fscrypt: add functions for direct I/O support

Encrypted files traditionally haven't supported DIO, due to the need to
encrypt/decrypt the data.  However, when the encryption is implemented
using inline encryption (blk-crypto) instead of the traditional
filesystem-layer encryption, it is straightforward to support DIO.

In preparation for supporting this, add the following functions:

- fscrypt_dio_supported() checks whether a DIO request is supported as
  far as encryption is concerned.  Encrypted files will only support DIO
  when inline encryption is used and the I/O request is properly
  aligned; this function checks these preconditions.

- fscrypt_limit_io_blocks() limits the length of a bio to avoid crossing
  a place in the file that a bio with an encryption context cannot
  cross due to a DUN discontiguity.  This function is needed by
  filesystems that use the iomap DIO implementation (which operates
  directly on logical ranges, so it won't use fscrypt_mergeable_bio())
  and that support FSCRYPT_POLICY_FLAG_IV_INO_LBLK_32.

Co-developed-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Satya Tangirala <satyat@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220128233940.79464-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
index 91ea9477e9bd..50d92d805bd8 100644
--- a/include/linux/fscrypt.h
+++ b/include/linux/fscrypt.h
@@ -714,6 +714,10 @@ bool fscrypt_mergeable_bio(struct bio *bio, const struct inode *inode,
 bool fscrypt_mergeable_bio_bh(struct bio *bio,
 			      const struct buffer_head *next_bh);
 
+bool fscrypt_dio_supported(struct kiocb *iocb, struct iov_iter *iter);
+
+u64 fscrypt_limit_io_blocks(const struct inode *inode, u64 lblk, u64 nr_blocks);
+
 #else /* CONFIG_FS_ENCRYPTION_INLINE_CRYPT */
 
 static inline bool __fscrypt_inode_uses_inline_crypto(const struct inode *inode)
@@ -742,6 +746,20 @@ static inline bool fscrypt_mergeable_bio_bh(struct bio *bio,
 {
 	return true;
 }
+
+static inline bool fscrypt_dio_supported(struct kiocb *iocb,
+					 struct iov_iter *iter)
+{
+	const struct inode *inode = file_inode(iocb->ki_filp);
+
+	return !fscrypt_needs_contents_encryption(inode);
+}
+
+static inline u64 fscrypt_limit_io_blocks(const struct inode *inode, u64 lblk,
+					  u64 nr_blocks)
+{
+	return nr_blocks;
+}
 #endif /* !CONFIG_FS_ENCRYPTION_INLINE_CRYPT */
 
 /**