Implement homeblocks_sig_check(): the drive signature check (Phase 8)

Real, complete verification logic -- not yet wired to any write path.
homeblocks_sig_check(dev, sig_start_fblock, out_sig) reads the 4
consecutive 1KB blkio forth-blocks the 4KB header spans, verifies
magic -> version -> CRC-64 in order, returns HOMEBLOCKS_SIG_OK/_BLANK/
_BAD_VERSION/_BAD_CRC/_READ_ERROR. Reuses block_subsystem.c's existing
CRC-64/ISO (compute_crc64, previously static/file-local, now exposed
via block_subsystem.h) rather than a second CRC implementation --
same algorithm already proven via per-block checksums. Takes the
header's starting block as a plain parameter rather than resolving it
internally: verifies a signature given a location, finding that
location (GPT-partition-relative) stays the caller's job.

Verified against the actual shipped code, not a reimplementation: a
standalone host test links the real homeblocks_sig.c against a fake
in-memory blkio_dev and exercises all four outcomes -- blank media,
a correctly-minted header (round-trips drive_uuid/minted_time_ns), a
flipped CRC, an unrecognized version. All four pass. A full
QEMU-hotplug live test isn't proportionate yet since nothing calls
this function from the live kernel path -- wiring it into the attach
path is the next punch-list item. Clean zero-warning compile and
clean boot on all three architectures confirms no build/link
regression from exposing compute_crc64 and adding the new source
file to every kernel build.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CXjAPTEKrgY2Mrk25KoLDn

FABRIC-3.md

@@ -212,10 +212,32 @@ decisions get added here, not to `FABRIC-2.md`. Follow the same discipline `FABR
   3-arch acceptance boot needed for this step; that starts with the signature-check
   implementation, the next punch-list item below.
 
-- [ ] Implement the signature check, called before any write path touches a newly-inserted
-  drive.
+- [x] **Implemented (2026-08-26): the check function itself, real and complete — not yet
+  wired to any write path.** `include/starkernel/homeblocks_sig.h` +
+  `src/starkernel/homeblocks_sig.c`: `homeblocks_sig_check(dev, sig_start_fblock, out_sig)`
+  reads the 4 consecutive 1KB `blkio` forth-blocks the 4KB header spans, verifies magic →
+  version → CRC-64 in order, returns one of `HOMEBLOCKS_SIG_OK`/`_BLANK`/`_BAD_VERSION`/
+  `_BAD_CRC`/`_READ_ERROR`. Reuses `block_subsystem.c`'s existing CRC-64/ISO
+  (`compute_crc64`, previously `static`/file-local, now exposed) rather than a second CRC
+  implementation — same algorithm already proven via per-block checksums. Takes the header's
+  starting block as a plain parameter rather than resolving it internally: this function
+  verifies a signature given a location; finding that location (GPT-partition-relative,
+  once a parser exists) stays the caller's job, not invented here.
 
-- [ ] Implement the warn-and-refuse behavior for blank/foreign/unrecognized media.
+  **Verified against the actual shipped code**, not a reimplementation: a standalone host
+  test links the real `homeblocks_sig.c` against a fake in-memory `blkio_dev` and exercises
+  all four outcomes — blank media → `BLANK`, a correctly-minted header → `OK` (round-trips
+  `drive_uuid`/`minted_time_ns` correctly), a flipped CRC → `BAD_CRC`, an unrecognized
+  version → `BAD_VERSION`. All four pass. A full QEMU-hotplug live test isn't proportionate
+  yet — nothing calls this function from the live kernel path (deliberately; wiring it into
+  the attach path is the next item below), so a live boot check has nothing to exercise.
+  Clean zero-warning compile and clean boot on all three architectures confirms no
+  build/link regression from exposing `compute_crc64` and adding the new source file to
+  every kernel build.
+
+- [ ] Implement the warn-and-refuse behavior for blank/foreign/unrecognized media — this is
+  where `homeblocks_sig_check()` above actually gets a live caller, wiring it into the real
+  drive-insertion path.
 
 - [ ] Extend `acl_pinned`'s one-way-ratchet mechanism (already exists, already proven, just
   needs applying) to gate zuse credential minting specifically — confirm whether this

Makefile.starkernel

@@ -402,7 +402,8 @@ LOADER_SRCS_BASE := \
     $(wildcard $(KERNEL_SRC)/usb/*.c) \
     $(KERNEL_SRC)/repl.c \
     $(KERNEL_SRC)/doe_log.c \
-    $(KERNEL_SRC)/heartbeat.c
+    $(KERNEL_SRC)/heartbeat.c \
+    $(KERNEL_SRC)/homeblocks_sig.c
 
 LOADER_ASM := \
     $(KERNEL_SRC)/arch/$(ARCH)/boot.S \
@@ -454,7 +455,8 @@ KERNEL_SRCS_BASE := \
     $(wildcard $(KERNEL_SRC)/arch/$(ARCH)/*.c) \
     $(KERNEL_SRC)/repl.c \
     $(KERNEL_SRC)/doe_log.c \
-    $(KERNEL_SRC)/heartbeat.c
+    $(KERNEL_SRC)/heartbeat.c \
+    $(KERNEL_SRC)/homeblocks_sig.c
 
 KERNEL_ASM := $(wildcard $(KERNEL_SRC)/arch/$(ARCH)/*.S)
 

include/block_subsystem.h

@@ -293,6 +293,12 @@ int blk_get_volume_meta(blk_volume_meta_t *meta);
 
 int blk_set_volume_meta(const blk_volume_meta_t *meta);
 
+/* CRC-64/ISO (poly 0x42F0E1EBA9EA3693), reflected, init/final all-ones --
+ * exposed for homeblocks_sig.c's drive-signature integrity check, which
+ * needs the exact same algorithm this file already uses for per-block
+ * checksums rather than a second, duplicate CRC implementation. */
+uint64_t compute_crc64(const uint8_t *data, size_t len);
+
 int blk_is_valid(uint32_t block_num);
 
 uint32_t blk_get_total_blocks(void);

include/starkernel/homeblocks_sig.h

@@ -118,6 +118,56 @@ typedef struct {
  * same discipline stadium.h's own header-size checks already use. */
 typedef char homeblocks_sig_size_check[(sizeof(homeblocks_sig_t) == 4096) ? 1 : -1];
 
+/*===========================================================================
+ * Signature check (FABRIC-3.md, Milestone 4)
+ *===========================================================================*/
+
+typedef enum {
+    HOMEBLOCKS_SIG_OK = 0,       /* magic, version, and crc all check out */
+    HOMEBLOCKS_SIG_BLANK,        /* magic does not match -- blank or foreign media */
+    HOMEBLOCKS_SIG_BAD_VERSION,  /* magic matches, version unrecognized */
+    HOMEBLOCKS_SIG_BAD_CRC,      /* magic+version match, crc fails -- corrupt or tampered */
+    HOMEBLOCKS_SIG_READ_ERROR    /* could not read from the device at all */
+} homeblocks_sig_result_t;
+
+/* Forward-declared, not included here -- avoids a hard dependency from this
+ * small format header onto blkio.h's full device/vtable machinery for
+ * callers that only need the struct layout (e.g. a future minting tool). */
+struct blkio_dev;
+
+/*
+ * homeblocks_sig_check - Read and verify the drive signature header.
+ *
+ * Reads 4 consecutive 1KB "forth blocks" (dev->read()'s own unit) starting
+ * at sig_start_fblock into a local 4KB buffer and interprets it as a
+ * homeblocks_sig_t. Deliberately takes the starting block as a plain
+ * parameter rather than resolving it internally -- this function verifies a
+ * signature given a location; finding that location (GPT-partition-relative
+ * today, once a GPT parser exists) is the caller's job, not invented here.
+ *
+ * @param dev              Open block device to read from.
+ * @param sig_start_fblock First of 4 consecutive forth-blocks holding the
+ *                         4KB header.
+ * @param out_sig          On HOMEBLOCKS_SIG_OK, populated with the verified
+ *                         header. Left unspecified on any other result.
+ * @return HOMEBLOCKS_SIG_OK, or the specific reason for refusal.
+ */
+homeblocks_sig_result_t homeblocks_sig_check(struct blkio_dev *dev,
+                                              uint32_t sig_start_fblock,
+                                              homeblocks_sig_t *out_sig);
+
+/*
+ * homeblocks_sig_compute_crc - CRC-64 over every field of `sig` up to but
+ * not including hdr_crc itself and the trailing padding -- the same
+ * boundary homeblocks_sig_check() verifies against and any future minting
+ * code must use when writing a fresh header. Exposed publicly since both
+ * directions (check and future mint) need the identical computation.
+ *
+ * @param sig  Header to checksum. hdr_crc and _pad are not read.
+ * @return The CRC-64 value that hdr_crc should hold for `sig` to verify.
+ */
+uint64_t homeblocks_sig_compute_crc(const homeblocks_sig_t *sig);
+
 #ifdef __cplusplus
 }
 #endif

src/block_subsystem.c

@@ -104,7 +104,7 @@ static void crc64_init(void) {
     crc64_inited = 1;
 }
 
-static inline uint64_t compute_crc64(const uint8_t *data, size_t len) {
+uint64_t compute_crc64(const uint8_t *data, size_t len) {
     if (!crc64_inited) crc64_init();
     uint64_t crc = 0xFFFFFFFFFFFFFFFFULL;
     for (size_t i = 0; i < len; i++) {

src/starkernel/homeblocks_sig.c

@@ -0,0 +1,87 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  This file is part of the StarForth project.
+
+  Licensed under the StarForth License, Version 1.0 (the "License");
+  you may not use this file except in compliance with the License.
+
+  You may obtain a copy of the License at:
+      https://github.com/star.4th@proton.me/StarForth/LICENSE.txt
+
+  This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
+  express or implied, including but not limited to the warranties of
+  merchantability, fitness for a particular purpose, and noninfringement.
+
+  See the License for the specific language governing permissions and
+  limitations under the License.
+*/
+
+/**
+ * homeblocks_sig.c - Drive signature check (FABRIC-3.md, Milestone 4).
+ * See starkernel/homeblocks_sig.h for the format and interface design.
+ */
+
+#include "starkernel/homeblocks_sig.h"
+
+#include <stddef.h>
+#include <string.h>
+
+#include "blkio.h"
+#include "block_subsystem.h" /* compute_crc64() -- same CRC-64/ISO this file's
+                              * check reuses rather than duplicating */
+
+uint64_t homeblocks_sig_compute_crc(const homeblocks_sig_t *sig) {
+    /* Covers every field up to but not including hdr_crc itself (and never
+     * _pad, which sits after it) -- offsetof is the exact boundary, not a
+     * hand-counted byte offset that could drift out of sync with the
+     * struct's own field list. */
+    size_t crc_span = offsetof(homeblocks_sig_t, hdr_crc);
+    return compute_crc64((const uint8_t *)sig, crc_span);
+}
+
+homeblocks_sig_result_t homeblocks_sig_check(struct blkio_dev *dev,
+                                              uint32_t sig_start_fblock,
+                                              homeblocks_sig_t *out_sig) {
+    uint8_t          buf[4096];
+    uint32_t         i;
+    homeblocks_sig_t local;
+    uint64_t         expected_crc;
+
+    if (!dev) return HOMEBLOCKS_SIG_READ_ERROR;
+
+    /* homeblocks_sig_t is exactly one 4KiB devblock; blkio's own unit is a
+     * 1KiB "forth block" (BLKIO_FORTH_BLOCK_SIZE), so the header spans 4
+     * consecutive reads starting at sig_start_fblock. */
+    for (i = 0; i < 4; i++) {
+        if (blkio_read((blkio_dev_t *)dev, sig_start_fblock + i,
+                        buf + (size_t)i * BLKIO_FORTH_BLOCK_SIZE) != BLKIO_OK) {
+            return HOMEBLOCKS_SIG_READ_ERROR;
+        }
+    }
+
+    /* Copy into a properly-aligned local rather than reinterpreting buf's
+     * address directly -- buf is only byte-aligned, and homeblocks_sig_t
+     * has uint64_t members; a raw cast would be a strict-aliasing and
+     * alignment violation for no benefit over one memcpy. */
+    memcpy(&local, buf, sizeof(local));
+
+    if (HOMEBLOCKS_SIG_GET_MAGIC(local.magic) != (uint32_t)(HOMEBLOCKS_SIG_MAGIC & 0xFFFFFFFFULL)) {
+        return HOMEBLOCKS_SIG_BLANK;
+    }
+
+    if (HOMEBLOCKS_SIG_GET_VERSION(local.magic) != HOMEBLOCKS_SIG_VERSION_0) {
+        return HOMEBLOCKS_SIG_BAD_VERSION;
+    }
+
+    expected_crc = homeblocks_sig_compute_crc(&local);
+    if (expected_crc != local.hdr_crc) {
+        return HOMEBLOCKS_SIG_BAD_CRC;
+    }
+
+    if (out_sig) *out_sig = local;
+    return HOMEBLOCKS_SIG_OK;
+}