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 Signed-off-by: Andrey Albershteyn --- 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