all modules: wrap Linux-only code in #ifdef __linux__ — full macOS build works

Every kernel-LPE module that uses Linux-only headers (splice, posix_fadvise,
linux/netlink.h, sys/ptrace.h, etc.) now follows the same #ifdef __linux__
pattern the new modules already used: Linux body in the ifdef, stub
detect/exploit/cleanup returning SKELETONKEY_PRECOND_FAIL on non-Linux,
platform-neutral rule strings + module struct + register fn left outside.

14 modules wrapped:
  dirty_pipe (already done above), af_packet, af_packet2,
  cgroup_release_agent, cls_route4, dirty_cow, fuse_legacy,
  netfilter_xtcompat, nf_tables, nft_fwd_dup, nft_payload,
  overlayfs, overlayfs_setuid, ptrace_traceme.

Several modules previously had ad-hoc partial stubs (af_packet2 faked
SIOCSIFFLAGS/MAP_LOCKED, netfilter_xtcompat faked sysv-msg syscalls,
the nft_* modules had 3 partial __linux__ islands each, fuse_legacy /
nf_tables had inner-only ifdef blocks) — all replaced with the uniform
outer-wrap shape from dirty_pipe / dirtydecrypt / fragnesia / pack2theroot.

Where a module includes core/kernel_range.h, core/finisher.h, or
core/offsets.h, those are now inside the ifdef block as well — silences
clangd's "unused-includes" LSP warning on macOS while keeping them
present for the real Linux build.

No exploit logic, constant, struct, shellcode byte, or rule string was
modified — only include placement and ifdef markers.

Build verification:
  macOS (local): make clean && make → Mach-O x86_64, 31 modules
                 registered, --scan reports each Linux-only module as
                 "Linux-only module — not applicable here".
  Linux (docker gcc:latest + libglib2.0-dev): make clean && make →
                 ELF 64-bit, 31 modules. Exploit code paths unchanged.

modules/overlayfs_cve_2021_3493/skeletonkey_modules.c

@@ -37,13 +37,16 @@
 
 #include "skeletonkey_modules.h"
 #include "../../core/registry.h"
-#include "../../core/kernel_range.h"
 
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <stdbool.h>
 #include <unistd.h>
+
+#ifdef __linux__
+
+#include "../../core/kernel_range.h"
 #include <fcntl.h>
 #include <sched.h>
 #include <sys/mount.h>
@@ -446,6 +449,28 @@ fail_workdir:
     return SKELETONKEY_EXPLOIT_FAIL;
 }
 
+#else  /* !__linux__ */
+
+/* Non-Linux dev builds: overlayfs / unshare(CLONE_NEWUSER|CLONE_NEWNS) /
+ * setxattr("security.capability") are all Linux-only. Stub out so the
+ * module still registers and the top-level `make` completes on
+ * macOS/BSD dev boxes. */
+static skeletonkey_result_t overlayfs_detect(const struct skeletonkey_ctx *ctx)
+{
+    if (!ctx->json)
+        fprintf(stderr, "[i] overlayfs: Linux-only module "
+                "(Ubuntu userns-overlayfs) — not applicable here\n");
+    return SKELETONKEY_PRECOND_FAIL;
+}
+static skeletonkey_result_t overlayfs_exploit(const struct skeletonkey_ctx *ctx)
+{
+    (void)ctx;
+    fprintf(stderr, "[-] overlayfs: Linux-only module — cannot run here\n");
+    return SKELETONKEY_PRECOND_FAIL;
+}
+
+#endif /* __linux__ */
+
 /* ----- Embedded detection rules ----- */
 
 static const char overlayfs_auditd[] =