src/inference/riscv64_no_seed_default.c: add riscv64 KASLR-disabled detection plugin

On non-EFI riscv64, KASLR requires a non-zero seed from the FDT
/chosen/kaslr-seed property. When the property is absent (ENOENT), the seed
is zero and the kernel loads at KERNEL_LINK_ADDR = 0xffffffff80000000.
Bilaterally pin text_base when EFI is absent, the FDT is mounted, and the
property is definitively missing.

Author: bcoles

src/inference/riscv64_no_seed_default.c

@@ -0,0 +1,106 @@
+// This file is part of KASLD - https://github.com/bcoles/kasld
+//
+// Inference plugin: riscv64 KASLR disabled when FDT kaslr-seed absent
+// (PRE_COLLECTION)
+//
+// On riscv64, arch/riscv/mm/init.c setup_vm() randomises only when the seed
+// is non-zero:
+//
+//   kaslr_seed = get_kaslr_seed_dt(dtb_va);
+//   if (IS_ENABLED(CONFIG_EFI))
+//       kaslr_seed ^= efi_kaslr_seed;
+//   if (!kaslr_seed)
+//       return;   // no KASLR; kernel loads at KERNEL_LINK_ADDR
+//
+// On a non-EFI system where the FDT has no /chosen/kaslr-seed property
+// (U-Boot not configured to provide entropy, e.g. missing
+// CONFIG_BOARD_RNG_SEED), get_kaslr_seed_dt() returns 0. With CONFIG_EFI
+// not enabled (non-EFI build) there is no efi_kaslr_seed contribution,
+// so kaslr_seed stays 0 and the kernel loads at the compile-time default
+// KERNEL_LINK_ADDR = 0xffffffff80000000.
+//
+// Detection guards (in order):
+//   1. access("/sys/firmware/efi", F_OK) == 0     → EFI present; skip.
+//      On EFI systems kaslr_seed = fdt_seed ^ efi_seed; absence of the FDT
+//      property alone does not disable KASLR.
+//   2. access("/proc/device-tree", F_OK) != 0     → no FDT mounted; seed
+//      state unknown; skip.
+//   3. access("/proc/device-tree/chosen/kaslr-seed", F_OK) == 0 → property
+//      present; KASLR may be active; fall back to riscv64_fdt_kaslr_seed.
+//
+// Trigger only on ENOENT (property absent), not on all-zero content.
+// If the kernel zeroes the property after consuming it (seed-wiping, as
+// noted in sysfs_devicetree_initrd.c), all-zero content is ambiguous
+// ("active seed wiped" vs "no seed ever provided") and is unsafe for
+// pinning without real-host confirmation.
+//
+// TODO: once seed-wiping behaviour is verified on a real riscv64 host, an
+// additional trigger can be added for the all-zero case.
+//
+// Phase: PRE_COLLECTION — access() checks need no component results.
+// Applicable: riscv64 only. See riscv64 H8.
+// ---
+// <bcoles@gmail.com>
+
+#define _POSIX_C_SOURCE 200809L
+
+#include "../include/kasld_inference.h"
+
+#include <errno.h>
+#include <stdio.h>
+#include <unistd.h>
+
+static void riscv64_no_seed_default_run(struct kasld_analysis_ctx *ctx) {
+#if (defined(__riscv) || defined(__riscv__)) && __riscv_xlen == 64
+
+  /* On EFI systems the combined seed (fdt ^ efi) can be non-zero even
+   * when the FDT property is absent; pinning would be wrong. */
+  if (access("/sys/firmware/efi", F_OK) == 0)
+    return;
+
+  /* FDT not mounted: seed state unknown. */
+  if (access("/proc/device-tree", F_OK) != 0)
+    return;
+
+  /* Property present: KASLR may be active; fall back to riscv64_fdt_kaslr_seed.
+   */
+  if (access("/proc/device-tree/chosen/kaslr-seed", F_OK) == 0)
+    return;
+
+  /* access() failed: only pin on ENOENT (property definitively absent).
+   * Any other error (EACCES, EIO) leaves seed state unknown. */
+  if (errno != ENOENT)
+    return;
+
+  /* Property absent (ENOENT): get_kaslr_seed_dt() returned 0; KASLR disabled.
+   */
+
+  const unsigned long link_addr = (unsigned long)KERNEL_LINK_ADDR;
+
+  /* Safety guard: only pin if KERNEL_LINK_ADDR is within the established
+   * window. */
+  if (link_addr < ctx->text_base_min || link_addr > ctx->text_base_max)
+    return;
+
+  if (verbose && !quiet)
+    fprintf(stderr,
+            "[layout] text_base pinned by riscv64_no_seed_default:"
+            " [%#lx, %#lx] -> %#lx"
+            " (non-EFI, FDT kaslr-seed absent -> KASLR disabled)\n",
+            ctx->text_base_min, ctx->text_base_max, link_addr);
+
+  ctx->text_base_min = link_addr;
+  ctx->text_base_max = link_addr;
+
+#else
+  (void)ctx;
+#endif /* riscv64 */
+}
+
+static const struct kasld_inference riscv64_no_seed_default = {
+    .name = "riscv64_no_seed_default",
+    .phase = KASLD_INFER_PHASE_PRE_COLLECTION,
+    .run = riscv64_no_seed_default_run,
+};
+
+KASLD_REGISTER_INFERENCE(riscv64_no_seed_default);