linux-ext4 - Re: [PATCH v3] ext4: fix underflow in ext4_max_bitmap

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Date:   Tue, 1 Mar 2022 12:36:18 +0100
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 v3] ext4: fix underflow in ext4_max_bitmap_size()

On Tue 01-03-22 19:17:04, Zhang Yi wrote:
> when ext4 filesystem is created with 64k block size, ^extent and
> ^huge_file features. the upper_limit would underflow during the
> computations in ext4_max_bitmap_size(). The problem is the size of block
> index tree for such large block size is more than i_blocks can carry.
> So fix the computation to count with this possibility. After this fix,
> the 'res' cannot overflow loff_t on the extreme case of filesystem with
> huge_files and 64K block size, so this patch also revert commit
> 75ca6ad408f4 ("ext4: fix loff_t overflow in ext4_max_bitmap_size()").
> 
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>

Thanks! The patch looks good. Feel free to add:

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

								Honza

> ---
> v3->v2: rewrite change log and use ppb to compute 'res' blocks.
> v2->v1: use DIV_ROUND_UP_ULL instead of DIV_ROUND_UP.
> 
>  fs/ext4/super.c | 46 +++++++++++++++++++++++++++++++---------------
>  1 file changed, 31 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index c5021ca0a28a..bfba62206a14 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3468,8 +3468,9 @@ static loff_t ext4_max_size(int blkbits, int has_huge_files)
>   */
>  static loff_t ext4_max_bitmap_size(int bits, int has_huge_files)
>  {
> -	unsigned long long upper_limit, res = EXT4_NDIR_BLOCKS;
> +	loff_t upper_limit, res = EXT4_NDIR_BLOCKS;
>  	int meta_blocks;
> +	unsigned int ppb = 1 << (bits - 2);
>  
>  	/*
>  	 * This is calculated to be the largest file size for a dense, block
> @@ -3501,27 +3502,42 @@ static loff_t ext4_max_bitmap_size(int bits, int has_huge_files)
>  
>  	}
>  
> +	/* Compute how many blocks we can address by block tree */
> +	res += ppb;
> +	res += ppb * ppb;
> +	res += ((loff_t)ppb) * ppb * ppb;
> +	/* 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 + meta_blocks <= upper_limit)
> +		goto check_lfs;
> +
> +	res = upper_limit;
> +	/* How many metadata blocks are needed for addressing upper_limit? */
> +	upper_limit -= EXT4_NDIR_BLOCKS;
>  	/* indirect blocks */
>  	meta_blocks = 1;
> +	upper_limit -= ppb;
>  	/* double indirect blocks */
> -	meta_blocks += 1 + (1LL << (bits-2));
> -	/* tripple indirect blocks */
> -	meta_blocks += 1 + (1LL << (bits-2)) + (1LL << (2*(bits-2)));
> -
> -	upper_limit -= meta_blocks;
> -	upper_limit <<= bits;
> -
> -	res += 1LL << (bits-2);
> -	res += 1LL << (2*(bits-2));
> -	res += 1LL << (3*(bits-2));
> +	if (upper_limit < ppb * ppb) {
> +		meta_blocks += 1 + DIV_ROUND_UP_ULL(upper_limit, ppb);
> +		res -= meta_blocks;
> +		goto check_lfs;
> +	}
> +	meta_blocks += 1 + ppb;
> +	upper_limit -= ppb * ppb;
> +	/* tripple indirect blocks for the rest */
> +	meta_blocks += 1 + DIV_ROUND_UP_ULL(upper_limit, ppb) +
> +		DIV_ROUND_UP_ULL(upper_limit, ppb*ppb);
> +	res -= meta_blocks;
> +check_lfs:
>  	res <<= bits;
> -	if (res > upper_limit)
> -		res = upper_limit;
> -
>  	if (res > MAX_LFS_FILESIZE)
>  		res = MAX_LFS_FILESIZE;
>  
> -	return (loff_t)res;
> +	return res;
>  }
>  
>  static ext4_fsblk_t descriptor_loc(struct super_block *sb,
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR