[PATCH 11/18] fsverity: pass the zero-hash value to the implementation

["Darrick J. Wong" <djwong@xxxxxxxxxx>]

From: Darrick J. Wong <djwong@xxxxxxxxxx>

Compute the hash of one filesystem block's worth of zeros and pass a
reference to this to the merkle tree read and write functions.  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@xxxxxxxxxx>
---
 fs/verity/enable.c           |    2 ++
 fs/verity/fsverity_private.h |    3 +++
 fs/verity/open.c             |    7 +++++++
 fs/verity/verify.c           |    2 ++
 include/linux/fsverity.h     |    8 ++++++++
 5 files changed, 22 insertions(+)


diff --git a/fs/verity/enable.c b/fs/verity/enable.c
index 233b20fb12ff5..8c6fe4b72b14e 100644
--- a/fs/verity/enable.c
+++ b/fs/verity/enable.c
@@ -52,6 +52,8 @@ static int write_merkle_tree_block(struct inode *inode, const u8 *buf,
 {
 	struct fsverity_writemerkle req = {
 		.inode = inode,
+		.zero_digest = params->zero_digest,
+		.digest_size = params->digest_size,
 	};
 	u64 pos = (u64)index << params->log_blocksize;
 	int err;
diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
index c1a306fd1f9b4..20208425e56fc 100644
--- a/fs/verity/fsverity_private.h
+++ b/fs/verity/fsverity_private.h
@@ -47,6 +47,9 @@ 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 aa71a4d3cbff1..c9d858d99f4ac 100644
--- a/fs/verity/open.c
+++ b/fs/verity/open.c
@@ -144,6 +144,13 @@ int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
 		goto out_err;
 	}
 
+	err = fsverity_hash_block(params, inode, page_address(ZERO_PAGE(0)),
+				   params->zero_digest);
+	if (err) {
+		fsverity_err(inode, "Error %d computing zero digest", err);
+		goto out_err;
+	}
+
 	params->tree_size = offset << log_blocksize;
 	params->tree_pages = PAGE_ALIGN(params->tree_size) >> PAGE_SHIFT;
 	return 0;
diff --git a/fs/verity/verify.c b/fs/verity/verify.c
index c4c5e1c082de5..0782a69d89f26 100644
--- a/fs/verity/verify.c
+++ b/fs/verity/verify.c
@@ -425,6 +425,8 @@ int fsverity_read_merkle_tree_block(struct inode *inode,
 			.level = level,
 			.num_levels = params->num_levels,
 			.ra_bytes = ra_bytes,
+			.zero_digest = params->zero_digest,
+			.digest_size = params->digest_size,
 		};
 
 		err = vops->read_merkle_tree_block(&req, block);
diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
index 710006552804d..dc8f85380b9c7 100644
--- a/include/linux/fsverity.h
+++ b/include/linux/fsverity.h
@@ -63,12 +63,16 @@ 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 of a merkle block-sized buffer of zeroes
+ * @digest_size: size of zero_digest, in bytes
  */
 struct fsverity_readmerkle {
 	struct inode *inode;
 	unsigned long ra_bytes;
 	int level;
 	int num_levels;
+	const u8 *zero_digest;
+	unsigned int digest_size;
 };
 
 #define FSVERITY_STREAMING_READ	(-1)
@@ -76,9 +80,13 @@ struct fsverity_readmerkle {
 /**
  * struct fsverity_writemerkle - Request to write a Merkle Tree block buffer
  * @inode: the inode to read
+ * @zero_digest: the hash of a merkle block-sized buffer of zeroes
+ * @digest_size: size of zero_digest, in bytes
  */
 struct fsverity_writemerkle {
 	struct inode *inode;
+	const u8 *zero_digest;
+	unsigned int digest_size;
 };
 
 /* Verity operations for filesystems */