# DF-0161 — m_tag_copy_chain reverses tag order: tprev misplaced

**Verdict:** REPRODUCED (source-only confirmation, Low severity)
**Impact:** none / defense-in-depth / latent (see below)
**Confidence:** certain
**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

## Mechanism (source-traced)

CONFIRMED by source trace. m_tag_copy_chain(:376-381): tprev=t is inside the else branch(:380). After first SLIST_INSERT_HEAD, tprev stays NULL → every tag inserted at head → destination reversed. SLIST_INSERT_AFTER is dead code. Upstream KAME/FreeBSD places tprev=t outside if/else.

## Kernel references (confirmed)
- `sys/kern/uipc_mbuf2.c:376`
- `sys/kern/uipc_mbuf2.c:379`
- `sys/kern/uipc_mbuf2.c:380`

## Fix

Move tprev=t outside the if/else to preserve tag order. Supersedes finding proposal.

The standalone git-apply-able diff is in `fix.diff`.

## Build validation

`fix.diff` was one of **50** diffs applied to a single combined
`make -j6 nativekernel KERNCONF=X86_64_GENERIC` build on the audit guest
(6.5-DEVELOPMENT #0, INVARIANTS ON). The combined build completed
**rc=0, 0 errors, 0 warnings** under `-Werror`, confirming this fix (and all
49 others) compile cleanly together.

- Combined build log (35649 lines): `findings/poc/DF-0161/../../_combined_build.log` (reference; full log at audit time).
- Combined kernel.stripped sha256: `9337c4e114e3a91edc02fee6d9eff48799b3c0926c1151d642b4573cb7911000`
- Build completed: 2026-07-22T22:33:21Z