Artemis genesis stamp: fix a BAM-corrupting offset before it ever ran (FABRIC-3.md §XXVI follow-on, Step 3)

Step 3: one-time artemis_sig_t genesis stamp, written once
kernel_main.c's virtio-blk path confirms Artemis's own disk, so the disk
image is later recognizable generically (repl.c's idle-loop USB-MSC scan,
built in the prior commit) regardless of which bus found it.

Correction made before this ever touched the real disk: the signature's
first design (committed in 29b6789) placed it at a fixed bottom-of-device
forth-block (4, devblock 1) -- copying homeblocks_sig_t's own convention,
which is safe for a raw identity thumbdrive but not for Artemis's own
disk. Artemis's disk is block_subsystem.c's own STFR/v2-formatted volume:
devblock 0 holds that format's header and devblock 1 is the FIRST
DEVBLOCK OF THE LIVE BAM (blk_compute_fresh_geometry(): bam_start = 1).
The original design would have overwritten Artemis's live allocation map
on the very first real boot. Caught via direct cross-reference against
block_subsystem.c before the genesis-stamp call site was ever run against
the real image -- no corruption occurred.

Fixed by moving the header to a fixed offset from the END of the device
instead (ARTEMIS_SIG_DEVBLOCK_FROM_TOP=64), the same top-of-device region
block_subsystem.c's own meta_fence_blocks reservation (128 devblocks)
already carves out for system metadata, and where Zuse's genesis marker/
eligibility list already live -- but computed independently via
blkio_info() rather than through blk_meta_zone_*(), since that accessor
needs an already-attached, format-detected slot, which is exactly the
state pre-attach generic discovery doesn't have yet. Picked well clear of
Zuse's two tenants (devblock_from_top 0 and 1+, open-ended) so the two
subsystems' independent math can never collide.

Also reordered kernel_main.c: rng_init() now runs before the Artemis
virtio-blk block (was after) -- the genesis stamp needs rng_get_bytes()
for disk_uuid, and the original order would have failed the stamp on
every boot.

Verified live: booted amd64 against the real disk/artemis.img twice --
first boot logs "Artemis: genesis signature stamped" (confirmed blank at
the target offset beforehand via a host-side read), second boot on the
now-stamped image logs no re-stamp (idempotent, CRC/read-back verified)
-- both boots and aarch64/riscv64 (against the same now-stamped image)
all still report "Artemis: 22998 data blocks" / "PASS: persist-read"
unchanged, confirming the BAM and data pool were never touched.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EXieurDfDSsDFdnSyusuWo

src/starkernel/artemis_sig.c

@@ -34,6 +34,9 @@
 #include "block_subsystem.h" /* compute_crc64() -- same CRC-64/ISO
                               * homeblocks_sig.c's own check reuses,
                               * not duplicated here either */
+#include "starkernel/rng.h"  /* rng_get_bytes() -- same entropy source
+                              * capsule_mint.c already uses for an
+                              * identity thumbdrive's own drive_uuid */
 
 uint64_t artemis_sig_compute_crc(const artemis_sig_t *sig) {
     /* Covers every field up to but not including hdr_crc itself (and never
@@ -43,15 +46,42 @@ uint64_t artemis_sig_compute_crc(const artemis_sig_t *sig) {
     return compute_crc64((const uint8_t *)sig, crc_span);
 }
 
+/* Resolve ARTEMIS_SIG_DEVBLOCK_FROM_TOP (a distance from the last physical
+ * devblock) to an absolute starting forth-block on `dev`, using only
+ * blkio_info() -- no attach or format-detection required, matching
+ * block_subsystem.c's own devblock_from_top arithmetic
+ * (total_devblocks - 1 - devblock_from_top) exactly, but computed
+ * independently since blk_meta_zone_*() isn't reachable pre-attach. The
+ * floor division (total forth-blocks / 4) matches block_subsystem.c's own
+ * udiv_floor(total_blkio_blocks_1k, 4) -- plain unsigned integer division
+ * already floors, so no separate helper is needed here.
+ *
+ * @return 0 and *out_fblock set on success; -1 if the device is too small
+ *         to hold the reserved offset at all (couldn't happen on a real
+ *         Artemis disk, but a foreign/tiny device must never underflow
+ *         into a wild block). */
+static int artemis_sig_start_fblock(struct blkio_dev *dev, uint32_t *out_fblock) {
+    blkio_info_t info;
+    if (blkio_info((blkio_dev_t *)dev, &info) != BLKIO_OK) return -1;
+
+    uint32_t total_devblocks = info.total_blocks / 4u;
+    if (total_devblocks < ARTEMIS_SIG_DEVBLOCK_FROM_TOP + 1u) return -1;
+
+    uint32_t devblock_idx = total_devblocks - 1u - ARTEMIS_SIG_DEVBLOCK_FROM_TOP;
+    *out_fblock = devblock_idx * 4u;
+    return 0;
+}
+
 artemis_sig_result_t artemis_sig_check(struct blkio_dev *dev,
-                                        uint32_t sig_start_fblock,
                                         artemis_sig_t *out_sig) {
     uint8_t          buf[4096];
     uint32_t         i;
+    uint32_t         sig_start_fblock;
     artemis_sig_t    local;
     uint64_t         expected_crc;
 
     if (!dev) return ARTEMIS_SIG_READ_ERROR;
+    if (artemis_sig_start_fblock(dev, &sig_start_fblock) != 0) return ARTEMIS_SIG_READ_ERROR;
 
     /* artemis_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
@@ -85,3 +115,47 @@ artemis_sig_result_t artemis_sig_check(struct blkio_dev *dev,
     if (out_sig) *out_sig = local;
     return ARTEMIS_SIG_OK;
 }
+
+int artemis_sig_genesis_stamp(struct blkio_dev *dev) {
+    if (!dev) return -1;
+
+    uint32_t sig_start_fblock;
+    if (artemis_sig_start_fblock(dev, &sig_start_fblock) != 0) return -1;
+
+    artemis_sig_t sig;
+    memset(&sig, 0, sizeof(sig));
+    sig.magic = ARTEMIS_SIG_PACK(ARTEMIS_SIG_VERSION_0);
+    if (rng_get_bytes(sig.disk_uuid, sizeof(sig.disk_uuid)) != 0) return -1;
+    sig.genesis_time_ns    = 0; /* no monotonic-ns source yet, same open
+                                 * item as homeblocks_sig_t's own
+                                 * minted_time_ns field */
+    sig.metadata_devblocks = 1; /* just this header devblock for now --
+                                 * grows to cover the log region once
+                                 * step 4 allocates it (a normal rewrite
+                                 * of this same one-header location) */
+    sig.log_region_offset    = 0; /* not yet allocated */
+    sig.log_region_devblocks = 0;
+    sig.hdr_crc = artemis_sig_compute_crc(&sig);
+
+    uint8_t buf[4096];
+    memcpy(buf, &sig, sizeof(sig));
+
+    uint32_t i;
+    for (i = 0; i < 4; i++) {
+        if (blkio_write((blkio_dev_t *)dev, sig_start_fblock + i,
+                         buf + (size_t)i * BLKIO_FORTH_BLOCK_SIZE) != BLKIO_OK) {
+            return -1;
+        }
+    }
+    (void)blkio_flush((blkio_dev_t *)dev);
+
+    /* Read-back verification, same discipline as capsule_mint.c's own
+     * verify_mint() -- a successful blkio_write() return only means the
+     * command chain completed, not that the bytes survive to be read
+     * back correctly. */
+    artemis_sig_t verify;
+    if (artemis_sig_check(dev, &verify) != ARTEMIS_SIG_OK) return -1;
+    if (memcmp(&verify, &sig, sizeof(sig)) != 0) return -1;
+
+    return 0;
+}

src/starkernel/kernel_main.c

@@ -63,6 +63,7 @@ EFI_RUNTIME_SERVICES *g_sk_runtime_services = NULL;
 #include "starkernel/capsule_loader.h"
 #include "starkernel/capsule_birth.h"  /* capsule_birth_mama, capsule_find_mama_init */
 #include "starkernel/capsule_zuse_boot.h" /* capsule_zuse_boot_load_root_pubkey */
+#include "starkernel/artemis_sig.h" /* artemis_sig_check/genesis_stamp */
 #include "starkernel/kmalloc.h"
 #include "starkernel/repl.h"
 #include "starkernel/pci.h"
@@ -589,6 +590,30 @@ static void kernel_main_deep(BootInfo *boot_info) {
     console_println("PCI: init...");
     pci_init(boot_info->acpi_table);
 
+    /* Phase 8: entropy. Real per-arch RNG doesn't cover all three
+     * architectures (amd64 RDRAND, riscv64 Zkr, but aarch64 has neither in
+     * QEMU's CPU models -- see vm_uuid.h's identical finding), so signing/
+     * keygen entropy comes from the unified rng_get_bytes() layer, whose
+     * v2.0.0 backend is the paravirtualized virtio-rng device. Unconditional
+     * call site, same graceful-noop precedent as virtio_blk_find_artemis()
+     * below -- boot proceeds either way, the device is only required once
+     * something actually calls rng_get_bytes().
+     *
+     * FABRIC-3.md §XXVI follow-on, 2026-09-13: moved ahead of the Artemis
+     * virtio-blk block below (was after it) -- artemis_sig_genesis_stamp()
+     * needs rng_get_bytes() for disk_uuid, and calling it before rng_init()
+     * ran would have failed the stamp on every single boot forever. Both
+     * calls only need pci_init() above; this reordering has no other
+     * dependency either way. */
+    {
+        int rrc = rng_init();
+        if (rrc == 0) {
+            console_println("entropy: ready");
+        } else {
+            console_println("entropy: not available (continuing without)");
+        }
+    }
+
     {
         static blkio_dev_t artemis_dev;
         int vrc = virtio_blk_find_artemis(&artemis_dev);
@@ -603,28 +628,40 @@ static void kernel_main_deep(BootInfo *boot_info) {
              * capsule_zuse_boot_load_root_pubkey()'s own doc comment for
              * why this is safe and separate from her live-session cert. */
             capsule_zuse_boot_load_root_pubkey((VM *)mama_vm);
+
+            /* FABRIC-3.md §XXVI follow-on, 2026-09-13: one-time
+             * artemis_sig_t genesis stamp, so this exact disk image can
+             * later be recognized generically (by content, not by which
+             * bus/vendor-ID scan happened to find it -- see repl.c's own
+             * idle-loop USB-MSC discovery, the reason this signature
+             * format exists at all). Safe to attempt unconditionally
+             * every boot: virtio_blk_find_artemis() only ever succeeds
+             * against the one dedicated PCI device, so a BLANK read here
+             * unambiguously means "never stamped," not "might be some
+             * other blank drive" -- and artemis_sig_check() returning
+             * anything other than BLANK (already stamped, or a version/
+             * CRC mismatch worth leaving alone rather than overwriting)
+             * skips the stamp. See artemis_sig.h's own doc comment for why
+             * this lands at a fence-relative top-of-device offset now,
+             * not a fixed bottom-of-device forth-block (that first attempt
+             * would have overwritten Artemis's own live BAM -- caught
+             * before ever being run against the real disk). */
+            {
+                artemis_sig_t asig;
+                artemis_sig_result_t art_rc = artemis_sig_check(&artemis_dev, &asig);
+                if (art_rc == ARTEMIS_SIG_BLANK) {
+                    if (artemis_sig_genesis_stamp(&artemis_dev) == 0) {
+                        console_println("Artemis: genesis signature stamped");
+                    } else {
+                        console_println("Artemis: genesis signature stamp FAILED");
+                    }
+                }
+            }
         } else {
             console_println("Artemis: no virtio-blk disk (continuing without)");
         }
     }
 
-    /* Phase 8: entropy. Real per-arch RNG doesn't cover all three
-     * architectures (amd64 RDRAND, riscv64 Zkr, but aarch64 has neither in
-     * QEMU's CPU models -- see vm_uuid.h's identical finding), so signing/
-     * keygen entropy comes from the unified rng_get_bytes() layer, whose
-     * v2.0.0 backend is the paravirtualized virtio-rng device. Unconditional
-     * call site, same graceful-noop precedent as virtio_blk_find_artemis()
-     * above -- boot proceeds either way, the device is only required once
-     * something actually calls rng_get_bytes(). */
-    {
-        int rrc = rng_init();
-        if (rrc == 0) {
-            console_println("entropy: ready");
-        } else {
-            console_println("entropy: not available (continuing without)");
-        }
-    }
-
     /* Zuse identity: SUPERSEDED 2026-08-28 (FABRIC-2.md §F.20/§F.21).
      * The one-shot block-fence mint-or-load that used to run here is
      * gone -- Zuse is thumbdrive-resident now (her seed never touches

src/starkernel/repl.c

@@ -496,8 +496,7 @@ static void sk_repl_idle(VM *active_vm)
             int is_artemis_disk = 0;
             if (sig_rc == HOMEBLOCKS_SIG_BLANK) {
                 artemis_sig_t asig;
-                artemis_sig_result_t art_rc =
-                    artemis_sig_check(usb_blk_dev, ARTEMIS_SIG_START_FBLOCK, &asig);
+                artemis_sig_result_t art_rc = artemis_sig_check(usb_blk_dev, &asig);
                 if (art_rc == ARTEMIS_SIG_OK) {
                     log_message(LOG_DEBUG, "xhci: USB drive recognized as Artemis's own disk");
                     is_artemis_disk = 1;