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LibGfx: Fix crash on decoding small gifs

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The LZW decode step will now copy and pad LZW data out to 4 bytes if there are
less than 4 bytes remaining in the buffer. This means it will now also work when
the total size of the LZW image data is less than 4 bytes.
This commit is contained in:
Peter Nelson 2020-04-25 17:30:09 +01:00 • committed by Andreas Kling
parent 421619fe2f
commit 5adf4901df
1 changed files with 11 additions and 11 deletions
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22
Libraries/LibGfx/GIFLoader.cpp
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@ -169,19 +169,19 @@ public:
u8 current_bit_offset = m_current_bit_index % 8;
u32 mask = (u32)(pow(2, m_code_size) - 1) << current_bit_offset;
// Make sure that we don't read bytes past the end of the data.
int bytes_past_end = current_byte_index + sizeof(mask) - m_lzw_bytes.size();
u8 bits_past_end = 0;
if (bytes_past_end > 0) {
current_byte_index -= bytes_past_end;
mask <<= bytes_past_end * 8;
bits_past_end = bytes_past_end * 8;
// Make a padded copy of the final bytes in the data to ensure we don't read past the end.
if (current_byte_index + sizeof(mask) > m_lzw_bytes.size()) {
u8 padded_last_bytes[sizeof(mask)] = { 0 };
for (int i = 0; current_byte_index + i < m_lzw_bytes.size(); ++i) {
padded_last_bytes[i] = m_lzw_bytes[current_byte_index + i];
}
const u32* addr = (const u32*)&padded_last_bytes;
m_current_code = (*addr & mask) >> current_bit_offset;
} else {
const u32* addr = (const u32*)&m_lzw_bytes.at(current_byte_index);
m_current_code = (*addr & mask) >> current_bit_offset;
}
ASSERT(current_byte_index + sizeof(mask) - 1 < m_lzw_bytes.size());
const u32* addr = (const u32*)&m_lzw_bytes.at(current_byte_index);
m_current_code = (*addr & mask) >> (current_bit_offset + bits_past_end);
if (m_current_code > m_code_table.size()) {
dbg() << "Corrupted LZW stream, invalid code: " << m_current_code << " at bit index: "
<< m_current_bit_index << ", code table size: " << m_code_table.size();