fscrypt: change fscrypt_dio_supported() to prepare for STATX_DIOALIGN

To prepare for STATX_DIOALIGN support, make two changes to
fscrypt_dio_supported().

First, remove the filesystem-block-alignment check and make the
filesystems handle it instead.  It previously made sense to have it in
fs/crypto/; however, to support STATX_DIOALIGN the alignment restriction
would have to be returned to filesystems.  It ends up being simpler if
filesystems handle this part themselves, especially for f2fs which only
allows fs-block-aligned DIO in the first place.

Second, make fscrypt_dio_supported() work on inodes whose encryption key
hasn't been set up yet, by making it set up the key if needed.  This is
required for statx(), since statx() doesn't require a file descriptor.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20220827065851.135710-4-ebiggers@kernel.org

fs/crypto/inline_crypt.c

@@ -401,46 +401,45 @@ bool fscrypt_mergeable_bio_bh(struct bio *bio,
 EXPORT_SYMBOL_GPL(fscrypt_mergeable_bio_bh);
 
 /**
- * fscrypt_dio_supported() - check whether a DIO (direct I/O) request is
- *			     supported as far as encryption is concerned
- * @iocb: the file and position the I/O is targeting
- * @iter: the I/O data segment(s)
+ * fscrypt_dio_supported() - check whether DIO (direct I/O) is supported on an
+ *			     inode, as far as encryption is concerned
+ * @inode: the inode in question
  *
  * Return: %true if there are no encryption constraints that prevent DIO from
  *	   being supported; %false if DIO is unsupported.  (Note that in the
  *	   %true case, the filesystem might have other, non-encryption-related
- *	   constraints that prevent DIO from actually being supported.)
+ *	   constraints that prevent DIO from actually being supported.  Also, on
+ *	   encrypted files the filesystem is still responsible for only allowing
+ *	   DIO when requests are filesystem-block-aligned.)
  */
-bool fscrypt_dio_supported(struct kiocb *iocb, struct iov_iter *iter)
+bool fscrypt_dio_supported(struct inode *inode)
 {
-	const struct inode *inode = file_inode(iocb->ki_filp);
-	const unsigned int blocksize = i_blocksize(inode);
+	int err;
 
 	/* If the file is unencrypted, no veto from us. */
 	if (!fscrypt_needs_contents_encryption(inode))
 		return true;
 
-	/* We only support DIO with inline crypto, not fs-layer crypto. */
-	if (!fscrypt_inode_uses_inline_crypto(inode))
-		return false;
-
 	/*
-	 * Since the granularity of encryption is filesystem blocks, the file
-	 * position and total I/O length must be aligned to the filesystem block
-	 * size -- not just to the block device's logical block size as is
-	 * traditionally the case for DIO on many filesystems.
+	 * We only support DIO with inline crypto, not fs-layer crypto.
 	 *
-	 * We require that the user-provided memory buffers be filesystem block
-	 * aligned too.  It is simpler to have a single alignment value required
-	 * for all properties of the I/O, as is normally the case for DIO.
-	 * Also, allowing less aligned buffers would imply that data units could
-	 * cross bvecs, which would greatly complicate the I/O stack, which
-	 * assumes that bios can be split at any bvec boundary.
+	 * To determine whether the inode is using inline crypto, we have to set
+	 * up the key if it wasn't already done.  This is because in the current
+	 * design of fscrypt, the decision of whether to use inline crypto or
+	 * not isn't made until the inode's encryption key is being set up.  In
+	 * the DIO read/write case, the key will always be set up already, since
+	 * the file will be open.  But in the case of statx(), the key might not
+	 * be set up yet, as the file might not have been opened yet.
 	 */
-	if (!IS_ALIGNED(iocb->ki_pos | iov_iter_alignment(iter), blocksize))
+	err = fscrypt_require_key(inode);
+	if (err) {
+		/*
+		 * Key unavailable or couldn't be set up.  This edge case isn't
+		 * worth worrying about; just report that DIO is unsupported.
+		 */
 		return false;
-
-	return true;
+	}
+	return fscrypt_inode_uses_inline_crypto(inode);
 }
 EXPORT_SYMBOL_GPL(fscrypt_dio_supported);
 

fs/ext4/file.c

@@ -40,8 +40,13 @@ static bool ext4_dio_supported(struct kiocb *iocb, struct iov_iter *iter)
 {
 	struct inode *inode = file_inode(iocb->ki_filp);
 
-	if (!fscrypt_dio_supported(iocb, iter))
-		return false;
+	if (IS_ENCRYPTED(inode)) {
+		if (!fscrypt_dio_supported(inode))
+			return false;
+		if (!IS_ALIGNED(iocb->ki_pos | iov_iter_alignment(iter),
+				i_blocksize(inode)))
+			return false;
+	}
 	if (fsverity_active(inode))
 		return false;
 	if (ext4_should_journal_data(inode))

fs/f2fs/f2fs.h

@@ -4498,7 +4498,7 @@ static inline bool f2fs_force_buffered_io(struct inode *inode,
 	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
 	int rw = iov_iter_rw(iter);
 
-	if (!fscrypt_dio_supported(iocb, iter))
+	if (!fscrypt_dio_supported(inode))
 		return true;
 	if (fsverity_active(inode))
 		return true;

include/linux/fscrypt.h

@@ -768,7 +768,7 @@ 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);
+bool fscrypt_dio_supported(struct inode *inode);
 
 u64 fscrypt_limit_io_blocks(const struct inode *inode, u64 lblk, u64 nr_blocks);
 
@@ -801,11 +801,8 @@ 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)
+static inline bool fscrypt_dio_supported(struct inode *inode)
 {
-	const struct inode *inode = file_inode(iocb->ki_filp);
-
 	return !fscrypt_needs_contents_encryption(inode);
 }