Date: Tue, 11 Jul 2006 09:54:00 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-arch@freebsd.org Cc: Craig Rodrigues <rodrigc@crodrigues.org>, Ceri Davies <ceri@submonkey.net>, Robert Watson <rwatson@freebsd.org>, freebsd-current@freebsd.org Subject: Re: [RFC] mount can figure out fstype automatically Message-ID: <200607110954.01691.jhb@freebsd.org> In-Reply-To: <20060711132752.GF65857@submonkey.net> References: <20060708152801.GA3671@crodrigues.org> <20060711124356.Y78628@fledge.watson.org> <20060711132752.GF65857@submonkey.net>
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On Tuesday 11 July 2006 09:27, Ceri Davies wrote:
> On Tue, Jul 11, 2006 at 12:45:18PM +0100, Robert Watson wrote:
> >
> > On Mon, 10 Jul 2006, Christoph Hellwig wrote:
> >
> > >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
> > >>maintaining a table of magic numbers?
> > >
> > >The feature is imensely useful. The implementation won't win any points
> > >for a clean design but works very well in practice. I think it's
> > >definitly better than probing in the kernel because letting a filesystem
> > >driver try to make sense of something that's not it's own format can lead
> > >to all kinds of funnies. Linux does this (iterating all filesystem types
> > >in kernel) for the special case of the root filesystem where mount(8) is
> > >not available, and it showeds various interesting bugs at least in the
fat
> > >driver.
> >
> > In both FreeBSD and Darwin, I've noticed that the kernel msdosfs code is
> > excessively permissive as to what it considers a FAT file system. This is
> > presumably necessary due to the enourmous diversity of FAT file systems
> > floating around, but it makes it a little too easy to cause msdos to trip
> > over layouts that violate its layout assumptions. :-) FAT is much more
> > reliably detected by looking at the partition type it lives in than by
> > 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
> md1
> % sudo newfs_msdos -f 1440 /dev/md1
> /dev/md1: 2847 sectors in 2847 FAT12 clusters (512 bytes/cluster)
> bps=512 spc=1 res=1 nft=2 rde=224 sec=2880 mid=0xf0 spf=9 spt=18 hds=2 hid=0
> % 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=0 heads=255 sectors/track=63 (16065 blks/cyl)
>
> parameters to be used for BIOS calculations are:
> cylinders=0 heads=255 sectors/track=63 (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
Dos floppies don't have an MBR (so fdisk on them is meaningless).
--
John Baldwin
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