{
 "finding_id": "DF-3249",
 "status": "not_reproduced",
 "reproduced": 0,
 "impact": "none",
 "confidence": "certain",
 "verdict": "Line-accurate confirmation; no runtime manifestation exists on x86 (shift count masks to 31; loc = -1/8 = 0; the loop body never executes because end clamps to -1 and i starts at -1, so back=0 and accounting stays correct). ext2_subr.c:175 sets start = bno - 1 for the backward scan; with cg-relative bno==0 (craftable by clearing bit 0 of a group bitmap) :180 evaluates bit = 1 << (start % NBBY) == 1 << -1 and loc = start/NBBY - undefined per C99. Pure hardening; the FFS lineage code shares the wart. Fix guards the backward scan with start >= 0; compiled in the F40FIX build.",
 "exploit_chain": "",
 "evidence": "VERDICT.md with path:line citations; run.log where a live demo exists; fix.diff",
 "kernel_refs": [
  "sys/vfs/ext2fs/ext2_subr.c:175",
  "sys/vfs/ext2fs/ext2_subr.c:180"
 ],
 "poc_changes": "n/a - no observable effect on x86",
 "attempts": 0,
 "guest_uname": "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",
 "runtime_sec": 30,
 "guest_dirty": 0,
 "build_cmd": "see build.sh",
 "run_cmd": "see run.sh",
 "code_hash": null,
 "notes": "",
 "recommended_fix": "skip the backward cluster scan when bno == 0 (start < 0)",
 "fix_status": "compile_validated",
 "fix_verdict": "fix.diff applied in the F40FIX verification kernel build (RC=0, zero -Werror errors); no runtime A/B for this item (line-certain hardening)"
}