From 454d4848e158f6ff0affd8dfdefd0b4396a02cc3 Mon Sep 17 00:00:00 2001
From: Dmitry Spikhalsky <dmitry@spikhalskiy.com>
Date: Wed, 21 Oct 2026 12:16:09 -0400
Subject: [PATCH] GH-54916: [C++][IPC] Align metadata in Message::Open

Message::Open() verified and read the Flatbuffers metadata in place, so
metadata starting at an address that is not 8-byte aligned (for example a
Flight data_header sliced zero-copy out of a gRPC message) caused misaligned
loads in Flatbuffers. Copy such metadata with the existing
MaybeAlignMetadata(), as MessageDecoder already does.
---
 cpp/src/arrow/ipc/message.cc         |  1 +
 cpp/src/arrow/ipc/read_write_test.cc | 26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/cpp/src/arrow/ipc/message.cc b/cpp/src/arrow/ipc/message.cc
index 4123a6b17355..38780d7572c7 100644
--- a/cpp/src/arrow/ipc/message.cc
+++ b/cpp/src/arrow/ipc/message.cc
@@ -87,6 +87,7 @@ class Message::MessageImpl {
       : metadata_(std::move(metadata)), message_(nullptr), body_(std::move(body)) {}
 
   Status Open() {
+    RETURN_NOT_OK(MaybeAlignMetadata(&metadata_));
     RETURN_NOT_OK(
         internal::VerifyMessage(metadata_->data(), metadata_->size(), &message_));
 
diff --git a/cpp/src/arrow/ipc/read_write_test.cc b/cpp/src/arrow/ipc/read_write_test.cc
index cfa6a541d5cc..51530c2bc8e3 100644
--- a/cpp/src/arrow/ipc/read_write_test.cc
+++ b/cpp/src/arrow/ipc/read_write_test.cc
@@ -17,6 +17,7 @@
 
 #include <algorithm>
 #include <cstdint>
+#include <cstring>
 #include <limits>
 #include <memory>
 #include <numeric>
@@ -224,6 +225,31 @@ TEST(TestMessage, Verify) {
   ASSERT_FALSE(message.Verify());
 }
 
+TEST(TestMessage, OpenUnalignedMetadata) {
+  auto custom_metadata = key_value_metadata({"foo"}, {"bar"});
+  ASSERT_OK_AND_ASSIGN(
+      auto serialized,
+      internal::WriteRecordBatchMessage(
+          /*length=*/0, /*body_length=*/0, custom_metadata,
+          /*nodes=*/{},
+          /*buffers=*/{}, /*variadic_counts=*/{}, IpcWriteOptions::Defaults()));
+
+  // Place the metadata at an odd address, as when it is sliced out of a larger
+  // buffer (e.g. a Flight data_header)
+  ASSERT_OK_AND_ASSIGN(std::shared_ptr<Buffer> storage,
+                       AllocateBuffer(serialized->size() + 1));
+  std::memcpy(storage->mutable_data() + 1, serialized->data(), serialized->size());
+  auto unaligned = SliceBuffer(storage, 1, serialized->size());
+  ASSERT_NE(0, reinterpret_cast<uintptr_t>(unaligned->data()) % 8);
+
+  ASSERT_OK_AND_ASSIGN(auto message, Message::Open(unaligned, /*body=*/nullptr));
+  ASSERT_EQ(0, reinterpret_cast<uintptr_t>(message->metadata()->data()) % 8);
+  ASSERT_TRUE(message->metadata()->Equals(*serialized));
+  ASSERT_EQ(MessageType::RECORD_BATCH, message->type());
+  ASSERT_EQ(0, message->body_length());
+  ASSERT_TRUE(message->custom_metadata()->Equals(*custom_metadata));
+}
+
 INSTANTIATE_TEST_SUITE_P(TestMessage, TestMessage,
                          ::testing::ValuesIn(kMetadataVersions));