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119 lines
4.7 KiB
C++
119 lines
4.7 KiB
C++
// Copyright 2016 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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// HpackVarintDecoder decodes HPACK variable length unsigned integers. These
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// integers are used to identify static or dynamic table index entries, to
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// specify string lengths, and to update the size limit of the dynamic table.
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//
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// The caller will need to validate that the decoded value is in an acceptable
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// range.
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//
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// In order to support naive encoders (i.e. which always output 5 extension
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// bytes for a uint32 that is >= prefix_mask), the decoder supports an an
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// encoding with up to 5 extension bytes, and a maximum value of 268,435,582
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// (4 "full" extension bytes plus the maximum for a prefix, 127). It could be
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// modified to support a lower maximum value (by requiring that extensions bytes
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// be "empty"), or a larger value if valuable for some reason I can't see.
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//
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// For details of the encoding, see:
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// http://httpwg.org/specs/rfc7541.html#integer.representation
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//
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// TODO(jamessynge): Consider dropping support for encodings of more than 4
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// bytes, including the prefix byte, as in practice we only see at most 3 bytes,
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// and 4 bytes would cover any desire to support large (but not ridiculously
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// large) header values.
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#ifndef NET_HTTP2_HPACK_DECODER_HPACK_VARINT_DECODER_H_
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#define NET_HTTP2_HPACK_DECODER_HPACK_VARINT_DECODER_H_
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#include "base/logging.h"
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#include "net/http2/decoder/decode_buffer.h"
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#include "net/http2/decoder/decode_status.h"
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#include "net/http2/platform/api/http2_export.h"
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#include "net/http2/platform/api/http2_string.h"
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namespace net {
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// Decodes an HPACK variable length unsigned integer, in a resumable fashion
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// so it can handle running out of input in the DecodeBuffer. Call Start or
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// StartExtended the first time (when decoding the byte that contains the
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// prefix), then call Resume later if it is necessary to resume. When done,
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// call value() to retrieve the decoded value.
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//
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// No constructor or destructor. Holds no resources, so destruction isn't
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// needed. Start and StartExtended handles the initialization of member
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// variables. This is necessary in order for HpackVarintDecoder to be part
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// of a union.
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class HTTP2_EXPORT_PRIVATE HpackVarintDecoder {
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public:
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// |prefix_value| is the first byte of the encoded varint.
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// |prefix_mask| is the mask of the valid bits, i.e. without the top 1 to 4
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// high-bits set, as appropriate for the item being decoded; must be a
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// contiguous sequence of set bits, starting with the low-order bits.
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DecodeStatus Start(uint8_t prefix_value,
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uint8_t prefix_mask,
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DecodeBuffer* db);
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// The caller has already determined that the encoding requires multiple
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// bytes, i.e. that the 4 to 7 low-order bits (the number determined by the
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// prefix length, a value not passed into this function) of the first byte are
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// are all 1. The caller passes in |prefix_mask|, which is 2^prefix_length-1.
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DecodeStatus StartExtended(uint8_t prefix_mask, DecodeBuffer* db);
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// Resume decoding a variable length integer after an earlier
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// call to Start or StartExtended returned kDecodeInProgress.
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DecodeStatus Resume(DecodeBuffer* db);
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uint32_t value() const;
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// This supports optimizations for the case of a varint with zero extension
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// bytes, where the handling of the prefix is done by the caller.
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void set_value(uint32_t v);
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// All the public methods below are for supporting assertions and tests.
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Http2String DebugString() const;
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// For benchmarking, these methods ensure the decoder
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// is NOT inlined into the caller.
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DecodeStatus StartForTest(uint8_t prefix_value,
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uint8_t prefix_mask,
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DecodeBuffer* db);
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DecodeStatus StartExtendedForTest(uint8_t prefix_mask, DecodeBuffer* db);
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DecodeStatus ResumeForTest(DecodeBuffer* db);
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static constexpr uint32_t MaxExtensionBytes() { return 5; }
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private:
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// Protection in case Resume is called when it shouldn't be.
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void MarkDone() {
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#ifndef NDEBUG
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// We support up to 5 extension bytes, so offset_ should never be > 28 when
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// it makes sense to call Resume().
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offset_ = MaxOffset() + 7;
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#endif
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}
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void CheckNotDone() const {
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#ifndef NDEBUG
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DCHECK_LE(offset_, MaxOffset());
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#endif
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}
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void CheckDone() const {
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#ifndef NDEBUG
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DCHECK_GT(offset_, MaxOffset());
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#endif
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}
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static constexpr uint32_t MaxOffset() {
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return 7 * (MaxExtensionBytes() - 1);
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}
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// These fields are initialized just to keep ASAN happy about reading
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// them from DebugString().
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uint32_t value_ = 0;
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uint32_t offset_ = 0;
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};
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} // namespace net
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#endif // NET_HTTP2_HPACK_DECODER_HPACK_VARINT_DECODER_H_
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