Luca Boccassi
cfca4412ca
Split out of https://github.com/systemd/systemd/pull/42983 ## Problem `dns_packet_append_name()` fails with `-EEXIST` once a name that was first appended at an offset beyond RFC 1035's 14-bit compression-pointer range (0x4000) occurs again in the same packet: the fall-through that re-emits the labels trips `hashmap_ensure_put()` on the existing map entry, and the whole packet build fails — reported by the caller as, for example, "Failed to build reply packet: File exists". The map is meant to hold only offsets a pointer can actually reference. Names past that range were being inserted anyway, so they were dead weight that could never be pointed at, and re-encountering one turned into an error. ## Fix Keep names at offsets ≥ 0x4000 out of the compression map entirely. "Mapped implies referenceable" becomes an invariant of the map rather than something callers have to reason about, and the `-EEXIST` collision cannot arise. A re-occurrence of such a name is emitted as labels, still compressed from the first suffix that *is* mapped in range — which is the only encoding RFC 1035 section 4.1.4 permits in that case, since a pointer must address ≤ 0x3FFF. ## Impact Any packet large enough to push a name past 16KiB hits this — it is not mDNS-specific, though that is where it was found: a DNS-SD announcement covering a few hundred published services crosses the threshold easily, and every record after it fails to encode. ## Test `test-dns-packet-append` gains coverage for the boundary: a name first written past the pointer range and then repeated, the same for a full resource record, the rollback path when an append fails mid-name, and the map invariant itself (asserted over the whole map, so a future change that inserts an unreferenceable offset is caught rather than silently reintroducing the bug). ## Testing status Builds clean with `-Dwerror=true`; `meson test --suite resolve` passes (19 ok, 1 skipped) at both commits.
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