[[Infowarrior] - NSA Said to Have Used Heartbleed Bug for Years]

Tue Apr 15 17:37:14 UTC 2014

I can't cite chapter and verse but I seem to remember this zeroing
problem was solved decades ago by just introducing a bit which said
this chunk of memory or disk is new (to this process) and not zeroed
but if there's any attempt to actually access it then read it back as
if it were filled with zeros, or alternatively zero it.

Sort of the complement of copy-on-write, you do it by lazy evaluation.

For a newly allocated disk sector for example you don't have to
actually zero it on the disk when allocated, you just return a block
of zeros if a process tries to read it (i.e,. the kernel mediates.)

Typically you allocate disk space (other than by writing blocks) by
doing a seek forward, maybe write a block, and then seek backwards to
access the virgin space in between.

But anything in that virgin space can be marked in kernel memory as
having to read back as zeros, no need to read anything at all from the
actual disk.

In fact, there's no need to actually allocate the block on disk which
is why we have the notion of files with "holes", see for example the
'tar' man or info page for discussion of dealing with file holes when
archiving. That is, whether to try to remember them as holes per se or
to actually write all the zeros.

It's important because it can create (it's a command line option) an
actual TB tar file which has expanded that hole into blocks of zeros,
a tar archive (e.g.) which might really be over a TB, or it might only
be a few blocks, header info plus info that the rest is just to be
treated as a TB hole. Or of course the tar archive could only appear
to be a TB file but really be a big hole itself.

This is not at all limited to tar, it's just some place it came up
very explicitly for the reasons I described.

Maybe that (lazy-evaluation of returning zeros) never got widely
implemented but I thought it showed up around BSD 4.3 ca 1984.

I think some of the models being presented her are somewhat
oversimplified.

-- 
        -Barry Shein

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