lib/zstd
1
0
Fork 0
mirror of https://github.com/facebook/zstd.git synced 2025-07-29 11:21:22 +03:00
Code Activity

fixed decoder behavior when nbSeqs==0 is encoded using 2 bytes

Browse Source 
The sequence section starts with a number, which tells how sequences are present in the section.
If this number if 0, the section automatically ends.

The number 0 can be represented using the 1 byte or the 2 bytes formats.
That's because the 2-bytes formats fully overlaps the 1 byte format.

However, when 0 is represented using the 2-bytes format,
the decoder was expecting the sequence section to continue,
and was looking for FSE tables, which is incorrect.

Fixed this behavior, in both the reference decoder and the educational behavior.

In practice, this behavior never happens,
because the encoder will always select the 1-byte format to represent 0,
since this is more efficient.

Completed the fix with a new golden sample for tests,
a clarification of the specification,
and a decoder errata paragraph.
This commit is contained in:
Yann Collet
2023-06-05 16:03:00 -07:00
parent 3e815f5b3a
commit 3732a08f5b
6 changed files with 44 additions and 18 deletions
Download Patch File Download Diff File Expand all files Collapse all files

13
doc/educational_decoder/zstd_decompress.c
View File

@ -1018,12 +1018,7 @@ static size_t decode_sequences(frame_context_t *const ctx, istream_t *in,
// This is a variable size field using between 1 and 3 bytes. Let's call its
// first byte byte0."
u8 header = IO_read_bits(in, 8);
if (header == 0) {
// "There are no sequences. The sequence section stops there.
// Regenerated content is defined entirely by literals section."
*sequences = NULL;
return 0;
} else if (header < 128) {
if (header < 128) {
// "Number_of_Sequences = byte0 . Uses 1 byte."
num_sequences = header;
} else if (header < 255) {
@ -1034,6 +1029,12 @@ static size_t decode_sequences(frame_context_t *const ctx, istream_t *in,
num_sequences = IO_read_bits(in, 16) + 0x7F00;
}
if (num_sequences == 0) {
// "There are no sequences. The sequence section stops there."
*sequences = NULL;
return 0;
}
*sequences = malloc(num_sequences * sizeof(sequence_command_t));
if (!*sequences) {
BAD_ALLOC();