linux-ext4 - Re: [PATCH v2 2/2] direct-io: fix stale data exposure from concurrent buffered read

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Date:	Wed, 11 May 2016 19:08:00 +0200
From:	Jan Kara <jack@...e.cz>
To:	Eryu Guan <guaneryu@...il.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	jmoyer@...hat.com
Subject: Re: [PATCH v2 2/2] direct-io: fix stale data exposure from
 concurrent buffered read

On Sun 08-05-16 02:31:50, Eryu Guan wrote:
> Currently direct writes inside i_size on a DIO_SKIP_HOLES filesystem are
> not allowed to allocate blocks(get_more_blocks() sets 'create' to 0
> before calling get_block() callback), if it's a sparse file, direct
> writes fall back to buffered writes to avoid stale data exposure from
> concurrent buffered read. But there're two cases that can result in
> stale data exposure are not correctly detected.
> 
> 1. The detection for "writing inside i_size" is not sufficient, writes
> can be treated as "extending writes" wrongly. For example, direct write
> 1FSB to a 1FSB sparse file on ext2/3/4, starting from offset 0, in this
> case it's writing inside i_size, but 'create' is non-zero, because
> 'block_in_file' and '(i_size_read(inode) >> blkbits' are both zero.
> 
> 2. Direct writes starting from or beyong i_size (not inside i_size) also
> could trigger block allocation and expose stale data. For example,
> consider a sparse file with i_size of 2k, and a write to offset 2k or 3k
> into the file, with a filesystem block size of 4k. (Thanks to Jeff Moyer
> for pointing this case out.)
> 
> The first problem can be demostrated by running ltp-aiodio test ADSP045
> many times. When testing on extN filesystems, I see test failures
> occasionally, buffered read could read non-zero (stale) data.
> 
> ADSP045: dio_sparse -a 4k -w 4k -s 2k -n 1
> 
> dio_sparse    0  TINFO  :  Dirtying free blocks
> dio_sparse    0  TINFO  :  Starting I/O tests
> non zero buffer at buf[0] => 0xffffffaa,ffffffaa,ffffffaa,ffffffaa
> non-zero read at offset 0
> dio_sparse    0  TINFO  :  Killing childrens(s)
> dio_sparse    1  TFAIL  :  dio_sparse.c:191: 1 children(s) exited abnormally
> 
> The second problem can also be reproduced easily by a hacked dio_sparse
> program, which accepts an option to specify the write offset.
> 
> What we should really do is to disable block allocation for writes that
> could result in filling holes inside i_size.
> 
> Signed-off-by: Eryu Guan <guaneryu@...il.com>

The patch looks good to me. Feel free to add:

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

								Honza

> ---
> 
> v2:
> - Fix the case Jeff pointed out as well
> - Update commit log
> 
>  fs/direct-io.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/direct-io.c b/fs/direct-io.c
> index 9d5aff9..5c13bbf 100644
> --- a/fs/direct-io.c
> +++ b/fs/direct-io.c
> @@ -632,8 +632,10 @@ static int get_more_blocks(struct dio *dio, struct dio_submit *sdio,
>  	map_bh->b_size = fs_count << i_blkbits;
>  
>  	/*
> -	 * For writes inside i_size on a DIO_SKIP_HOLES filesystem we
> -	 * forbid block creations: only overwrites are permitted.
> +	 * For writes that could fill holes inside i_size on a
> +	 * DIO_SKIP_HOLES filesystem we forbid block creations: only
> +	 * overwrites are permitted.
> +	 *
>  	 * We will return early to the caller once we see an
>  	 * unmapped buffer head returned, and the caller will fall
>  	 * back to buffered I/O.
> @@ -644,7 +646,7 @@ static int get_more_blocks(struct dio *dio, struct dio_submit *sdio,
>  	 */
>  	create = dio->rw & WRITE;
>  	if (dio->flags & DIO_SKIP_HOLES) {
> -		if (block_in_file < (i_size_read(inode) >> blkbits))
> +		if (fs_startblk <= ((i_size_read(inode) - 1) >> i_blkbits))
>  			create = 0;
>  	}
>  
> -- 
> 2.5.5
> 
> --
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-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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