linux-ext4 - Re: [PATCH] ext4: remove redundant check for encrypted file on dio write path

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Date:   Wed, 24 May 2017 10:10:41 +0200
From:   Jan Kara <jack@...e.cz>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
        Theodore Ts'o <tytso@....edu>, linux-fscrypt@...r.kernel.org,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] ext4: remove redundant check for encrypted file on dio
 write path

On Tue 23-05-17 09:13:54, Eric Biggers wrote:
> Hi Jan,
> 
> On Tue, May 23, 2017 at 10:24:10AM +0200, Jan Kara wrote:
> > On Mon 22-05-17 17:53:16, Eric Biggers wrote:
> > > From: Eric Biggers <ebiggers@...gle.com>
> > > 
> > > Currently we don't allow direct I/O on encrypted regular files, so in
> > > such cases we return 0 early in ext4_direct_IO().  There was also an
> > > additional BUG_ON() check in ext4_direct_IO_write(), but it can never
> > > be hit because of the earlier check for the exact same condition in
> > > ext4_direct_IO().  There was also no matching check on the read path,
> > > which made the write path specific check seem very ad-hoc.
> > > 
> > > Just remove the unnecessary BUG_ON().
> > > 
> > > Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> > 
> > Yeah, the check is rather before the BUG_ON so I guess that there's no
> > big point in the BUG_ON. When looking at this code I have one question
> > though:
> > 
> > So when you mount the filesystem with 'dioread_nolock', do overwriting
> > direct write to the file, and just after we do inode_unlock() in
> > ext4_direct_IO_write() someone calls EXT4_IOC_SET_ENCRYPTION_POLICY
> > ioctl on the file, the BUG_ON could actually trigger. So I think you
> > need to wait for outstanding direct IO for the file when setting
> > encryption policy. Likely in ext4_set_context() or maybe in the generic
> > fscrypt code (you need to wait after acquiring inode_lock), I'm not
> > sure how other filesystems using fscrypt handle this and whether it
> > would make more sense in the generic code or in ext4 specific one.
> > 
> 
> That's not possible because the ioctl can only set an encryption policy
> on a directory, and specifically an empty one.  Other files can only
> acquire an encryption policy through inheritance.  There have been
> thoughts about implementing "in-place" encryption but it's not something
> we currently support.

Ah, good. Thanks for explanation. Then you can add:

Reviewed-by: Jan Kara <jack@...e.cz>

to your patch.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR