linux-ext4 - Re: [PATCH] ext4: check incompatible mount options when mounting ext2/3 [V2]

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Message-ID: <20131031140040.GD14337@orion.maiolino.org>
Date:	Thu, 31 Oct 2013 12:00:41 -0200
From:	Carlos Maiolino <cmaiolino@...hat.com>
To:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: check incompatible mount options when mounting
 ext2/3 [V2]

Hi guys, my apologies to have not touched this conversation for a while, got
busy with other stuff.

So, let me continue this.

On Wed, Jan 23, 2013 at 02:00:37PM +0100, Jan Kara wrote:
> On Wed 23-01-13 07:22:49, Carlos Maiolino wrote:
> > On Wed, Jan 23, 2013 at 11:39:17AM +0100, Jan Kara wrote:
> > > On Tue 22-01-13 11:07:58, Carlos Maiolino wrote:
> > > > This checks for incompatible mounting options when using ext4 module to mount
> > > > ext3 or ext2 filesystems.
> > > > 
> > > > Sets two new flags to group ext4 mount options that are incompatible with ext2
> > > > and ext3, and then add two functions -- check_ext2/3_incompat_mount() -- to
> > > > check and warn/fail mount, if any of these options are being used.
> > > > 
> > > > I believe, some options like those expecting an argument needs to be checked
> > > > during parsing time.
> > > > 
> > > > barrier mount, although it has a flag, when mounting an ext2fs, where
> > > > barriers are not supported (afaik), should also be checked during parse
> > > > time, otherwise the BARRIER mount flag will be set.
> > > > 
> > > > I didn't add all mount options I believe to need to raise a warning, just
> > > > those with a flag set on superblock, another flags should be added after a
> > > > discussion to reach a concensus of all ext2/3 options that should be rejected by
> > > > ext4 mount.
> > >   Thinking about it a bit more I'm not sure if restricting mount options is
> > > the right thing to start with.  IMHO what we should restrict is mounting
> > > filesystem with certain *features* as ext3/ext2. So e.g. filesystem with
> > > EXT4_FEATURE_INCOMPAT_EXTENTS cannot be mounted as ext2 or ext3. Similarly
> > > as currently we forbid mounting ext3 filesystem with
> > > EXT3_FEATURE_INCOMPAT_RECOVER as ext2... This should avoid the confusion
> > > which could arise when someone reports problems with "ext3" filesystem but
> > > actually has some of the ext4 features enabled.
> > > 
> > This is interesting, but I wonder if not restricting mount options, but
> > features, would open a 'window' to let users change their filesystem on-disk
> > format without know what they are doing, but I might be wrong.
>   If there are mount options that enable features, then these should be
> disallowed for ext2/ext3 mounts. But I think we already got rid of these
> traps on users...
> 
> 								Honza
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR

Jan, I agree with the idea of check for incompat features and I can work on that
with no problem, but I still think we need to also ensure some mount options are
not allowed to be used with some specific ExtFS versions wheter it change the
disk format (we probably got rid of these as you said) or not. Not sure if my
previous (V2) to restrict mount options lacks something or need to be reworked
in some way (of course now it should to fit new kernel versions).

As an example, commit=<secs> option doesn't make sense to be used on ext2
filesystems, but you can mount an ext2 FS with this option with no problem.


-- 
Carlos
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