count; i++)
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eras_pos[i] = loc[i] - pad;
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}
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return count;
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}
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