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Date:	Thu, 8 Oct 2015 11:08:17 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Jan Kara <jack@...e.com>, linux-ext4 <linux-ext4@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>,
	"Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH] ext2: don't mount filesystems with compat features we
 know only ext4 supports

On Wed, Oct 07, 2015 at 03:29:55PM +0200, Jan Kara wrote:
> On Wed 30-09-15 09:36:35, Darrick J. Wong wrote:
> > On Wed, Sep 30, 2015 at 10:41:26AM +0200, Jan Kara wrote:
> > > On Mon 28-09-15 15:36:26, Darrick J. Wong wrote:
> > > > The ext2 mount code never checks the compat features against the ones
> > > > it knows about.  This is correct behavior since compat features are
> > > > supposed to be rw-compatible with old drivers; however, for certain
> > > > configurations (journalled rootfs) we probably want the ext4 driver
> > > > to load, not ext2.
> > > > 
> > > > Reported-by: Dave Chinner <david@...morbit.com>
> > > > Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> > > 
> > > Isn't this a bit too harsh? I agree we should at least warn (and that's
> > > probably regardless of EXT4 config option) but just refusing mount looks
> > > too much to me...
> > 
> > I admit that refusing the mount might be a bit much; the goal here was
> > merely to make it so that if the FS has a journal and ext4 was turned on,
> > hopefully ext2 rejects the mount and ext4 will probe it next.
> > 
> > <shrug>
> 
> OK, after some more thought about this I agree that your solution is
> probably the best. I'd say it's exceedingly rare that someone would like to
> mount ext3 filesystem using ext2 driver when he has ext4 available.
> Definitely more rare than someone wanting to mount clean ext3 filesystem
> and unexpectedly using ext2 driver for that. So my:

Now that I've thought about it more, I don't think that it's a good idea to
forever forbid mounting ext3 on ext2.ko.  I'll send a patch to change the
default probing order along with an advisory to specify rootfstype= if you
compile both ext4 and ext2 and want specifically ext2 to drive.

That said, I still think that it's rare that anyone would really want to mount
ext3 with ext2.ko other than perhaps as a desperate last resort to get data off
the filesystem.  Perhaps this patch will turn into something requiring readonly
mounting if ext4 was built and has_journal is on... after changing the default
probe order.

--D

> 
> Acked-by: Jan Kara <jack@...e.com>
> 
> Ted, can you please pick up the patch? Thanks!
> 
> 								Honza
> > > > ---
> > > >  fs/ext2/ext2.h  |    6 +++++-
> > > >  fs/ext2/super.c |   13 ++++++++++---
> > > >  2 files changed, 15 insertions(+), 4 deletions(-)
> > > > 
> > > > diff --git a/fs/ext2/ext2.h b/fs/ext2/ext2.h
> > > > index 8d15feb..ce508b1 100644
> > > > --- a/fs/ext2/ext2.h
> > > > +++ b/fs/ext2/ext2.h
> > > > @@ -547,7 +547,11 @@ struct ext2_super_block {
> > > >  #define EXT2_FEATURE_INCOMPAT_META_BG		0x0010
> > > >  #define EXT2_FEATURE_INCOMPAT_ANY		0xffffffff
> > > >  
> > > > -#define EXT2_FEATURE_COMPAT_SUPP	EXT2_FEATURE_COMPAT_EXT_ATTR
> > > > +#define EXT2_FEATURE_COMPAT_SUPP	(EXT2_FEATURE_COMPAT_DIR_PREALLOC| \
> > > > +					 EXT2_FEATURE_COMPAT_IMAGIC_INODES| \
> > > > +					 EXT2_FEATURE_COMPAT_EXT_ATTR| \
> > > > +					 EXT2_FEATURE_COMPAT_RESIZE_INO| \
> > > > +					 EXT2_FEATURE_COMPAT_DIR_INDEX)
> > > >  #define EXT2_FEATURE_INCOMPAT_SUPP	(EXT2_FEATURE_INCOMPAT_FILETYPE| \
> > > >  					 EXT2_FEATURE_INCOMPAT_META_BG)
> > > >  #define EXT2_FEATURE_RO_COMPAT_SUPP	(EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER| \
> > > > diff --git a/fs/ext2/super.c b/fs/ext2/super.c
> > > > index 900e19c..4efb018 100644
> > > > --- a/fs/ext2/super.c
> > > > +++ b/fs/ext2/super.c
> > > > @@ -896,6 +896,16 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
> > > >  	 * previously didn't change the revision level when setting the flags,
> > > >  	 * so there is a chance incompat flags are set on a rev 0 filesystem.
> > > >  	 */
> > > > +#ifdef CONFIG_EXT4_FS
> > > > +	/* Journalled FS should mount with ext4 if it's available */
> > > > +	features = EXT2_HAS_COMPAT_FEATURE(sb, ~EXT2_FEATURE_COMPAT_SUPP);
> > > > +	if (features) {
> > > > +		ext2_msg(sb, KERN_ERR,	"error: won't mount because of "
> > > > +		       "unsupported optional features (%x); try ext4",
> > > > +			le32_to_cpu(features));
> > > > +		goto failed_mount;
> > > > +	}
> > > > +#endif
> > > >  	features = EXT2_HAS_INCOMPAT_FEATURE(sb, ~EXT2_FEATURE_INCOMPAT_SUPP);
> > > >  	if (features) {
> > > >  		ext2_msg(sb, KERN_ERR,	"error: couldn't mount because of "
> > > > 
> > > -- 
> > > Jan Kara <jack@...e.com>
> > > SUSE Labs, CR
> > > --
> > > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
> > > the body of a message to majordomo@...r.kernel.org
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> > --
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> -- 
> Jan Kara <jack@...e.com>
> SUSE Labs, CR
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