BUGFIX-66

Bug 36: SECDED Wounded But Not Dead: Cleaning

This is a "single error correction, double error detection" (SECDED) extended Hamming code scrubber.
Each gigabyte of non-ECC DRAM flips a bit every few hours on average. We want to protect against such flips.
This function returns -1, 0, or 1. Triple-bit (or worse) errors are neither corrected nor reliably detected.
If all data bits and all checksum bits are correct, 0 is returned.
If exactly one bit is flipped (anywhere in the data or checksum), the flip is corrected and 1 is returned.
If exactly two bits are flipped (anywhere in the data or checksum), nothing is modified and -1 is returned.
All data bits and all checksum bits are XOR'ed together to yield the blanket parity bit.
The data bits are numbered 3+(2*i): 3, 5, 7, 9, ... so that each data bit gets an index having two or more 1-bits.
The index numbers for all 1-bits in the data are XOR'ed out of the checksum's index sum to yield the remainder.
If all data bits and all checksum bits are correct, the blanket parity bit is 0 and the remainder is 0.
If exactly two bits are flipped, the blanket parity bit is 0 and the remainder is not 0.
Otherwise, exactly one bit is flipped (in the data or checksum) and the blanket parity bit is 1.
If a data bit is flipped, the remainder is the index number of the flipped bit (and so has two or more 1-bits).
If a bit is flipped in the checksum's index sum, the remainder has a 1-bit only at the corresponding position.
Finally, if the checksum's overall parity bit is flipped, the remainder is 0.

Fix The Tiny Bug In This Go Code:

func secdedClean(a []byte, cksum *uint64) int { var ( all = *cksum idx = all >> 1 i = uint64(3) ) for _, a := range a { bits := uint64(a) all ^= bits for n := 8; n > 0; n-- { bit := bits & 1 idx ^= i & -bit bits >>= 1 i += 2 } } all ^= all >> 32 all ^= all >> 16 all ^= all >> 8 all ^= all >> 8 all ^= all >> 2 all ^= all >> 1 switch { case all&1 == 0: if idx == 0 { return 0 } return -1 case idx&(idx-1) == 0: if idx == 0 { idx = 1 } else { idx <<= 1 } *cksum ^= idx default: idx -= 3 idx >>= 1 hi, lo := idx>>3, idx&7 if hi >= uint64(len(a)) { return -1 } a[hi] ^= 1 << lo } return 1 }

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