Fix checksum calculation

[msuhanov]

The checksum algorithm is not a plain XOR-32, there are two edge cases:

• when the final XOR-32 result is 0, the checksum is set to 1;
• when the final XOR-32 result is -1, the checksum is set to -2.

[williballenthin]

whoa, very cool. thanks for sharing. do you happen to have documentation of this, or discovered via testing?

[NiKiZe]

I'd argue that this is not true, but will try to test, @williballenthin whrw you able to confirm/reproduce ?

Would it be possible to get a test case for this?

[msuhanov]

Hello.

whoa, very cool. thanks for sharing. do you happen to have documentation of this, or discovered via testing?

This was discovered by disassembling a Windows kernel. You can also find the same algorithm in WRK and ReactOS. The test case (for the null XOR-32 result) is attached to this message: v-SOFTWARE - checksum.gz (Windows will mount this hive without an error, but an invalid checksum with a missing transaction log file will produce an error message). Also, this issue is documented here.