[TUHS] Bugs in V6 'dcheck'

Noel Chiappa jnc at mercury.lcs.mit.edu
Sat May 31 23:15:04 AEST 2014

So it turns out the 'dcheck' distributed with V6 has two (well, three, but
the third one was only a potential problem for me) bugs it.

The first was a fence-post error on a table clearing operation; it could
cause the entry for the last inode of the disk in the constructed table of
directory entry counts to start with a non-zero count when a second disk was
scanned. However, it was only triggered in very specific circumstances:

- A larger disk was listed before a smaller one (either in the command line,
    or compiled in)
- The inode on the larger disk corresponding to the last inode on the smaller
    one was in use

I can understand how they never ran across this one.

The other one, however, which was an un-initalized variable, should have
bitten them anytime they had more than one disk listed! It caused the
constructed table of directory entry counts to be partially or wholly
(depending on the size of the two disks) blank in all disks after the first
one, causing numerous (bogus) error reports.

(It was also amusing to find an un-used procedure in the source; it looks
like dcheck was written starting with the code for 'icheck' - which explains
the second bug; since the logic in icheck is subtly different, that variable
_is_ set properly in icheck.)

How this bug never bit them I cannot understand - unless they saw it, and
couldn't be bothered to find and fix it!

To me, it's completely amazing to find such a serious bug in such a critical
piece of widely-distributd code! A lesson for archaeologists...

Anyway, a fixed version is here:


if anyone cares/needs it.


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