Let filesystem iterate over hashes in the block and check if these are
hashes of zeroed data blocks. XFS will use this to decide if it want to
store tree block full of these hashes.

Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
---
 fs/btrfs/verity.c        | 6 +++++-
 fs/ext4/verity.c         | 4 +++-
 fs/f2fs/verity.c         | 4 +++-
 fs/verity/enable.c       | 4 +++-
 include/linux/fsverity.h | 6 +++++-
 5 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/verity.c b/fs/btrfs/verity.c
index cd96fac4739f..30a2d206299f 100644
--- a/fs/btrfs/verity.c
+++ b/fs/btrfs/verity.c
@@ -776,11 +776,15 @@ static struct page *btrfs_read_merkle_tree_page(struct inode *inode,
  * @buf:	Merkle tree block to write
  * @pos:	the position of the block in the Merkle tree (in bytes)
  * @size:	the Merkle tree block size (in bytes)
+ * @zero_digest:	the hash of a merkle block-sized buffer of zeroes
+ * @digest_size:	size of zero_digest, in bytes
  *
  * Returns 0 on success or negative error code on failure
  */
 static int btrfs_write_merkle_tree_block(struct file *file, const void *buf,
-					 u64 pos, unsigned int size)
+					 u64 pos, unsigned int size,
+					 const u8 *zero_digest,
+					 unsigned int digest_size)
 {
 	struct inode *inode = file_inode(file);
 	loff_t merkle_pos = merkle_file_pos(inode);
diff --git a/fs/ext4/verity.c b/fs/ext4/verity.c
index 5caa658adc12..3507af8a0979 100644
--- a/fs/ext4/verity.c
+++ b/fs/ext4/verity.c
@@ -372,7 +372,9 @@ static void ext4_readahead_merkle_tree(struct inode *inode, pgoff_t index,
 }
 
 static int ext4_write_merkle_tree_block(struct file *file, const void *buf,
-					u64 pos, unsigned int size)
+					u64 pos, unsigned int size,
+					const u8 *zero_digest,
+					unsigned int digest_size)
 {
 	pos += ext4_verity_metadata_pos(file_inode(file));
 
diff --git a/fs/f2fs/verity.c b/fs/f2fs/verity.c
index 92ebcc19cab0..b3b3e71604ac 100644
--- a/fs/f2fs/verity.c
+++ b/fs/f2fs/verity.c
@@ -270,7 +270,9 @@ static void f2fs_readahead_merkle_tree(struct inode *inode, pgoff_t index,
 }
 
 static int f2fs_write_merkle_tree_block(struct file *file, const void *buf,
-					u64 pos, unsigned int size)
+					u64 pos, unsigned int size,
+					const u8 *zero_digest,
+					unsigned int digest_size)
 {
 	pos += f2fs_verity_metadata_pos(file_inode(file));
 
diff --git a/fs/verity/enable.c b/fs/verity/enable.c
index 42dfed1ce0ce..ad4ff71d7dd9 100644
--- a/fs/verity/enable.c
+++ b/fs/verity/enable.c
@@ -50,7 +50,9 @@ static int write_merkle_tree_block(struct file *file, const u8 *buf,
 	int err;
 
 	err = inode->i_sb->s_vop->write_merkle_tree_block(file, buf, pos,
-							  params->block_size);
+							  params->block_size,
+							  params->zero_digest,
+							  params->digest_size);
 	if (err)
 		fsverity_err(inode, "Error %d writing Merkle tree block %lu",
 			     err, index);
diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
index addee462dcc2..6a97a5a73f93 100644
--- a/include/linux/fsverity.h
+++ b/include/linux/fsverity.h
@@ -124,6 +124,8 @@ struct fsverity_operations {
 	 * @buf: the Merkle tree block to write
 	 * @pos: the position of the block in the Merkle tree (in bytes)
 	 * @size: the Merkle tree block size (in bytes)
+	 * @zero_digest: the hash of a merkle block-sized buffer of zeroes
+	 * @digest_size: size of zero_digest, in bytes
 	 *
 	 * This is only called between ->begin_enable_verity() and
 	 * ->end_enable_verity().
@@ -131,7 +133,9 @@ struct fsverity_operations {
 	 * Return: 0 on success, -errno on failure
 	 */
 	int (*write_merkle_tree_block)(struct file *file, const void *buf,
-				       u64 pos, unsigned int size);
+				       u64 pos, unsigned int size,
+				       const u8 *zero_digest,
+				       unsigned int digest_size);
 
 	/**
 	 * Notify the filesystem that file data is corrupt.
-- 
2.51.2