[executorch][flat_tensor] flat tensor header

Pull Request resolved: #7764

Parse a FlatTensor header. The FlatTensor header contains:
- flatbuffer offset
- flatbuffer size
- segment base offset
- segment data size (total size of all segments)

Mostly taken from extended_header.cpp/.h
ghstack-source-id: 262543309

Differential Revision: [D67064570](https://our.internmc.facebook.com/intern/diff/D67064570/)

Author: lucylq

extension/flat_tensor/serialize/flat_tensor_header.cpp

@@ -0,0 +1,113 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <executorch/extension/flat_tensor/serialize/flat_tensor_header.h>
+
+#include <cinttypes>
+#include <cstring>
+
+#include <executorch/runtime/core/error.h>
+#include <executorch/runtime/core/result.h>
+
+#pragma clang diagnostic ignored "-Wdeprecated"
+
+namespace executorch {
+using runtime::Error;
+using runtime::Result;
+namespace extension {
+namespace {
+
+/// The expected location of the header length field relative to the beginning
+/// of the header.
+static constexpr size_t kHeaderLengthOffset = FlatTensorHeader::kMagicSize;
+
+/// The expected location of the flatbuffer_offset field relative to the
+/// beginning of the header.
+static constexpr size_t kHeaderFlatbufferStartOffset =
+    kHeaderLengthOffset + sizeof(uint32_t);
+
+/// The expected location of the flatbuffer_size field relative to the beginning
+/// of the header.
+static constexpr size_t kHeaderFlatbufferSizeOffset =
+    kHeaderFlatbufferStartOffset + sizeof(uint64_t);
+
+/// The expected location of the segment_base_offset field relative to the
+/// beginning of the header.
+static constexpr size_t kHeaderSegmentBaseOffsetOffset =
+    kHeaderFlatbufferSizeOffset + sizeof(uint64_t);
+
+/// The expected location of the segment_data_size field relative to the
+/// beginning of the header.
+static constexpr size_t kHeaderSegmentDataSizeOffset =
+    kHeaderSegmentBaseOffsetOffset + sizeof(uint64_t);
+
+/**
+ * The size of the header that covers the fields known of by this version of
+ * the code. It's ok for a header to be larger as long as the fields stay in
+ * the same place, but this code will ignore any new fields.
+ */
+static constexpr size_t kMinimumHeaderLength =
+    kHeaderSegmentDataSizeOffset + sizeof(uint64_t);
+
+/// Interprets the 4 bytes at `data` as a little-endian uint32_t.
+uint32_t GetUInt32LE(const uint8_t* data) {
+  return (uint32_t)data[0] | ((uint32_t)data[1] << 8) |
+      ((uint32_t)data[2] << 16) | ((uint32_t)data[3] << 24);
+}
+
+/// Interprets the 8 bytes at `data` as a little-endian uint64_t.
+uint64_t GetUInt64LE(const uint8_t* data) {
+  return (uint64_t)data[0] | ((uint64_t)data[1] << 8) |
+      ((uint64_t)data[2] << 16) | ((uint64_t)data[3] << 24) |
+      ((uint64_t)data[4] << 32) | ((uint64_t)data[5] << 40) |
+      ((uint64_t)data[6] << 48) | ((uint64_t)data[7] << 56);
+}
+
+} // namespace
+
+/* static */ Result<FlatTensorHeader> FlatTensorHeader::Parse(
+    const void* data,
+    size_t size) {
+  if (size < FlatTensorHeader::kNumHeadBytes) {
+    return Error::InvalidArgument;
+  }
+  const uint8_t* header = reinterpret_cast<const uint8_t*>(data);
+
+  // Check magic bytes.
+  if (std::memcmp(
+          header, FlatTensorHeader::kMagic, FlatTensorHeader::kMagicSize) !=
+      0) {
+    return Error::NotFound;
+  }
+
+  // Check header length.
+  uint32_t header_length = GetUInt32LE(header + kHeaderLengthOffset);
+  if (header_length < kMinimumHeaderLength) {
+    ET_LOG(
+        Error,
+        "FlatTensor header length %" PRIu32 " < %zu",
+        header_length,
+        kMinimumHeaderLength);
+    return Error::InvalidExternalData;
+  }
+
+  // The header is present and apparently valid.
+  return FlatTensorHeader{
+      /*flatbuffer_offset=*/GetUInt64LE(header + kHeaderFlatbufferStartOffset),
+      /*flatbuffer_size=*/GetUInt64LE(header + kHeaderFlatbufferSizeOffset),
+      /*segment_base_offset=*/
+      GetUInt64LE(header + kHeaderSegmentBaseOffsetOffset),
+      /*segment_data_size=*/GetUInt64LE(header + kHeaderSegmentDataSizeOffset),
+  };
+}
+
+// Define storage for the static.
+constexpr char FlatTensorHeader::kMagic[kMagicSize];
+
+} // namespace extension
+} // namespace executorch

extension/flat_tensor/serialize/flat_tensor_header.h

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <executorch/runtime/core/result.h>
+
+namespace executorch {
+namespace extension {
+
+/**
+ * A FlatTensor header found at the beginning of a flat_tensor-serialized blob.
+ */
+struct FlatTensorHeader {
+  /**
+   * To find the header, callers should provide at least this many bytes of the
+   * head of the serialized FlatTensor data.
+   */
+  static constexpr size_t kNumHeadBytes = 48;
+
+  /**
+   * The magic bytes that identify the header. This should be in sync with
+   * the magic in executorch/extension/flat_tensor/serialize/serialize.py
+   *
+   * This is the canonical definition of the expected value. If the header
+   * layout ever changes in a compatibility-breaking way, increment the digits
+   * in the magic. But, doing so will prevent older binaries from recognizing
+   * the presence of the header. The compatibility-preserving way to make
+   * changes is to increase the header's length field and add new fields at the
+   * end.
+   */
+  static constexpr size_t kMagicSize = 4;
+  static constexpr char kMagic[kMagicSize] = {'F', 'H', '0', '1'};
+
+  /**
+   * Look for and parse a FlatTensorHeader in the provided data.
+   *
+   * @param[in] data The contents of the beginning of the serialized binary
+   *     FlatTensor data, starting at offset 0 (i.e., the head of the file).
+   * @param[in] size Length of `data` in bytes. Must be >= kNumHeadBytes or this
+   *     call will fail.
+   *
+   * @returns a FlatTensorHeader if the header was found and is valid. Returns
+   * an error if size was too short, if the header was not found, or if the
+   *     header appeared to be corrupt.
+   */
+  static runtime::Result<FlatTensorHeader> Parse(const void* data, size_t size);
+
+  /// Offset of the FlatTensor flatbuffer in the serialized binary.
+  uint64_t flatbuffer_offset;
+
+  /// Size of the flatbuffer in bytes.
+  uint64_t flatbuffer_size;
+
+  /// The offset in bytes of the first segment.
+  uint64_t segment_base_offset;
+
+  /// Size of all the segment data, in bytes.
+  uint64_t segment_data_size;
+};
+
+} // namespace extension
+} // namespace executorch

extension/flat_tensor/serialize/serialize.py

@@ -93,6 +93,8 @@ class FlatTensorConfig:
 class FlatTensorHeader:
     # Class constants.
     # The magic bytes that should be at the beginning of the header.
+    # This should be in sync with the magic in 
+    # executorch/extension/flat_tensor/serialize/flat_tensor_header.h
     EXPECTED_MAGIC: ClassVar[bytes] = b"FH01"
     EXPECTED_LENGTH: ClassVar[int] = (
         # Header magic

extension/flat_tensor/serialize/targets.bzl

@@ -34,3 +34,11 @@ def define_common_targets():
         },
         exported_external_deps = ["flatbuffers-api"],
     )
+
+    runtime.cxx_library(
+        name = "flat_tensor_header",
+        srcs = ["flat_tensor_header.cpp"],
+        exported_headers = ["flat_tensor_header.h"],
+        visibility = ["//executorch/..."],
+        exported_deps = ["//executorch/runtime/core:core"],
+    )

extension/flat_tensor/test/TARGETS

@@ -1,7 +1,13 @@
+# Any targets that should be shared between fbcode and xplat must be defined in
+# targets.bzl. This file can contain fbcode-only targets.
+
 load("@fbcode_macros//build_defs:python_unittest.bzl", "python_unittest")
+load(":targets.bzl", "define_common_targets")
 
 oncall("executorch")
 
+define_common_targets()
+
 python_unittest(
     name = "serialize",
     srcs = [

extension/flat_tensor/test/flat_tensor_header_test.cpp

@@ -0,0 +1,91 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <executorch/extension/flat_tensor/serialize/flat_tensor_header.h>
+
+#include <gtest/gtest.h>
+
+#include <executorch/runtime/core/result.h>
+#include <executorch/runtime/platform/runtime.h>
+
+using namespace ::testing;
+using executorch::extension::FlatTensorHeader;
+using executorch::runtime::Error;
+using executorch::runtime::Result;
+
+class FlatTensorHeaderTest : public ::testing::Test {
+ protected:
+  void SetUp() override {
+    // Since these tests cause ET_LOG to be called, the PAL must be initialized
+    // first.
+    executorch::runtime::runtime_init();
+  }
+};
+
+/**
+ * An example, valid flat_tensor header.
+ *
+ * This data is intentionally fragile. If the header layout or magic changes,
+ * this test data must change too. The layout of the header is a contract, not
+ * an implementation detail.
+ */
+// clang-format off
+constexpr char kExampleHeaderData[] = {
+  // Magic bytes
+  'F', 'H', '0', '1',
+  // uint32_t header size (little endian)
+  0x40, 0x00, 0x00, 0x00,
+  // uint64_t flatbuffer_offset
+  0x71, 0x61, 0x51, 0x41, 0x31, 0x21, 0x11, 0x01,
+  // uint64_t flatbuffer_size
+  0x72, 0x62, 0x52, 0x42, 0x32, 0x22, 0x12, 0x02,
+  // uint64_t segment_base_offset
+  0x73, 0x63, 0x53, 0x43, 0x33, 0x23, 0x13, 0x03,
+  // uint64_t segment_data_size
+  0x74, 0x64, 0x54, 0x44, 0x34, 0x24, 0x14, 0x04,
+};
+
+constexpr uint64_t kExampleFlatbufferOffset = 0x0111213141516171;
+constexpr uint64_t kExampleFlatbufferSize = 0x0212223242526272;
+constexpr uint64_t kExampleSegmentBaseOffset = 0x0313233343536373; 
+constexpr uint64_t kExampleSegmentDataSize = 0x0414243444546474;
+
+/**
+ * Returns fake serialized FlatTensor data that contains kExampleHeaderData at
+ * the expected offset.
+ */
+std::vector<uint8_t> CreateExampleFlatTensorHeader() {
+  // Allocate memory representing the FlatTensor header.
+  std::vector<uint8_t> ret(FlatTensorHeader::kNumHeadBytes);
+  // Write non-zeros into it to make it more obvious if we read outside the
+  // header.
+  memset(ret.data(), 0x55, ret.size());
+  // Copy the example header into the right offset.
+  memcpy(
+      ret.data(),
+      kExampleHeaderData,
+      sizeof(kExampleHeaderData));
+  return ret;
+}
+
+TEST_F(FlatTensorHeaderTest, ValidHeaderParsesCorrectly) {
+  std::vector<uint8_t> flat_tensor = CreateExampleFlatTensorHeader();
+
+  Result<FlatTensorHeader> header = FlatTensorHeader::Parse(flat_tensor.data(), flat_tensor.size());
+
+  // The header should be present.
+  ASSERT_EQ(header.error(), Error::Ok);  
+
+  // Since each byte of these fields is unique, success demonstrates that the
+  // endian-to-int conversion is correct and looks at the expected bytes of the
+  // header.
+  EXPECT_EQ(header->flatbuffer_offset, kExampleFlatbufferOffset);
+  EXPECT_EQ(header->flatbuffer_size, kExampleFlatbufferSize);
+  EXPECT_EQ(header->segment_base_offset, kExampleSegmentBaseOffset);
+  EXPECT_EQ(header->segment_data_size, kExampleSegmentDataSize);
+}

extension/flat_tensor/test/targets.bzl

@@ -0,0 +1,18 @@
+load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")
+
+def define_common_targets():
+    """Defines targets that should be shared between fbcode and xplat.
+
+    The directory containing this targets.bzl file should also contain both
+    TARGETS and BUCK files that call this function.
+    """
+
+    runtime.cxx_test(
+        name = "flat_tensor_header_test",
+        srcs = [
+            "flat_tensor_header_test.cpp",
+        ],
+        deps = [
+            "//executorch/extension/flat_tensor/serialize:flat_tensor_header",
+        ],
+    )

runtime/core/error.h

@@ -79,6 +79,9 @@ enum class Error : error_code_t {
   /// Error caused by the contents of a program.
   InvalidProgram = 0x23,
 
+  /// Error caused by the contents of external data.
+  InvalidExternalData = 0x24,
+
   /*
    * Delegate errors.
    */