linux-kernel - Re: [PATCH RFC 2/8] fs/ext4: Disallow verity if inode is DAX

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Message-ID: <20200415191451.GA2305801@iweiny-DESK2.sc.intel.com>
Date:   Wed, 15 Apr 2020 12:14:52 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-kernel@...r.kernel.org,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jeff Moyer <jmoyer@...hat.com>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH RFC 2/8] fs/ext4: Disallow verity if inode is DAX

On Wed, Apr 15, 2020 at 02:00:02PM +0200, Jan Kara wrote:
> On Mon 13-04-20 21:00:24, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > Verity and DAX are incompatible.  Changing the DAX mode due to a verity
> > flag change is wrong without a corresponding address_space_operations
> > update.
> > 
> > Make the 2 options mutually exclusive by returning an error if DAX was
> > set first.
> > 
> > (Setting DAX is already disabled if Verity is set first.)
> > 
> > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> > ---
> >  fs/ext4/verity.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/fs/ext4/verity.c b/fs/ext4/verity.c
> > index dc5ec724d889..ce3f9a198d3b 100644
> > --- a/fs/ext4/verity.c
> > +++ b/fs/ext4/verity.c
> > @@ -113,6 +113,9 @@ static int ext4_begin_enable_verity(struct file *filp)
> >  	handle_t *handle;
> >  	int err;
> >  
> > +	if (WARN_ON_ONCE(IS_DAX(inode)))
> > +		return -EINVAL;
> > +
> 
> Hum, one question, is there a reason for WARN_ON_ONCE()? If I understand
> correctly, user could normally trigger this, couldn't he?

Ok.  I did not think this through but I did think about this.  I was following
the code from the encryption side which issues a warning and was thinking that
would be a good way to alert the user they are doing something wrong...

I think you are right about both of them but we also need to put something in
the verity, dax, and ...  (I can't find a file in Documentation which talks
about encryption right off) documentation files....  For verity something like.

<quote>
Verity and DAX
--------------

Verity and DAX are not compatible and attempts to set both of these flags on a
file will fail.
</quote>

And the same thing in the DAX doc?

And where would be appropriate for the encrypt doc?

Ira

> 
> 								Honza
> 
> >  	if (ext4_verity_in_progress(inode))
> >  		return -EBUSY;
> >  
> > -- 
> > 2.25.1
> > 
> -- 
> Jan Kara <jack@...e.com>
> SUSE Labs, CR