[PATCH v4 04/25] fsverity: introduce fsverity_folio_zero_hash()

[Andrey Albershteyn <aalbersh@kernel.org>]

Helper to pre-fill folio with hashes of empty blocks. This will be used
by iomap to synthesize blocks full of zero hashes on the fly.

Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
---
 fs/verity/pagecache.c    | 22 ++++++++++++++++++++++
 include/linux/fsverity.h |  2 ++
 2 files changed, 24 insertions(+)

diff --git a/fs/verity/pagecache.c b/fs/verity/pagecache.c
index 1819314ecaa3..1d94bf73f38c 100644
--- a/fs/verity/pagecache.c
+++ b/fs/verity/pagecache.c
@@ -2,6 +2,7 @@
 /*
  * Copyright 2019 Google LLC
  */
+#include "fsverity_private.h"
 
 #include <linux/export.h>
 #include <linux/fsverity.h>
@@ -56,3 +57,24 @@ void generic_readahead_merkle_tree(struct inode *inode, pgoff_t index,
 		folio_put(folio);
 }
 EXPORT_SYMBOL_GPL(generic_readahead_merkle_tree);
+
+/**
+ * fsverity_folio_zero_hash() - fill folio with hashes of zero data block
+ * @folio:	folio to fill
+ * @poff:	offset in the folio to start
+ * @plen:	length of the range to fill with hashes
+ * @vi:		fsverity info
+ */
+void fsverity_folio_zero_hash(struct folio *folio, size_t poff, size_t plen,
+			      struct fsverity_info *vi)
+{
+	size_t offset = poff;
+
+	WARN_ON_ONCE(!IS_ALIGNED(poff, vi->tree_params.digest_size));
+	WARN_ON_ONCE(!IS_ALIGNED(plen, vi->tree_params.digest_size));
+
+	for (; offset < (poff + plen); offset += vi->tree_params.digest_size)
+		memcpy_to_folio(folio, offset, vi->tree_params.zero_digest,
+				vi->tree_params.digest_size);
+}
+EXPORT_SYMBOL_GPL(fsverity_folio_zero_hash);
diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
index 8ba7806b225e..b490b2c8a393 100644
--- a/include/linux/fsverity.h
+++ b/include/linux/fsverity.h
@@ -321,5 +321,7 @@ void fsverity_cleanup_inode(struct inode *inode);
 struct page *generic_read_merkle_tree_page(struct inode *inode, pgoff_t index);
 void generic_readahead_merkle_tree(struct inode *inode, pgoff_t index,
 				   unsigned long nr_pages);
+void fsverity_folio_zero_hash(struct folio *folio, size_t poff, size_t plen,
+			      struct fsverity_info *vi);
 
 #endif	/* _LINUX_FSVERITY_H */
-- 
2.51.2