Re: [RFC] mount can figure out fstype automatically

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.

Robert N M Watson
Computer Laboratory
University of Cambridge