Date: Tue, 11 Jul 2006 14:27:52 +0100 From: Ceri Davies <ceri@submonkey.net> To: Robert Watson <rwatson@FreeBSD.org> Cc: Christoph Hellwig <hch@infradead.org>, Craig Rodrigues <rodrigc@crodrigues.org>, freebsd-current@freebsd.org, freebsd-arch@freebsd.org Subject: Re: [RFC] mount can figure out fstype automatically Message-ID: <20060711132752.GF65857@submonkey.net> In-Reply-To: <20060711124356.Y78628@fledge.watson.org> References: <20060708152801.GA3671@crodrigues.org> <44AFD7DF.8090002@errno.com> <20060708174606.GA29602@infradead.org> <44B2A51A.4040103@samsco.org> <20060710202219.GA29786@infradead.org> <20060711124356.Y78628@fledge.watson.org>
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On Tue, Jul 11, 2006 at 12:45:18PM +0100, Robert Watson wrote:
>=20
> On Mon, 10 Jul 2006, Christoph Hellwig wrote:
>=20
> >On Mon, Jul 10, 2006 at 01:06:02PM -0600, Scott Long wrote:
> >>So in your opinion and experience, what are the pros and cons of=20
> >>maintaining a table of magic numbers?
> >
> >The feature is imensely useful. The implementation won't win any points=
=20
> >for a clean design but works very well in practice. I think it's=20
> >definitly better than probing in the kernel because letting a filesystem=
=20
> >driver try to make sense of something that's not it's own format can lea=
d=20
> >to all kinds of funnies. Linux does this (iterating all filesystem type=
s=20
> >in kernel) for the special case of the root filesystem where mount(8) is=
=20
> >not available, and it showeds various interesting bugs at least in the f=
at=20
> >driver.
>=20
> In both FreeBSD and Darwin, I've noticed that the kernel msdosfs code is=
=20
> excessively permissive as to what it considers a FAT file system. This i=
s=20
> presumably necessary due to the enourmous diversity of FAT file systems=
=20
> floating around, but it makes it a little too easy to cause msdos to trip=
=20
> over layouts that violate its layout assumptions. :-) FAT is much more=
=20
> reliably detected by looking at the partition type it lives in than by=20
> looking at the bytes that appear inside the partition, I believe.
Assuming that there is a valid partition type. I don't really know what
this makes, but there's a valid FAT filesystem on it:
% truncate -s 1440k floppy
% sudo mdconfig -a -f floppy=20
md1
% sudo newfs_msdos -f 1440 /dev/md1
/dev/md1: 2847 sectors in 2847 FAT12 clusters (512 bytes/cluster)
bps=3D512 spc=3D1 res=3D1 nft=3D2 rde=3D224 sec=3D2880 mid=3D0xf0 spf=3D9 s=
pt=3D18 hds=3D2 hid=3D0
% sudo mount -t msdos /dev/md1 /mnt
% df -h /mnt
Filesystem Size Used Avail Capacity Mounted on
/dev/md1 1.4M 1.0K 1.4M 0% /mnt
{ceri@shrike}-{~} % fdisk /dev/md1
******* Working on device /dev/md1 *******
parameters extracted from in-core disklabel are:
cylinders=3D0 heads=3D255 sectors/track=3D63 (16065 blks/cyl)
parameters to be used for BIOS calculations are:
cylinders=3D0 heads=3D255 sectors/track=3D63 (16065 blks/cyl)
Media sector size is 512
Warning: BIOS sector numbering starts with sector 1
Information from DOS bootblock is:
The data for partition 1 is:
<UNUSED>
The data for partition 2 is:
<UNUSED>
The data for partition 3 is:
<UNUSED>
The data for partition 4 is:
<UNUSED>
Ceri
--=20
That must be wonderful! I don't understand it at all.
-- Moliere
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