[RFC PATCH 08/10] ftrfs: add CRC32 checksumming and Reed-Solomon FEC skeleton

[Aurelien DESBRIERES <aurelien@hackers.camp>]

From: Aurélien DESBRIERES <aurelien@hackers.camp>

Implement data integrity layer:

- ftrfs_crc32(): CRC32 wrapper via kernel crc32_le(), used for
  per-inode and per-superblock checksums
- init_gf_tables(): initialize GF(2^8) Galois Field lookup tables
  (primitive polynomial 0x1d) for Reed-Solomon arithmetic
- gf_mul(): GF(2^8) multiplication via log/exp tables
- ftrfs_rs_encode(): systematic Reed-Solomon encoder over
  FTRFS_SUBBLOCK_DATA bytes with FTRFS_RS_PARITY check symbols

The RS encoder operates on sub-blocks within each 4096-byte data
block. Decoding (error correction) is not yet implemented.

Signed-off-by: Aurélien DESBRIERES <aurelien@hackers.camp>
---
 fs/ftrfs/edac.c | 84 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100644 fs/ftrfs/edac.c

diff --git a/fs/ftrfs/edac.c b/fs/ftrfs/edac.c
new file mode 100644
index 000000000..ebe676c98
--- /dev/null
+++ b/fs/ftrfs/edac.c
@@ -0,0 +1,84 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * FTRFS — EDAC layer: CRC32 + Reed-Solomon FEC
+ * Author: roastercode - Aurelien DESBRIERES <aurelien@hackers.camp>
+ */
+
+#include <linux/kernel.h>
+#include <linux/crc32.h>
+#include <linux/slab.h>
+#include "ftrfs.h"
+
+/* Reed-Solomon FEC context */
+static uint8_t gf_exp[256];
+static uint8_t gf_log[256];
+static bool rs_initialized;
+
+/* Initialize Galois Field tables */
+static void init_gf_tables(void)
+{
+	uint8_t x = 1;
+
+	for (int i = 0; i < 255; i++) {
+		gf_exp[i] = x;
+		gf_log[x] = i;
+		x = (x << 1) ^ ((x & 0x80) ? 0x1d : 0);
+	}
+	gf_exp[255] = gf_exp[0];
+	rs_initialized = true;
+}
+
+/* Galois Field multiplication */
+static uint8_t gf_mul(uint8_t a, uint8_t b)
+{
+	if (a == 0 || b == 0)
+		return 0;
+	return gf_exp[(gf_log[a] + gf_log[b]) % 255];
+}
+
+/* Reed-Solomon encoding */
+int ftrfs_rs_encode(uint8_t *data, uint8_t *parity)
+{
+	uint8_t msg[FTRFS_SUBBLOCK_DATA + FTRFS_RS_PARITY];
+
+	if (!rs_initialized)
+		init_gf_tables();
+
+	memset(msg, 0, sizeof(msg));
+	memcpy(msg, data, FTRFS_SUBBLOCK_DATA);
+
+	for (int i = 0; i < FTRFS_SUBBLOCK_DATA; i++) {
+		uint8_t feedback = gf_mul(msg[i], gf_exp[FTRFS_RS_PARITY]);
+
+		if (feedback != 0) {
+			for (int j = 1; j <= FTRFS_RS_PARITY; j++)
+				msg[FTRFS_SUBBLOCK_DATA + j - 1] ^= gf_mul(msg[i], gf_exp[j]);
+		}
+	}
+
+	memcpy(parity, msg + FTRFS_SUBBLOCK_DATA, FTRFS_RS_PARITY);
+	return 0;
+}
+
+/* Reed-Solomon decoding (simplified for now) */
+int ftrfs_rs_decode(uint8_t *data, uint8_t *parity)
+{
+	if (!rs_initialized)
+		init_gf_tables();
+
+	/* For now, assume no errors (full decoding to be implemented) */
+	return 0;
+}
+
+/*
+ * ftrfs_crc32 - compute CRC32 checksum
+ * @buf: data buffer
+ * @len: length in bytes
+ *
+ * Returns CRC32 checksum. Uses kernel's hardware-accelerated CRC32
+ * (same as ext4/btrfs).
+ */
+__u32 ftrfs_crc32(const void *buf, size_t len)
+{
+	return crc32_le(0xFFFFFFFF, buf, len) ^ 0xFFFFFFFF;
+}
-- 
2.52.0