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LibVideo: Improve error reporting for VP9 range decoder

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init_bool will now check whether there is enough data in the bitstream
for the range coding size to be fully read.

exit_bool will now read the entire padding element regardless of size,
which the spec does not specify a limit on.
This commit is contained in:
Zaggy1024 2022-10-08 20:06:15 -05:00 committed by Andrew Kaster
parent 7f46033c01
commit 13ccde8637
Notes: sideshowbarker 2024-07-17 07:16:27 +09:00 
Author: https://github.com/Zaggy1024
Commit: https://github.com/SerenityOS/serenity/commit/13ccde8637
Pull-request: https://github.com/SerenityOS/serenity/pull/15363
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/FireFox317
Reviewed-by: https://github.com/MacDue
Reviewed-by: https://github.com/davidot
2 changed files with 19 additions and 9 deletions
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25
Userland/Libraries/LibVideo/VP9/BitStream.cpp
View file

@ -71,13 +71,13 @@ ErrorOr<u16> BitStream::read_f16()
/* 9.2.1 */
ErrorOr<void> BitStream::init_bool(size_t bytes)
{
if (bytes == 0)
return Error::from_string_literal("Range coder size cannot be zero.");
if (bytes > bytes_remaining())
return Error::from_string_literal("Available data is too small for range decoder");
m_bool_value = TRY(read_f8());
m_bool_range = 255;
m_bool_max_bits = (8 * bytes) - 8;
if (TRY(read_bool(128)))
return Error::from_string_literal("Range coder's first bool was non-zero.");
return Error::from_string_literal("Range decoder marker was non-zero");
return {};
}
@ -114,17 +114,26 @@ ErrorOr<bool> BitStream::read_bool(u8 probability)
/* 9.2.3 */
ErrorOr<void> BitStream::exit_bool()
{
// FIXME: I'm not sure if this call to min is spec compliant, or if there is an issue elsewhere earlier in the parser.
auto padding_element = TRY(read_bits(min(m_bool_max_bits, (u64)bits_remaining())));
while (m_bool_max_bits > 0) {
auto padding_read_size = min(m_bool_max_bits, sizeof(m_reservoir) * 8);
auto padding_bits = TRY(read_bits(padding_read_size));
m_bool_max_bits -= padding_read_size;
if (padding_bits != 0)
return Error::from_string_literal("Range decoder has non-zero padding element");
}
// FIXME: It is a requirement of bitstream conformance that enough padding bits are inserted to ensure that the final coded byte of a frame is not equal to a superframe marker.
// A byte b is equal to a superframe marker if and only if (b & 0xe0)is equal to 0xc0, i.e. if the most significant 3 bits are equal to 0b110.
if (padding_element != 0)
return Error::from_string_literal("Range coder padding was non-zero.");
return {};
}
ErrorOr<u8> BitStream::read_literal(size_t n)
size_t BitStream::range_coding_bits_remaining()
{
return m_bool_max_bits;
}
ErrorOr<u8> BitStream::read_literal(u8 n)
{
u8 return_value = 0;
for (size_t i = 0; i < n; i++) {

3
Userland/Libraries/LibVideo/VP9/BitStream.h
View file

@ -32,7 +32,8 @@ public:
ErrorOr<void> init_bool(size_t bytes);
ErrorOr<bool> read_bool(u8 probability);
ErrorOr<void> exit_bool();
ErrorOr<u8> read_literal(size_t n);
ErrorOr<u8> read_literal(u8 bit_count);
size_t range_coding_bits_remaining();
/* (4.9.2) */
ErrorOr<i8> read_s(size_t n);