linux-ext4 - Re: [PATCH] ext2: correct max file size computing

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Date:   Fri, 25 Feb 2022 13:57:47 +0100
From:   Jan Kara <jack@...e.cz>
To:     Zhang Yi <yi.zhang@...wei.com>
Cc:     linux-ext4@...r.kernel.org, jack@...e.cz, yukuai3@...wei.com
Subject: Re: [PATCH] ext2: correct max file size computing

On Sat 12-02-22 13:05:32, Zhang Yi wrote:
> We need to calculate the max file size accurately if the total blocks
> that can address by block tree exceed the upper_limit. But this check is
> not correct now, it only compute the total data blocks but missing
> metadata blocks are needed. So in the case of "data blocks < upper_limit
> && total blocks > upper_limit", we will get wrong result. Fortunately,
> this case could not happen in reality, but it's confused and better to
> correct the computing.
> 
>   bits   data blocks   metadatablocks   upper_limit
>   10        16843020            66051    2147483647
>   11       134480396           263171    1073741823
>   12      1074791436          1050627     536870911 (*)
>   13      8594130956          4198403     268435455 (*)
>   14     68736258060         16785411     134217727 (*)
>   15    549822930956         67125251      67108863 (*)
>   16   4398314962956        268468227      33554431 (*)
> 
>   [*] Need to calculate in depth.
> 
> Fixes: 1c2d14212b15 ("ext2: Fix underflow in ext2_max_size()")
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>

Thanks for the cleanup! I've merged the patch to my tree.

								Honza

> ---
>  fs/ext2/super.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext2/super.c b/fs/ext2/super.c
> index 94f1fbd7d3ac..6d4f5ef74766 100644
> --- a/fs/ext2/super.c
> +++ b/fs/ext2/super.c
> @@ -753,8 +753,12 @@ static loff_t ext2_max_size(int bits)
>  	res += 1LL << (bits-2);
>  	res += 1LL << (2*(bits-2));
>  	res += 1LL << (3*(bits-2));
> +	/* Compute how many metadata blocks are needed */
> +	meta_blocks = 1;
> +	meta_blocks += 1 + ppb;
> +	meta_blocks += 1 + ppb + ppb * ppb;
>  	/* Does block tree limit file size? */
> -	if (res < upper_limit)
> +	if (res + meta_blocks <= upper_limit)
>  		goto check_lfs;
>  
>  	res = upper_limit;
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR