v2.0.1: G.4 amd64 RDRAND backend behind rng_get_bytes() (SER5 entropy)
First real per-arch RNG backend, added to the v2.0.0 unified entry point in src/starkernel/rng/rng.c, #if-guarded to amd64: CPUID.01H:ECX[30] RDRAND detection + inline-asm rdrand draws feeding rdrand_fill() (whole-byte emission from the low end; a partial final draw is discarded -- throwing away entropy is always safe). Probe order honors the release policy: virtio-rng is tried first, so the QEMU path stays on virtio-rng unchanged; RDRAND is the fallback only real hardware (which has no virtio-rng device) reaches. QEMU-verified both ways on amd64: with virtio-rng present -> "rng: backend = virtio-rng" (unchanged); with virtio-rng absent and RDRAND exposed (-cpu max) -> "rng: backend = rdrand" + "entropy: ready" + Zuse attach confirmed. rdrand_fill()'s exact logic host-proven: fills 32-byte/16-byte buffers and yields differing draws run-to-run (non-deterministic). aarch64/riscv64 builds unaffected (guarded off). riscv64 Zkr and aarch64 peripheral-RNG backends remain parked for their real boards. FABRIC-3.md G.4 amd64 slice marked BUILT + QEMU-verified.
This commit is contained in:
Robert Allan James
+13
@@ -3413,6 +3413,19 @@ there.
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primitive) returns genuinely non-deterministic bytes (two boots differ), the Zuse mint
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path seeded from it produces a valid distinct cert per boot when bleached, and QEMU
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behavior is unchanged.
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- **v2.0.1 — amd64 RDRAND backend BUILT + QEMU-verified 2026-08-29 (SER5 slice).** The first
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real per-arch backend lands in `src/starkernel/rng/rng.c`, behind the v2.0.0 unified
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entry point, `#if`-guarded to amd64: CPUID.01H:ECX[30] detection + inline-asm `rdrand`
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draws feeding `rdrand_fill()` (whole-byte emission, partial draw discarded — throwing
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away entropy is always safe). Probe order honors the policy: virtio-rng is tried first,
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so the QEMU path stays on virtio-rng unchanged; RDRAND is the fallback that only real
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hardware (which has no virtio-rng) reaches. QEMU-verified both ways on amd64: with
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virtio-rng present → `rng: backend = virtio-rng` (unchanged); with virtio-rng absent and
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RDRAND exposed (`-cpu max`) → `rng: backend = rdrand` + `entropy: ready` + Zuse attach
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confirmed. `rdrand_fill()`'s exact logic host-proven to fill 32-byte/16-byte buffers and
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produce differing draws run-to-run (non-deterministic). aarch64/riscv64 builds unaffected
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(guarded off). Still parked for their real boards: riscv64 Zkr (RNDR), aarch64 peripheral
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RNG. Full "two distinct certs per bleached boot" proof remains v2.5.0/real-board.
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#### G.5 [v2.5.0] Real-machine boot validation (SER5 / RasPi 6 / Milk-V)
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@@ -1,5 +1,5 @@
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# Capsule Block Manifest — Auto-generated
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<!-- Generated by mkcapsule --manifest 2026-08-29T14:11:01Z -->
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<!-- Generated by mkcapsule --manifest 2026-08-29T14:16:48Z -->
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<!-- DO NOT EDIT — re-run mkcapsule --manifest to refresh. -->
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<!-- Hand-written justifications and immutability notes live -->
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<!-- in MANIFEST.md alongside this auto-generated index. -->
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Binary file not shown.
@@ -6,8 +6,16 @@
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v2.0.0: virtio-rng is the sole active backend (QEMU-only, uniform across all
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three arches). The backend-selection switch in rng_get_bytes() is the exact
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seam where the v2.5.0 real per-arch drivers (amd64 RDRAND, riscv64 Zkr,
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aarch64 peripheral RNG) plug in without touching the call path.
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seam where the v2.5.0 real per-arch drivers plug in without touching the call
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path.
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v2.0.1 (G.4, metadata): the real per-arch backends begin to land here, each
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guarded by its ISA. amd64 RDRAND is first (SER5). Backend probe order matters
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and honors the release policy: virtio-rng is tried first so the QEMU path
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stays on virtio-rng unchanged; the real CPU primitive (RDRAND on amd64) is
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the fallback that only real hardware reaches, because real boards have no
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virtio-rng device. Each backend prints its identity at rng_init() so a boot
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log states exactly which source serves entropy.
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*/
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#ifndef __STARKERNEL__
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@@ -19,23 +27,80 @@
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#include "starkernel/console.h"
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/* Number of entropy backends known to this build. Each maps to one case in
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* rng_init() and rng_get_bytes(). v2.0.0 has exactly one: virtio-rng. */
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* rng_init() and rng_get_bytes(). v2.0.0: virtio-rng. v2.0.1 adds the amd64
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* RDRAND backend (real-hardware/SER5 path). */
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enum {
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RNG_BACKEND_NONE = 0,
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RNG_BACKEND_VIRTIO, /* virtio-rng (QEMU, all three arches) */
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#if defined(__x86_64__) || defined(__amd64__)
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RNG_BACKEND_RDRAND, /* amd64 RDRAND (real hardware, e.g. Beelink SER5) */
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#endif
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};
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static int g_rng_backend = RNG_BACKEND_NONE;
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/* --- amd64 RDRAND backend (G.4) ---------------------------------------- */
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#if defined(__x86_64__) || defined(__amd64__)
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/* CPUID.01H:ECX bit 30 = RDRAND supported. */
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static inline int rdrand_available(void) {
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uint32_t eax, ebx, ecx, edx;
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__asm__ volatile ("cpuid"
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: "=a"(eax), "=b"(ebx), "=c"(ecx), "=d"(edx)
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: "a"(1u), "c"(0u));
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(void)eax; (void)ebx; (void)edx;
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return (int)((ecx >> 30) & 1u);
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}
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/* Execute RDRAND; returns 1 on a valid draw (CF=1), 0 otherwise. */
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static inline int rdrand64(uint64_t *out) {
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uint8_t ok;
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__asm__ volatile ("rdrand %0; setc %1"
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: "=r"(*out), "=qm"(ok)
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:
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: "cc");
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return (int)ok;
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}
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static int rdrand_fill(uint8_t *buf, size_t n) {
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size_t filled = 0;
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uint32_t attempts = 0;
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const uint32_t MAX_ATTEMPTS = 128u; /* generous: hardware RDRAND is fast */
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while (filled < n) {
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uint64_t w;
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if (!rdrand64(&w)) {
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if (++attempts > MAX_ATTEMPTS) return RNG_ERR_NO_BACKEND;
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continue;
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}
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/* Emit whole bytes from the low end. The last (partial) draw is
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* simply discarded — throwing away entropy is always safe. */
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for (size_t i = 0; i < sizeof(w) && filled < n; i++)
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buf[filled++] = (uint8_t)(w >> (8 * i));
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}
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return 0;
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}
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#endif /* amd64 */
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int rng_init(void) {
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/* Probe backends in priority order; first success wins. virtio-rng is
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* the sole backend at v2.0.0; v2.5.0 adds real per-arch drivers here. */
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* tried first so the QEMU path stays on virtio-rng unchanged; the real
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* per-arch primitive is the fallback that only real hardware reaches. */
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if (virtio_rng_init() == 0) {
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g_rng_backend = RNG_BACKEND_VIRTIO;
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console_println("rng: backend = virtio-rng");
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return 0;
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}
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#if defined(__x86_64__) || defined(__amd64__)
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if (rdrand_available()) {
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g_rng_backend = RNG_BACKEND_RDRAND;
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console_println("rng: backend = rdrand");
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return 0;
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}
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#endif
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/* Refuse loudly: never fall through to a deterministic seed. */
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g_rng_backend = RNG_BACKEND_NONE;
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console_println(
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@@ -49,12 +114,17 @@ int rng_ready(void) {
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}
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int rng_get_bytes(uint8_t *buf, size_t n) {
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if (!buf || n == 0) return RNG_ERR_NO_BACKEND;
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switch (g_rng_backend) {
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case RNG_BACKEND_VIRTIO:
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return virtio_rng_get_bytes(buf, n);
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#if defined(__x86_64__) || defined(__amd64__)
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case RNG_BACKEND_RDRAND:
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return rdrand_fill(buf, n);
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#endif
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default:
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/* v2.5.0 real per-arch cases land here. No backend: refuse loudly,
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* never return a deterministic throwaway. */
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/* No backend: refuse loudly, never return a deterministic
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* throwaway. */
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return RNG_ERR_NO_BACKEND;
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}
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}
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