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LibCompress: Use the correct LZMA repetition offset in all cases

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Tim Schumacher 2023-03-29 14:46:36 +02:00 committed by Andreas Kling
parent 9ccb0fc1d8
commit 023c64011c
Notes: sideshowbarker 2024-07-17 02:42:21 +09:00 
Author: https://github.com/timschumi
Commit: https://github.com/SerenityOS/serenity/commit/023c64011c
Pull-request: https://github.com/SerenityOS/serenity/pull/18082
Reviewed-by: https://github.com/kleinesfilmroellchen ✅
2 changed files with 14 additions and 3 deletions
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16
Userland/Libraries/LibCompress/Lzma.cpp
View file

@ -321,7 +321,7 @@ ErrorOr<void> LzmaDecompressor::decode_literal_to_output_buffer()
// Testing `(State > 7)` with actual test files yields errors, so the reference implementation appears to be the correct one.
if (m_state >= 7) {
u8 matched_byte = 0;
auto read_bytes = TRY(m_dictionary->read_with_seekback({ &matched_byte, sizeof(matched_byte) }, m_rep0 + 1));
auto read_bytes = TRY(m_dictionary->read_with_seekback({ &matched_byte, sizeof(matched_byte) }, current_repetition_offset()));
VERIFY(read_bytes.size() == sizeof(matched_byte));
do {
@ -455,6 +455,16 @@ ErrorOr<u32> LzmaDecompressor::decode_normalized_match_distance(u16 normalized_m
return (distance_prefix << number_of_alignment_bits) | TRY(decode_symbol_using_reverse_bit_tree(number_of_alignment_bits, m_alignment_bit_probabilities));
}
u32 LzmaDecompressor::current_repetition_offset() const
{
// LZMA never needs to read at offset 0 (i.e. the actual read head of the buffer).
// Instead, the values are remapped so that the rep-value n starts reading n + 1 bytes back.
// The special rep-value 0xFFFFFFFF is reserved for marking the end of the stream,
// so this should never overflow.
VERIFY(m_rep0 < NumericLimits<u32>::max());
return m_rep0 + 1;
}
ErrorOr<Bytes> LzmaDecompressor::read_some(Bytes bytes)
{
while (m_dictionary->used_space() < bytes.size() && m_dictionary->empty_space() != 0) {
@ -517,7 +527,7 @@ ErrorOr<Bytes> LzmaDecompressor::read_some(Bytes bytes)
}
u8 byte;
auto read_bytes = TRY(m_dictionary->read_with_seekback({ &byte, sizeof(byte) }, m_rep0 + 1));
auto read_bytes = TRY(m_dictionary->read_with_seekback({ &byte, sizeof(byte) }, current_repetition_offset()));
VERIFY(read_bytes.size() == sizeof(byte));
auto written_bytes = m_dictionary->write({ &byte, sizeof(byte) });
@ -600,7 +610,7 @@ ErrorOr<Bytes> LzmaDecompressor::read_some(Bytes bytes)
// "Also the decoder must check that "rep0" value is not larger than dictionary size
// and is not larger than the number of already decoded bytes."
if (m_rep0 > m_dictionary->seekback_limit())
if (current_repetition_offset() > m_dictionary->seekback_limit())
return Error::from_string_literal("rep0 value is larger than the possible lookback size");
// "Then the decoder must copy match bytes as described in

1
Userland/Libraries/LibCompress/Lzma.h
View file

@ -147,6 +147,7 @@ private:
u32 m_rep1 { 0 };
u32 m_rep2 { 0 };
u32 m_rep3 { 0 };
u32 current_repetition_offset() const;
static constexpr size_t maximum_number_of_position_bits = 4;
static constexpr size_t number_of_states = 12;