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From: bruner@uicsrd.csrd.uiuc.edu (John Bruner)
Newsgroups: comp.unix.wizards
Subject: Re: Can directory files have holes in them ?
Message-ID: <1989Oct11.131534.7553@ux1.cso.uiuc.edu>
Date: 11 Oct 89 13:15:34 GMT
References: <1212@accuvax.nwu.edu> <442@ecs.soton.ac.uk> <1240@virtech.UUCP> <20044@mimsy.UUCP>
Sender: news@ux1.cso.uiuc.edu (News)
Reply-To: bruner@uicsrd.csrd.uiuc.edu (John Bruner)
Organization: Center for Supercomputing Research and Development, Univ. of Illinois
Lines: 30

In article <20044@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:
>The original question was why dump is careful not to look at holes
>in directories.  The only possible answer is `paranoia'.

The paranoid code in dump and the kernel was added because of a
problem I ran into at Purdue on either a V7 PDP-11 or 4.1BSD VAX
(probably an 11/70) several years ago.  A hardware problem caused
a block pointer in a directory inode to be zeroed.  (I do not
remember the exact circumstances, except that it was a hardware
problem and it did not totally destroy the disc.)  "fsck" passed
the filesystem without a complaint, but any attempt to access the
directory (which I recall was something innocuous like "/usr/bin")
caused a panic.

In those days I believe that namei() did something like

	bp = bread(ip->i_dev, bmap(ip, lbn, B_READ));

The bug was that namei() didn't check whether bmap() returned -1
because holes in directories were "impossible".  My fix was to report
the hole on the console and allocate a new block if the filesystem
was mounted read/write; otherwise, it just skipped to the next block.
As I recall, "fsck" had code which checked for holes in files, but it
was commented out.  I turned on the check for directories.
I sent mail to Berkeley, and they integrated these changes into the
4.2BSD filesystem code (although they removed the code which filled
in the hole).
--
John Bruner	Center for Supercomputing R&D, University of Illinois
	bruner@uicsrd.csrd.uiuc.edu	(217) 244-4476