Phase 8: single kernel/module build with ALL 9 fix.diffs applied + KCFLAGS=-Werror.
Guest: DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul  2 06:02:54 UTC 2026     root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC  x86_64 (with-src snapshot, INVARIANTS ON, gcc 8.3)

Each fix.diff was applied to /usr/src/sys via 'patch -p1 --forward' (all 9 applied OK),
then every affected module built with KCFLAGS=-Werror:

  Module dir                 | Result
  ---------------------------+-----------------------------------------------
  dev/raid/iir               | iir.ko            rc=0, 0 errors/warnings   (DF-2028)
  dev/raid/mfi               | mfi.ko + mfip.ko  rc=0, 0 errors/warnings   (DF-2016, DF-2031)
  dev/raid/tws               | tws.ko            rc=0, 0 errors/warnings   (DF-2032)
  dev/drm/drm                | drm.ko (780KB)    rc=0, 0 errors/warnings   (DF-2017, DF-2018, DF-2020)
  dev/drm/amd/amdgpu         | amdgpu.ko (3.7MB) rc=0, 0 errors/warnings   (DF-2021, DF-2022)

This finding (DF-2018) belongs to module 'dev/<...>/dev/drm/drm' -> drm.ko (780KB) built, rc=0, 0 errors/warnings
Patched source(s) verified to compile with -Werror: drm_auth.c, drm_sysctl.c

Conclusion: fix.diff for DF-2018 APPLIES and COMPILES clean (-Werror). Since the vulnerable
driver cannot attach on this guest (no matching HW), fix_status is reported as
'not_testable' for the runtime before/after, but the fix is validated at the
compile level and traced to close the cited code path.