linux-ext4 - Re: [PATCH v2 4/4] ext4: prevent getting empty inode buffer

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Date:   Thu, 19 Aug 2021 12:35:04 +0200
From:   Jan Kara <jack@...e.cz>
To:     Zhang Yi <yi.zhang@...wei.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, jack@...e.cz, yukuai3@...wei.com
Subject: Re: [PATCH v2 4/4] ext4: prevent getting empty inode buffer

On Thu 19-08-21 14:57:04, Zhang Yi wrote:
> In ext4_get_inode_loc(), we may skip IO and get an zero && uptodate
> inode buffer when the inode monopolize an inode block for performance
> reason. For most cases, ext4_mark_iloc_dirty() will fill the inode
> buffer to make it fine, but we could miss this call if something bad
> happened. Finally, __ext4_get_inode_loc_noinmem() may probably get an
> empty inode buffer and trigger ext4 error.
> 
> For example, if we remove a nonexistent xattr on inode A,
> ext4_xattr_set_handle() will return ENODATA before invoking
> ext4_mark_iloc_dirty(), it will left an uptodate but zero buffer. We
> will get checksum error message in ext4_iget() when getting inode again.
> 
>   EXT4-fs error (device sda): ext4_lookup:1784: inode #131074: comm cat: iget: checksum invalid
> 
> Even worse, if we allocate another inode B at the same inode block, it
> will corrupt the inode A on disk when write back inode B.
> 
> So this patch postpone the init and mark buffer uptodate logic until
> before filling correct inode data in ext4_do_update_inode() if skip read
> I/O, ensure the buffer is real uptodate.
> 
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>

Looks good. Just some language fixes below. Feel free to add:

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

> ---
>  fs/ext4/inode.c | 26 +++++++++++++++++++++++---
>  1 file changed, 23 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index d0d7a5295bf9..02d910c9d8f1 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -4367,9 +4367,11 @@ static int __ext4_get_inode_loc(struct super_block *sb, unsigned long ino,
>  		}
>  		brelse(bitmap_bh);
>  		if (i == start + inodes_per_block) {
> -			/* all other inodes are free, so skip I/O */
> -			memset(bh->b_data, 0, bh->b_size);
> -			set_buffer_uptodate(bh);
> +			/*
> +			 * All other inodes are free, skip I/O. Return
> +			 * un-inited buffer (which is postponed until

I'd repharse this sentence as: Return uninitialized buffer immediately,
initialization is postponed until shortly before we fill inode contents.

> +			 * before filling inode data) immediately.
> +			 */
>  			unlock_buffer(bh);
>  			goto has_buffer;
>  		}
> @@ -5026,6 +5028,24 @@ static int ext4_do_update_inode(handle_t *handle,
>  	gid_t i_gid;
>  	projid_t i_projid;
>  
> +	/*
> +	 * If the buffer is not uptodate, it means all information of inode
								   ^^^^^^^^
of the inode is

> +	 * in memory and we got this buffer without reading the block. We
> +	 * must be cautious that once we mark the buffer as uptodate, we
> +	 * rely on filling in the correct inode data later in this function.
> +	 * Otherwise if we getting the left falsepositive buffer when

I'd rephrase this sentence as: Otherwise if we left uptodate buffer without
copying proper inode contents, we could corrupt the inode on disk after
allocating another inode in the same block.

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