linux-ext4 - Re: [PATCH v2] ext4: fix underflow in ext4_max_bitmap

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Date:   Fri, 25 Feb 2022 13:38:51 +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 v2] ext4: fix underflow in ext4_max_bitmap_size()

On Fri 25-02-22 18:28:37, Zhang Yi wrote:
> The same to commit 1c2d14212b15 ("ext2: Fix underflow in ext2_max_size()")
> in ext2 filesystem, ext4 driver has the same issue with 64K block size
> and ^huge_file, fix this issue the same as ext2. This patch also revert
> commit 75ca6ad408f4 ("ext4: fix loff_t overflow in ext4_max_bitmap_size()")
> because it's no longer needed.
> 
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>

Thanks for the patch. I would not refer to ext2 patch in the changelog - it
is better to have it self-contained. AFAIU the problem is that (meta_blocks
> upper_limit) for 64k blocksize and ^huge_file and so upper_limit would
underflow during the computations, am I right?

Also two comments below:

> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index c5021ca0a28a..95608c2127e7 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)
>  
>  	}
>  
> -	/* indirect blocks */
> -	meta_blocks = 1;
> -	/* 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;
> -
> +	/* Compute how many blocks we can address by block tree */
>  	res += 1LL << (bits-2);
>  	res += 1LL << (2*(bits-2));
>  	res += 1LL << (3*(bits-2));

When you have the 'ppb' convenience variable, perhaps you can update this
math to:

	res = EXT4_NDIR_BLOCKS + ppb + ppb*ppb + ((long long)ppb)*ppb*ppb;

It is easier to understand and matches how you compute meta_blocks as well.

> +	/* 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 */
> +	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;

Cannot this overflow loff_t again? I mean if upper_limit == (1 << 48) - 1
and we have 64k blocksize, 'res' will be larger than (1 << 47) and thus 
res << 16 will be greater than 1 << 63 => negative... Am I missing
something?

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