[TUHS] Bugs in V6 'dcheck'

Clem Cole clemc at ccc.com
Sun Jun 1 05:48:09 AEST 2014

btw.  there is a v6 version of fsck floating around.  CMU was still running v6 when Ted did his OYOC.  he later moved it to unix/TS (aka v7) which was the release mechanics out side of BTL.   I'm wonder if I can find a readable copy. 

funny I remember the first time fsck saved our butts     we had had power failure and corrupted two file systems on the EE dept 11/34 I was running Icheck/dcheck and the like trying patch the system and getting it running a gaining.  this new grad student (tjk) says he has something he wanted to try that he wrote over the summer /he had started writing at michiga.     the machine was dead Ted had it on a tape and getting it on to an rk05 was a trick  - we ended up doing it on the IUS system in CS.  

once it was running we were in awe   man that program migrated to all the CMU pdp11s within hours

> On May 31, 2014, at 9:15 AM, jnc at mercury.lcs.mit.edu (Noel Chiappa) wrote:
> 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:
>  http://ana-3.lcs.mit.edu/~jnc/tech/unix/ucmd/dcheck.c
> if anyone cares/needs it.
>    Noel
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