Re: [RFC] Reed-Solomon Code: Update no_eras to the actual number of errors
From: Kees Cook
Date: Thu Jun 25 2020 - 01:35:51 EST
On Wed, Jun 24, 2020 at 09:10:53PM -0700, Aiden Leong wrote:
> Corr and eras_pos are updated to actual correction pattern and erasure
> positions, but no_eras is not.
>
> When this library is used to recover lost bytes, we normally memset the
> lost trunk of bytes to zero as a placeholder. Unfortunately, if the lost
> byte is zero, b[i] is zero too. Without correct no_eras, users won't be
> able to determine the valid length of corr and eras_pos.
>
> Signed-off-by: Aiden Leong <aiden.leong@xxxxxxxxx>
> ---
> drivers/md/dm-verity-fec.c | 2 +-
> fs/pstore/ram_core.c | 2 +-
> include/linux/rslib.h | 4 ++--
> lib/reed_solomon/decode_rs.c | 20 ++++++++++++++------
> lib/reed_solomon/reed_solomon.c | 4 ++--
> lib/reed_solomon/test_rslib.c | 18 ++++++++++++------
> 6 files changed, 32 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
> index fb41b4f23c48..ae8366a50244 100644
> --- a/drivers/md/dm-verity-fec.c
> +++ b/drivers/md/dm-verity-fec.c
> @@ -50,7 +50,7 @@ static int fec_decode_rs8(struct dm_verity *v, struct dm_verity_fec_io *fio,
> for (i = 0; i < v->fec->roots; i++)
> par[i] = fec[i];
>
> - return decode_rs8(fio->rs, data, par, v->fec->rsn, NULL, neras,
> + return decode_rs8(fio->rs, data, par, v->fec->rsn, NULL, &neras,
> fio->erasures, 0, NULL);
> }
>
> diff --git a/fs/pstore/ram_core.c b/fs/pstore/ram_core.c
> index aa8e0b65ff1a..fcc661a60640 100644
> --- a/fs/pstore/ram_core.c
> +++ b/fs/pstore/ram_core.c
> @@ -115,7 +115,7 @@ static int persistent_ram_decode_rs8(struct persistent_ram_zone *prz,
>
> for (i = 0; i < prz->ecc_info.ecc_size; i++)
> prz->ecc_info.par[i] = ecc[i];
> - return decode_rs8(prz->rs_decoder, data, prz->ecc_info.par, len,
> + return decode_rs8(prz->rs_decoder, data, prz->ecc_info.par, &len,
> NULL, 0, NULL, 0, NULL);
This looks like the wrong arg changed -- did you compile test this?
> }
>
> diff --git a/include/linux/rslib.h b/include/linux/rslib.h
> index 238bb85243d3..80662abc9af7 100644
> --- a/include/linux/rslib.h
> +++ b/include/linux/rslib.h
> @@ -64,7 +64,7 @@ int encode_rs8(struct rs_control *rs, uint8_t *data, int len, uint16_t *par,
> #endif
> #ifdef CONFIG_REED_SOLOMON_DEC8
> int decode_rs8(struct rs_control *rs, uint8_t *data, uint16_t *par, int len,
> - uint16_t *s, int no_eras, int *eras_pos, uint16_t invmsk,
> + uint16_t *s, int *no_eras, int *eras_pos, uint16_t invmsk,
> uint16_t *corr);
> #endif
>
> @@ -75,7 +75,7 @@ int encode_rs16(struct rs_control *rs, uint16_t *data, int len, uint16_t *par,
> #endif
> #ifdef CONFIG_REED_SOLOMON_DEC16
> int decode_rs16(struct rs_control *rs, uint16_t *data, uint16_t *par, int len,
> - uint16_t *s, int no_eras, int *eras_pos, uint16_t invmsk,
> + uint16_t *s, int *no_eras, int *eras_pos, uint16_t invmsk,
> uint16_t *corr);
> #endif
>
> diff --git a/lib/reed_solomon/decode_rs.c b/lib/reed_solomon/decode_rs.c
> index 805de84ae83d..122bc08eb75f 100644
> --- a/lib/reed_solomon/decode_rs.c
> +++ b/lib/reed_solomon/decode_rs.c
> @@ -24,6 +24,7 @@
> int count = 0;
> int num_corrected;
> uint16_t msk = (uint16_t) rs->nn;
> + int no_eras_orig = no_eras ? *no_eras : 0;
To avoid code churn, I would name this int no_eras, and change the arg
to something like no_eras_ptr:
int no_eras = no_eras_ptr ? *no_eras_ptr : 0;
>
> /*
> * The decoder buffers are in the rs control struct. They are
> @@ -106,11 +107,11 @@
> memset(&lambda[1], 0, nroots * sizeof(lambda[0]));
> lambda[0] = 1;
>
> - if (no_eras > 0) {
> + if (no_eras_orig > 0) {
> /* Init lambda to be the erasure locator polynomial */
> lambda[1] = alpha_to[rs_modnn(rs,
> prim * (nn - 1 - (eras_pos[0] + pad)))];
> - for (i = 1; i < no_eras; i++) {
> + for (i = 1; i < no_eras_orig; 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]];
> @@ -129,8 +130,8 @@
> * Begin Berlekamp-Massey algorithm to determine error+erasure
> * locator polynomial
> */
> - r = no_eras;
> - el = no_eras;
> + r = no_eras_orig;
> + el = no_eras_orig;
> while (++r <= nroots) { /* r is the step number */
> /* Compute discrepancy at the r-th step in poly-form */
> discr_r = 0;
> @@ -158,8 +159,8 @@
> } else
> t[i + 1] = lambda[i + 1];
> }
> - if (2 * el <= r + no_eras - 1) {
> - el = r + no_eras - el;
> + if (2 * el <= r + no_eras_orig - 1) {
> + el = r + no_eras_orig - el;
> /*
> * 2 lines below: B(x) <-- inv(discr_r) *
> * lambda(x)
> @@ -312,14 +313,21 @@
> eras_pos[j++] = loc[i] - pad;
> }
> }
> + if (no_eras > 0)
> + *no_eras = j;
Is this meant to be "if (j > 0)" or "if (no_eras != NULL)" ? It's
uncommon to use > 0 for a pointer value.
> } else if (data && par) {
> /* Apply error to data and parity */
> + j = 0;
> 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];
> + if (b[i])
> + j++;
> }
> + if (no_eras > 0)
> + *no_eras = j;
I assume it's a pointer test, so both would be:
if (no_eras_ptr != NULL)
*no_eras_ptr = j;
> }
>
> return num_corrected;
> diff --git a/lib/reed_solomon/reed_solomon.c b/lib/reed_solomon/reed_solomon.c
> index bbc01bad3053..b2c811674c98 100644
> --- a/lib/reed_solomon/reed_solomon.c
> +++ b/lib/reed_solomon/reed_solomon.c
> @@ -359,7 +359,7 @@ EXPORT_SYMBOL_GPL(encode_rs8);
> * errors. The count includes errors in the parity.
> */
> int decode_rs8(struct rs_control *rsc, uint8_t *data, uint16_t *par, int len,
> - uint16_t *s, int no_eras, int *eras_pos, uint16_t invmsk,
> + uint16_t *s, int *no_eras, int *eras_pos, uint16_t invmsk,
> uint16_t *corr)
> {
> #include "decode_rs.c"
> @@ -410,7 +410,7 @@ EXPORT_SYMBOL_GPL(encode_rs16);
> * errors. The count includes errors in the parity.
> */
> int decode_rs16(struct rs_control *rsc, uint16_t *data, uint16_t *par, int len,
> - uint16_t *s, int no_eras, int *eras_pos, uint16_t invmsk,
> + uint16_t *s, int *no_eras, int *eras_pos, uint16_t invmsk,
> uint16_t *corr)
> {
> #include "decode_rs.c"
> diff --git a/lib/reed_solomon/test_rslib.c b/lib/reed_solomon/test_rslib.c
> index 4eb29f365ece..b30a4aea8796 100644
> --- a/lib/reed_solomon/test_rslib.c
> +++ b/lib/reed_solomon/test_rslib.c
> @@ -258,7 +258,7 @@ static void compute_syndrome(struct rs_control *rsc, uint16_t *data,
>
> /* Test up to error correction capacity */
> static void test_uc(struct rs_control *rs, int len, int errs,
> - int eras, int trials, struct estat *stat,
> + int *eras, int trials, struct estat *stat,
> struct wspace *ws, int method)
> {
> int dlen = len - rs->codec->nroots;
> @@ -327,8 +327,11 @@ static int ex_rs_helper(struct rs_control *rs, struct wspace *ws,
> pr_info(" %s\n", desc[method]);
>
> for (errs = 0; errs <= nroots / 2; errs++)
> - for (eras = 0; eras <= nroots - 2 * errs; eras++)
> - test_uc(rs, len, errs, eras, trials, &stat, ws, method);
> + for (eras = 0; eras <= nroots - 2 * errs; eras++) {
> + int no_eras = ers;
> +
> + test_uc(rs, len, errs, &no_eras, trials, &stat, ws, method);
> + }
>
> if (v >= V_CSUMMARY) {
> pr_info(" Decodes wrong: %d / %d\n",
> @@ -364,7 +367,7 @@ static int exercise_rs(struct rs_control *rs, struct wspace *ws,
>
> /* Tests for correct behaviour beyond error correction capacity */
> static void test_bc(struct rs_control *rs, int len, int errs,
> - int eras, int trials, struct bcstat *stat,
> + int *eras, int trials, struct bcstat *stat,
> struct wspace *ws)
> {
> int nroots = rs->codec->nroots;
> @@ -420,8 +423,11 @@ static int exercise_rs_bc(struct rs_control *rs, struct wspace *ws,
> eras = 0;
>
> cutoff = nroots <= len - errs ? nroots : len - errs;
> - for (; eras <= cutoff; eras++)
> - test_bc(rs, len, errs, eras, trials, &stat, ws);
> + for (; eras <= cutoff; eras++) {
> + int no_eras = eras;
> +
> + test_bc(rs, len, errs, &no_eras, trials, &stat, ws);
> + }
> }
>
> if (v >= V_CSUMMARY) {
> --
> 2.25.1
>
Otherwise, yeah, cool. Looks good.
--
Kees Cook