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LibCompress/Brotli: Update the lookback buffer with uncompressed data

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We previously skipped updating the lookback buffer when copying
uncompressed data, which resulted in a wrong total byte count.
With a wrong total byte count, our decompressor implementation
ended up choosing a wrong offset into the dictionary.
This commit is contained in:
Tim Schumacher 2024-01-01 13:46:50 +01:00 • committed by Andreas Kling
parent c3167afa3a
commit 707a36dd79
2 changed files with 61 additions and 0 deletions
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57
Tests/LibCompress/TestBrotli.cpp
View file

@ -6,9 +6,66 @@
#include <LibTest/TestCase.h>
#include <AK/BitStream.h>
#include <AK/MaybeOwned.h>
#include <AK/MemoryStream.h>
#include <LibCompress/Brotli.h>
#include <LibCore/File.h>
TEST_CASE(dictionary_use_after_uncompressed_block)
{
// This input file contains one block of uncompressed data ("WHF") and then invokes
// a copy command that, together with the default distance, results in a dictionary
// lookup-and-copy ("categories").
// That in particular isn't a special combination, but dictionary indices depend on
// the count of bytes that have been decompressed so far, and we previously had
// a bug where uncompressed data was unaccounted for.
auto stream = make<AllocatingMemoryStream>();
// Brotli operates on bits instead of bytes, so we can't easily use a well-documented byte array.
// Instead, assemble the test case on-the-fly via a bit stream.
auto stream_in = LittleEndianOutputBitStream { MaybeOwned<Stream>(*stream) };
MUST(stream_in.write_bits(0b0u, 1u)); // WBITS = 16
MUST(stream_in.write_bits(0b0u, 1u)); // ISLAST = false
MUST(stream_in.write_bits(0b00u, 2u)); // MNIBBLES = 4
MUST(stream_in.write_bits(2u, 16u)); // MLEN - 1 = 2
MUST(stream_in.write_bits(0b1u, 1u)); // ISUNCOMPRESSED = true
MUST(stream_in.align_to_byte_boundary());
MUST(stream_in.write_until_depleted("WHF"sv.bytes())); // Literal uncompressed data
MUST(stream_in.write_bits(0b1u, 1u)); // ISLAST = true
MUST(stream_in.write_bits(0b0u, 1u)); // ISLASTEMPTY = false
MUST(stream_in.write_bits(0b00u, 2u)); // MNIBBLES = 4
MUST(stream_in.write_bits(9u, 16u)); // MLEN - 1 = 9
MUST(stream_in.write_bits(0b0u, 1u)); // NBLTYPESL = 1
MUST(stream_in.write_bits(0b0u, 1u)); // NBLTYPESI = 1
MUST(stream_in.write_bits(0b0u, 1u)); // NBLTYPESD = 1
MUST(stream_in.write_bits(0b00u, 2u)); // NPOSTFIX = 0
MUST(stream_in.write_bits(0b0000u, 4u)); // NDIRECT = 0
MUST(stream_in.write_bits(0b10u, 2u)); // CMODE[0] = 2
MUST(stream_in.write_bits(0b0u, 1u)); // NTREESL = 1
MUST(stream_in.write_bits(0b0u, 1u)); // NTREESD = 1
MUST(stream_in.write_bits(0b01u, 2u)); // literal_codes[0] hskip = 1
MUST(stream_in.write_bits(0b00u, 2u)); // literal_codes[0] number of symbols - 1 = 0
MUST(stream_in.write_bits(0u, 8u)); // literal_codes[0] symbols[0] = 0 (unused)
MUST(stream_in.write_bits(0b01u, 2u)); // iac_codes[0] hskip = 1
MUST(stream_in.write_bits(0b00u, 2u)); // iac_codes[0] number of symbols - 1 = 0
MUST(stream_in.write_bits(64u, 10u)); // iac_codes[0] symbols[0] = 64 (index = 1, insert_offset = 0, copy_offset = 0)
MUST(stream_in.write_bits(0b01u, 2u)); // distance_codes[0] hskip = 1
MUST(stream_in.write_bits(0b00u, 2u)); // distance_codes[0] number of symbols - 1 = 0
MUST(stream_in.write_bits(0u, 6u)); // distance_codes[0] symbols[0] = 0 (unused)
MUST(stream_in.align_to_byte_boundary());
MUST(stream_in.flush_buffer_to_stream());
auto decompressor = Compress::BrotliDecompressionStream { MaybeOwned<Stream>(*stream) };
auto buffer = TRY_OR_FAIL(decompressor.read_until_eof());
EXPECT_EQ(buffer.span(), "WHFcategories"sv.bytes());
}
static void run_test(StringView const file_name)
{
// This makes sure that the tests will run both on target and in Lagom.

4
Userland/Libraries/LibCompress/Brotli.cpp
View file

@ -735,6 +735,10 @@ ErrorOr<Bytes> BrotliDecompressionStream::read_some(Bytes output_buffer)
if (uncompressed_bytes.is_empty())
return Error::from_string_literal("eof");
// TODO: Replace the home-grown LookbackBuffer with AK::CircularBuffer.
for (auto c : uncompressed_bytes)
m_lookback_buffer.value().write(c);
m_bytes_left -= uncompressed_bytes.size();
bytes_read += uncompressed_bytes.size();