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// Copyright 2018 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include <cstdint>
#include <vector>
#include "base/component_export.h"
#include "base/containers/span.h"
#include "mojo/public/cpp/bindings/struct_traits.h"
#include "mojo/public/cpp/system/buffer.h"
#include "third_party/abseil-cpp/absl/types/optional.h"
namespace mojo_base {
class BigBuffer;
class BigBufferView;
namespace internal {
// Internal helper used by BigBuffer when backed by shared memory.
class COMPONENT_EXPORT(MOJO_BASE) BigBufferSharedMemoryRegion {
BigBufferSharedMemoryRegion(mojo::ScopedSharedBufferHandle buffer_handle,
size_t size);
BigBufferSharedMemoryRegion(BigBufferSharedMemoryRegion&& other);
BigBufferSharedMemoryRegion(const BigBufferSharedMemoryRegion&) = delete;
BigBufferSharedMemoryRegion& operator=(const BigBufferSharedMemoryRegion&) =
BigBufferSharedMemoryRegion& operator=(BigBufferSharedMemoryRegion&& other);
void* memory() const { return buffer_mapping_.get(); }
size_t size() const { return size_; }
mojo::ScopedSharedBufferHandle TakeBufferHandle();
friend class mojo_base::BigBuffer;
friend class mojo_base::BigBufferView;
size_t size_;
mojo::ScopedSharedBufferHandle buffer_handle_;
mojo::ScopedSharedBufferMapping buffer_mapping_;
} // namespace internal
// BigBuffer represents a potentially large sequence of bytes. When passed over
// mojom (as a mojo_base::mojom::BigBuffer union), it may serialize as either an
// inlined array of bytes or as a shared buffer handle with the payload copied
// into the corresponding shared memory region. This makes it easier to send
// payloads of varying and unbounded size over IPC without fear of hitting
// message size limits.
// A BigBuffer may be (implicitly) constructed over any span of bytes, and it
// exposes simple |data()| and |size()| accessors akin to what common container
// types provide. Users do not need to be concerned with the actual backing
// storage used to implement this interface.
// SECURITY NOTE: When shmem is backing the message, it may be writable in the
// sending process while being read in the receiving process. If a BigBuffer is
// received from an untrustworthy process, you should make a copy of the data
// before processing it to avoid time-of-check time-of-use (TOCTOU) bugs.
// The |size()| of the data cannot be manipulated.
static constexpr size_t kMaxInlineBytes = 64 * 1024;
enum class StorageType {
// Defaults to empty kBytes storage.
BigBuffer(BigBuffer&& other);
// Constructs a BigBuffer over an existing span of bytes. Intentionally
// implicit for convenience. Always copies the contents of |data| into some
// internal storage.
BigBuffer(base::span<const uint8_t> data);
// Helper for implicit conversion from byte vectors.
BigBuffer(const std::vector<uint8_t>& data);
// Constructs a BigBuffer from an existing shared memory region. Not intended
// for general-purpose use.
explicit BigBuffer(internal::BigBufferSharedMemoryRegion shared_memory);
// Constructs a BigBuffer with the given size. The contents of buffer memory
// are uninitialized. Buffers constructed this way must be filled completely
// before transfer to avoid leaking information to less privileged processes.
explicit BigBuffer(size_t size);
BigBuffer(const BigBuffer&) = delete;
BigBuffer& operator=(const BigBuffer&) = delete;
BigBuffer& operator=(BigBuffer&& other);
// Returns a pointer to the data stored by this BigBuffer, regardless of
// backing storage type.
uint8_t* data();
const uint8_t* data() const;
// Returns the size of the data stored by this BigBuffer, regardless of
// backing storage type.
size_t size() const;
StorageType storage_type() const { return storage_type_; }
base::span<const uint8_t> byte_span() const {
DCHECK_EQ(storage_type_, StorageType::kBytes);
return base::make_span(bytes_.get(), bytes_size_);
internal::BigBufferSharedMemoryRegion& shared_memory() {
DCHECK_EQ(storage_type_, StorageType::kSharedMemory);
return shared_memory_.value();
friend class BigBufferView;
StorageType storage_type_;
std::unique_ptr<uint8_t[]> bytes_;
size_t bytes_size_;
absl::optional<internal::BigBufferSharedMemoryRegion> shared_memory_;
// Similar to BigBuffer, but doesn't *necessarily* own the buffer storage.
// Namely, if constructed over a small enough span of memory, it will simply
// retain a reference to that memory. This is generally only safe to use for
// serialization and deserialization.
BigBufferView(BigBufferView&& other);
// Constructs a BigBufferView over |bytes|. If |bytes| is large enough, this
// will allocate shared memory and copy the contents there. Otherwise this
// will retain an unsafe reference to |bytes| and must therefore not outlive
// |bytes|.
explicit BigBufferView(base::span<const uint8_t> bytes);
BigBufferView(const BigBufferView&) = delete;
BigBufferView& operator=(const BigBufferView&) = delete;
BigBufferView& operator=(BigBufferView&& other);
base::span<const uint8_t> data() const;
// Explicitly retains a reference to |bytes| as the backing storage for this
// view. Does not copy and therefore |bytes| must remain valid throughout the
// view's lifetime. Used for deserialization.
void SetBytes(base::span<const uint8_t> bytes);
// Explictly adopts |shared_memory| as the backing storage for this view. Used
// for deserialization.
void SetSharedMemory(internal::BigBufferSharedMemoryRegion shared_memory);
// Converts to a BigBuffer which owns the viewed data. May have to copy data.
[[nodiscard]] static BigBuffer ToBigBuffer(BigBufferView view);
BigBuffer::StorageType storage_type() const { return storage_type_; }
base::span<const uint8_t> bytes() const {
DCHECK_EQ(storage_type_, BigBuffer::StorageType::kBytes);
return bytes_;
internal::BigBufferSharedMemoryRegion& shared_memory() {
DCHECK_EQ(storage_type_, BigBuffer::StorageType::kSharedMemory);
return shared_memory_.value();
static BigBufferView CreateInvalidForTest();
BigBuffer::StorageType storage_type_ = BigBuffer::StorageType::kBytes;
base::span<const uint8_t> bytes_;
absl::optional<internal::BigBufferSharedMemoryRegion> shared_memory_;
} // namespace mojo_base