Re: [PATCH 10/13] fsverity: pass the zero-hash value to the implementation

Darrick J. Wong
2024-04-24T19:02:47Z

On Thu, Apr 04, 2024 at 10:57:50PM -0400, Eric Biggers wrote:
> On Fri, Mar 29, 2024 at 05:35:17PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> >
> > Compute the hash of a data block full of zeros, and then supply this to
> > the merkle tree read and write methods. A subsequent xfs patch will use
>
> This should say "hash of a block", not "hash of a data block". What you
> actually care about is the hash of a Merkle tree block, not the hash of a data
> block. Yet, there is no difference in how the hashes are calculated for the two
> types of blocks, so we should simply write "hash of a block".

I think I could go further with the precision of the description --

"Compute the hash of one filesystem block's worth of zeroes. Any merkle
tree block containing only this hash can be elided at write time, and
its contents synthesized at read time."

I don't think this is going to happen very often above the leaf levels
of the merkle tree, but as written there's nothing to prevent the
elision of internal nodes. Also note that the elision can happen for
internal nodes even when merkle tree blocksize != i_blocksize.

> > diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
> > index de8798f141d4a..195a92f203bba 100644
> > --- a/fs/verity/fsverity_private.h
> > +++ b/fs/verity/fsverity_private.h
> > @@ -47,6 +47,8 @@ struct merkle_tree_params {
> > u64 tree_size; /* Merkle tree size in bytes */
> > unsigned long tree_pages; /* Merkle tree size in pages */
> >
> > + u8 zero_digest[FS_VERITY_MAX_DIGEST_SIZE]; /* hash of zeroed data block */
>
> Similarly, "block" instead of "data block".

How about "the hash of an i_blocksize-sized buffer of zeroes" for all
three?

--D

> > diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
> > index 5dacd30d65353..761a0b76eefec 100644
> > --- a/include/linux/fsverity.h
> > +++ b/include/linux/fsverity.h
> > @@ -66,6 +66,8 @@ struct fsverity_blockbuf {
> > * if the page at @block->offset isn't already cached.
> > * Implementations may ignore this argument; it's only a
> > * performance optimization.
> > + * @zero_digest: the hash for a data block of zeroes
>
> Likewise.
>
> > /**
> > @@ -81,12 +85,16 @@ struct fsverity_readmerkle {
> > * @level: level of the block; level 0 are the leaves
> > * @num_levels: number of levels in the tree total
> > * @log_blocksize: log2 of the size of the block
> > + * @zero_digest: the hash for a data block of zeroes
> > + * @digest_size: size of zero_digest
>
> Likewise.
>
> - Eric
>