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

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Message-ID: <20200416012901.GA816@sol.localdomain>
Date:   Wed, 15 Apr 2020 18:29:01 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Jan Kara <jack@...e.cz>, 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 12:14:52PM -0700, Ira Weiny wrote:
> 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?
> 

Documentation/filesystems/fscrypt.rst mentions that DAX isn't supported on
encrypted files, but it doesn't say what happens if someone tries to do it
anyway.  Feel free to improve the documentation.

- Eric