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From:   Andreas Dilger <adilger@dilger.ca>
Message-Id: <F009F595-F69B-4430-BF18-71E4EE022883@dilger.ca>
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Subject: Re: [PATCH 2/2] ext4: make sure ext4_append() always allocates new
 block
Date:   Sat, 9 Jul 2022 11:32:28 -0600
In-Reply-To: <20220704142721.157985-2-lczerner@redhat.com>
Cc:     Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Theodore Ts'o <tytso@mit.edu>
To:     Lukas Czerner <lczerner@redhat.com>
References: <20220704142721.157985-1-lczerner@redhat.com>
 <20220704142721.157985-2-lczerner@redhat.com>
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On Jul 4, 2022, at 8:27 AM, Lukas Czerner <lczerner@redhat.com> wrote:
> 
> ext4_append() must always allocate a new block, otherwise we run the
> risk of overwriting existing directory block corrupting the directory
> tree in the process resulting in all manner of problems later on.
> 
> Add a sanity check to see if the logical block is already allocated and
> error out if it is.
> 
> Signed-off-by: Lukas Czerner <lczerner@redhat.com>

Reviewed-by: Andreas Dilger <adilger@dilger.ca>

> ---
> fs/ext4/namei.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
> 
> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> index cf460aa4f81d..4af441494e09 100644
> --- a/fs/ext4/namei.c
> +++ b/fs/ext4/namei.c
> @@ -54,6 +54,7 @@ static struct buffer_head *ext4_append(handle_t *handle,
> 					struct inode *inode,
> 					ext4_lblk_t *block)
> {
> +	struct ext4_map_blocks map;
> 	struct buffer_head *bh;
> 	int err;
> 
> @@ -63,6 +64,21 @@ static struct buffer_head *ext4_append(handle_t *handle,
> 		return ERR_PTR(-ENOSPC);
> 
> 	*block = inode->i_size >> inode->i_sb->s_blocksize_bits;
> +	map.m_lblk = *block;
> +	map.m_len = 1;
> +
> +	/*
> +	 * We're appending new directory block. Make sure the block is not
> +	 * allocated yet, otherwise we will end up corrupting the
> +	 * directory.
> +	 */
> +	err = ext4_map_blocks(NULL, inode, &map, 0);
> +	if (err < 0)
> +		return ERR_PTR(err);
> +	if (err) {
> +		EXT4_ERROR_INODE(inode, "Logical block already allocated");
> +		return ERR_PTR(-EFSCORRUPTED);
> +	}
> 
> 	bh = ext4_bread(handle, inode, *block, EXT4_GET_BLOCKS_CREATE);
> 	if (IS_ERR(bh))
> --
> 2.35.3
> 


Cheers, Andreas


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