Re: [PATCH v3 04/35] fsverity: generate and store zero-block hash

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Feb 18, 2026 at 12:19:04AM +0100, Andrey Albershteyn wrote:
> Compute the hash of one filesystem block's worth of zeros. A filesystem
> implementation can decide to elide merkle tree blocks containing only
> this hash and synthesize the contents at read time.
> 
> Let's pretend that there's a file containing six data blocks and whose
> merkle tree looks roughly like this:
> 
> root
>  +--leaf0
>  |   +--data0
>  |   +--data1
>  |   `--data2
>  `--leaf1
>      +--data3
>      +--data4
>      `--data5
> 
> If data[0-2] are sparse holes, then leaf0 will contain a repeating
> sequence of @zero_digest.  Therefore, leaf0 need not be written to disk
> because its contents can be synthesized.
> 
> A subsequent xfs patch will use this to reduce the size of the merkle
> tree when dealing with sparse gold master disk images and the like.
> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
> ---
>  fs/verity/fsverity_private.h | 3 +++
>  fs/verity/open.c             | 3 +++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
> index 6e6854c19078..35636c1e2c41 100644
> --- a/fs/verity/fsverity_private.h
> +++ b/fs/verity/fsverity_private.h
> @@ -53,6 +53,9 @@ struct merkle_tree_params {
>  	u64 tree_size;			/* Merkle tree size in bytes */
>  	unsigned long tree_pages;	/* Merkle tree size in pages */
>  
> +	/* the hash of a merkle block-sized buffer of zeroes */
> +	u8 zero_digest[FS_VERITY_MAX_DIGEST_SIZE];
> +
>  	/*
>  	 * Starting block index for each tree level, ordered from leaf level (0)
>  	 * to root level ('num_levels - 1')
> diff --git a/fs/verity/open.c b/fs/verity/open.c
> index 0483db672526..94407a37aa08 100644
> --- a/fs/verity/open.c
> +++ b/fs/verity/open.c
> @@ -153,6 +153,9 @@ int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
>  		goto out_err;
>  	}
>  
> +	fsverity_hash_block(params, page_address(ZERO_PAGE(0)),
> +			    params->zero_digest);

Hrm.  At first I thought "Now that xfs supports blocksize > pagesize
you actually need to compute this with a zeroed folio."  But then I
remembered that merkle tree blocksize != filesystem blocksize.  So I
guess as long as *fsverity* doesn't support merkle tree blocksize >
pagesize then this is still ok?

--D

> +
>  	params->tree_size = offset << log_blocksize;
>  	params->tree_pages = PAGE_ALIGN(params->tree_size) >> PAGE_SHIFT;
>  	return 0;
> -- 
> 2.51.2
> 
>