[I] [C++][FlightRPC] ipc::Message::Open reads unaligned Flatbuffers metadata in place [arrow]

["Spikhalskiy (via GitHub)" <gi...@apache.org>]

Spikhalskiy opened a new issue, #54916:
URL: https://github.com/apache/arrow/issues/54916

   ### Describe the bug, including details regarding any error messages, version, and platform.
   
   `ipc::Message::Open(metadata, body)` verifies and reads the Flatbuffers metadata in place. When the metadata buffer is not 8-byte aligned, every Flatbuffers access is a misaligned load (UB, reported by `-fsanitize=alignment`).
   
   Flight hits this on every read: `DeserializeFlightData` slices `data_header` zero-copy out of the gRPC message (`ReadBytesZeroCopy`), and `FlightData::OpenMessage()` passes it straight to `Message::Open()`. `message.cc` already has `MaybeAlignMetadata()` for exactly this ("to avoid potential UBSAN issues from Flatbuffers"), but only `MessageDecoder` calls it.
   
   Reproduction (on main, built with `-fsanitize=alignment`): copy a serialized record batch message to an odd address and call `ipc::Message::Open()` on it. UBSan reports misaligned accesses in `VerifyMessage` and `GetKeyValueMetadata`:
   
   ```
   flatbuffers/buffer.h:186:20: runtime error: load of misaligned address 0x000756c38081 for type 'uoffset_t' (aka 'unsigned int'), which requires 4 byte alignment
       #3 in arrow::ipc::internal::VerifyMessage(...) metadata_internal.h:190
       #4 in arrow::ipc::Message::MessageImpl::Open() message.cc:90
   ```
   
   Suggested fix: call `MaybeAlignMetadata(&metadata_)` at the start of `MessageImpl::Open()`. Already-aligned metadata is not copied. (Unaligned bodies are a separate matter, covered by `ensure_alignment`, #32276.)
   
   ### Component(s)
   
   C++, FlightRPC


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