blob: 5ad78880eabda1a0aad9642fc0688950d75669d6 [file] [log] [blame]
// Copyright 2016 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// HpackVarintDecoder decodes HPACK variable length unsigned integers. These
// integers are used to identify static or dynamic table index entries, to
// specify string lengths, and to update the size limit of the dynamic table.
// The caller will need to validate that the decoded value is in an acceptable
// range.
// In order to support naive encoders (i.e. which always output 5 extension
// bytes for a uint32 that is >= prefix_mask), the decoder supports an an
// encoding with up to 5 extension bytes, and a maximum value of 268,435,582
// (4 "full" extension bytes plus the maximum for a prefix, 127). It could be
// modified to support a lower maximum value (by requiring that extensions bytes
// be "empty"), or a larger value if valuable for some reason I can't see.
// For details of the encoding, see:
// TODO(jamessynge): Consider dropping support for encodings of more than 4
// bytes, including the prefix byte, as in practice we only see at most 3 bytes,
// and 4 bytes would cover any desire to support large (but not ridiculously
// large) header values.
#include <cstdint>
#include "base/logging.h"
#include "net/third_party/http2/decoder/decode_buffer.h"
#include "net/third_party/http2/decoder/decode_status.h"
#include "net/third_party/http2/platform/api/http2_export.h"
#include "net/third_party/http2/platform/api/http2_string.h"
namespace http2 {
// Decodes an HPACK variable length unsigned integer, in a resumable fashion
// so it can handle running out of input in the DecodeBuffer. Call Start or
// StartExtended the first time (when decoding the byte that contains the
// prefix), then call Resume later if it is necessary to resume. When done,
// call value() to retrieve the decoded value.
// No constructor or destructor. Holds no resources, so destruction isn't
// needed. Start and StartExtended handles the initialization of member
// variables. This is necessary in order for HpackVarintDecoder to be part
// of a union.
class HTTP2_EXPORT_PRIVATE HpackVarintDecoder {
// |prefix_value| is the first byte of the encoded varint.
// |prefix_length| is number of bits in the first byte that are used for
// encoding the integer. |db| is the rest of the buffer, that is, not
// including the first byte.
DecodeStatus Start(uint8_t prefix_value,
uint8_t prefix_length,
DecodeBuffer* db);
// The caller has already determined that the encoding requires multiple
// bytes, i.e. that the 3 to 7 low-order bits (the number determined by
// |prefix_length|) of the first byte are are all 1. |db| is the rest of the
// buffer, that is, not including the first byte.
DecodeStatus StartExtended(uint8_t prefix_length, DecodeBuffer* db);
// Resume decoding a variable length integer after an earlier
// call to Start or StartExtended returned kDecodeInProgress.
DecodeStatus Resume(DecodeBuffer* db);
uint32_t value() const;
// This supports optimizations for the case of a varint with zero extension
// bytes, where the handling of the prefix is done by the caller.
void set_value(uint32_t v);
// All the public methods below are for supporting assertions and tests.
Http2String DebugString() const;
// For benchmarking, these methods ensure the decoder
// is NOT inlined into the caller.
DecodeStatus StartForTest(uint8_t prefix_value,
uint8_t prefix_length,
DecodeBuffer* db);
DecodeStatus StartExtendedForTest(uint8_t prefix_length, DecodeBuffer* db);
DecodeStatus ResumeForTest(DecodeBuffer* db);
static constexpr uint32_t MaxExtensionBytes() { return 5; }
// Protection in case Resume is called when it shouldn't be.
void MarkDone() {
#ifndef NDEBUG
// We support up to 5 extension bytes, so offset_ should never be > 28 when
// it makes sense to call Resume().
offset_ = MaxOffset() + 7;
void CheckNotDone() const {
#ifndef NDEBUG
DCHECK_LE(offset_, MaxOffset());
void CheckDone() const {
#ifndef NDEBUG
DCHECK_GT(offset_, MaxOffset());
static constexpr uint32_t MaxOffset() {
return 7 * (MaxExtensionBytes() - 1);
// These fields are initialized just to keep ASAN happy about reading
// them from DebugString().
uint32_t value_ = 0;
uint32_t offset_ = 0;
} // namespace http2