Re: [PATCH 3/3] [fs/sysv] V7: Add support for non-PDP11 v7 filesystems

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Jul 19, 2010 at 04:58:51PM +0200, Christoph Hellwig wrote:
> On Mon, Jul 19, 2010 at 07:16:42PM +0200, Lubomir Rintel wrote:
> > A mount-time option was added that makes it possible to override the
> > endianness and an attempt is made to autodetect it (which seems easy,
> > given the disk addresses are 3-byte.
> > 
> > No attempt is made to detect big-endian filesystems -- were there any?
> > Tested with PDP-11 v7 filesystems and PC-IX maintenance floppy.
> 
> Do you actually need the mount option?  We get away just fine with
> it for sysv filesystems.  And if not I'd be consistent and accept the
> options for both sysv and v7 filesystems.

Actually, it shouldn't be too hard to detect the damn thing even without magic.
Look - we always have the inode table starting at block 2, so on-disk root
inode is guaranteed to be found correctly.  Now, suppose we'd mistaken
l-e for pdp or vice versa; the half-words of i_size would get swapped.
What could pass both tests?  Suppose the right size is a * 65536 + b;
then we have: a and b are both multiples of 16 and at least one is non-zero.
So all we need is to reject root directories bigger than 1Mb.  And posted
patches do reject that (and lower than that, actually).

So I'd rather see a variant without that option.  Simply get both bh, then
try the same sanity checks with LE and PDP used for s_bytesex.  And use
one that works - we _know_ that it's impossible to have both pass at the
same time.

I'm fine with the rest of patch series as is; Lubomir, could you redo the
last one that way and resend?