Re: [PATCH v3 3/3] direct-io: defer alignment check until after the EOF check

[Jan Kara <jack@suse.cz>]

On Sat 05-09-20 01:20:23, Gabriel Krisman Bertazi wrote:
> Prior to commit 9fe55eea7e4b ("Fix race when checking i_size on direct
> i/o read"), an unaligned direct read past end of file would trigger EOF,
> since generic_file_aio_read detected this read-at-EOF condition and
> skipped the direct IO read entirely, returning 0. After that change, the
> read now reaches dio_generic, which detects the misalignment and returns
> EINVAL.
> 
> This consolidates the generic direct-io to follow the same behavior of
> filesystems.  Apparently, this fix will only affect ocfs2 since other
> filesystems do this verification before calling do_blockdev_direct_IO,
> with the exception of f2fs, which has the same bug, but is fixed in the
> next patch.
> 
> it can be verified by a read loop on a file that does a partial read
> before EOF (On file that doesn't end at an aligned address).  The
> following code fails on an unaligned file on filesystems without
> prior validation without this patch, but not on btrfs, ext4, and xfs.
> 
>   while (done < total) {
>     ssize_t delta = pread(fd, buf + done, total - done, off + done);
>     if (!delta)
>       break;
>     ...
>   }
> 
> Fix this regression by moving the misalignment check to after the EOF
> check added by commit 74cedf9b6c60 ("direct-io: Fix negative return from
> dio read beyond eof").
> 
> Based on a patch by Jamie Liu.
> 
> Reported-by: Jamie Liu <jamieliu@google.com>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>

The patch looks good to me. You can add:

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

								Honza

> ---
>  fs/direct-io.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/direct-io.c b/fs/direct-io.c
> index c17efe58f1c9..82838cca934b 100644
> --- a/fs/direct-io.c
> +++ b/fs/direct-io.c
> @@ -1165,14 +1165,6 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
>  	 * the early prefetch in the caller enough time.
>  	 */
>  
> -	if (align & blocksize_mask) {
> -		if (bdev)
> -			blkbits = blksize_bits(bdev_logical_block_size(bdev));
> -		blocksize_mask = (1 << blkbits) - 1;
> -		if (align & blocksize_mask)
> -			return -EINVAL;
> -	}
> -
>  	/* watch out for a 0 len io from a tricksy fs */
>  	if (iov_iter_rw(iter) == READ && !count)
>  		return 0;
> @@ -1200,6 +1192,14 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
>  		goto fail_dio;
>  	}
>  
> +	if (align & blocksize_mask) {
> +		if (bdev)
> +			blkbits = blksize_bits(bdev_logical_block_size(bdev));
> +		blocksize_mask = (1 << blkbits) - 1;
> +		if (align & blocksize_mask)
> +			goto fail_dio;
> +	}
> +
>  	if (dio->flags & DIO_LOCKING && iov_iter_rw(iter) == READ) {
>  		struct address_space *mapping = iocb->ki_filp->f_mapping;
>  
> -- 
> 2.28.0
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR