[PATCH v4 03/25] fsverity: generate and store zero-block hash

[Andrey Albershteyn <aalbersh@kernel.org>]

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