amd64/89202: Kernel crash when accessing filesystem

[Bruce Evans]

On Thu, 17 Nov 2005, Ivo Janssen wrote:

> I recreated the filesystem without the huge -f (avg filesize) option,
> which we had initially set to 1Gb:
> # newfs -U -O2 -b 8192 -f 1024
> # tunefs -e 20480  (10x the default)
>
> With the defaults of avg filesize of 16384, we don't see a crash. I
> then started playing with tunefs.
> Increasing the avg filesize to -f 16777216 (16Mb), we don't see a crash.
> Increasing the avg filesize to -f 33554432 (32Mb), we repro the crash.

I have had the following comment about this bug (but haven't fixed it) for
many years

% Index: ffs_alloc.c
% ===================================================================
% RCS file: /home/ncvs/src/sys/ufs/ffs/ffs_alloc.c,v
% retrieving revision 1.121
% diff -u -2 -r1.121 ffs_alloc.c
% --- ffs_alloc.c	16 Jun 2004 09:47:25 -0000	1.121
% +++ ffs_alloc.c	28 Oct 2004 15:12:47 -0000
% @@ -964,4 +991,11 @@
%  		minbfree = 1;
%  	cgsize = fs->fs_fsize * fs->fs_fpg;
% +	/*
% +	 * XXX the following multiplication can overflow, since newfs can
% +	 * be abused to set fs_avgfilesize and fs_avgfpdir to preposterous
% +	 * values.  For the particular preposterous values of (64M, 16),
% +	 * the multiplication overflows to 0 and then dirsize = 0 sometimes
% +	 * causes division by 0.
% +	 */
%  	dirsize = fs->fs_avgfilesize * fs->fs_avgfpdir;
%  	curdirsize = avgndir ? (cgsize - avgbfree * fs->fs_bsize) / avgndir : 0;

This bug is easy to work around by not changing fs->fs_avgfilesize or
fs->fs_avgfpdir.  Tuning them has limited useful effects.

Bruce