Re: [PATCH] direct-io: Fix negative return from dio read beyond eof

[Jan Kara <jack@suse.cz>]

On Thu 19-11-15 21:25:34, Jan Kara wrote:
> Assume a filesystem with 4KB blocks. When a file has size 1000 bytes and
> we issue direct IO read at offset 1024, blockdev_direct_IO() reads the
> tail of the last block and the logic for handling short DIO reads in
> dio_complete() results in a return value -24 (1000 - 1024) which
> obviously confuses userspace.
> 
> Fix the problem by bailing out early once we sample i_size and can
> reliably check that direct IO read starts beyond i_size.
> 
> Reported-by: Avi Kivity <avi@scylladb.com>
> Fixes: 9fe55eea7e4b444bafc42fa0000cc2d1d2847275
> CC: stable@vger.kernel.org
> CC: Steven Whitehouse <swhiteho@redhat.com>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/direct-io.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> Avi, this patch fixes the issue for me.

Jens, can you pick up this fix please? Thanks!

								Honza

> diff --git a/fs/direct-io.c b/fs/direct-io.c
> index 18e7554cf94c..08094c9d8172 100644
> --- a/fs/direct-io.c
> +++ b/fs/direct-io.c
> @@ -1163,6 +1163,15 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
>  		}
>  	}
>  
> +	/* Once we sampled i_size check for reads beyond EOF */
> +	dio->i_size = i_size_read(inode);
> +	if (iov_iter_rw(iter) == READ && offset >= dio->i_size) {
> +		if (dio->flags & DIO_LOCKING)
> +			mutex_unlock(&inode->i_mutex);
> +		kmem_cache_free(dio_cache, dio);
> +		goto out;
> +	}
> +
>  	/*
>  	 * For file extending writes updating i_size before data writeouts
>  	 * complete can expose uninitialized blocks in dumb filesystems.
> @@ -1216,7 +1225,6 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
>  	sdio.next_block_for_io = -1;
>  
>  	dio->iocb = iocb;
> -	dio->i_size = i_size_read(inode);
>  
>  	spin_lock_init(&dio->bio_lock);
>  	dio->refcount = 1;
> -- 
> 2.1.4
> 
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-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR