[PATCH 07/13] iomap: align the direct I/O bios of encrypted files to the block size

[Baokun Li <libaokun@linux.alibaba.com>]

A bio that carries an encryption context must cover a whole number of
data units: its data unit number is generated from its first data unit
(see bio_crypt_set_ctx()), and it is only split at data unit
boundaries.

iomap attaches the encryption context of an encrypted file to the bios
that it builds for direct I/O, but it only cuts them at the logical
block size of the device.  A bio ends where the bio_vecs that fit into
it end, not at a fixed number of bytes: the extraction stops when the
bio is out of bio_vecs or when the iterator is exhausted, and a
bio_vec can cover a whole folio.

A bio can end in the middle of a data unit, and a data unit that is
larger than a page makes this reachable without any misaligned input
at all: the extraction then stops in the middle of a folio, and a cut
at a page granularity inside such a data unit is not data unit aligned
even when the position, the length and the buffer are.

The fallback rejects such a bio once it reaches the block layer: one
of its bio_vecs is not data unit aligned, so a direct I/O that should
work fails.

By contrast, the equivalent check on the hardware path lives in the
splitting code, which a small single segment bio skips, so a bio that
starts in the middle of a data unit can reach the hardware without
being rejected.

Fix this by cutting the bios of an encrypted file at the block size,
like the IOMAP_DIO_FSBLOCK_ALIGNED flag does for the filesystems that
pass it.  fscrypt rejects a data unit size larger than the block size,
so a bio that is cut at block size boundaries covers a whole number of
data units.

This also makes the file systems that don't pass the flag follow the
contract, e.g. f2fs, which passes direct I/O that is not aligned to
the logical block size of the device on to iomap expecting it to fail
with -EINVAL, while iomap checks only the file position and the
length, so such a request can reach bio submission.

Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/iomap/direct-io.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index 8b4039d16ce8..9b6cdf04dced 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -410,6 +410,16 @@ static inline unsigned int iomap_dio_alignment(struct inode *inode,
 {
 	if (dio_flags & IOMAP_DIO_FSBLOCK_ALIGNED)
 		return i_blocksize(inode);
+
+	/*
+	 * A bio of an encrypted file must cover a whole number of data
+	 * units.  The data unit size of an encryption policy never exceeds
+	 * the block size, so cutting the bios at block size boundaries is
+	 * enough.
+	 */
+	if (IS_ENCRYPTED(inode))
+		return i_blocksize(inode);
+
 	return bdev_logical_block_size(bdev);
 }
 
-- 
2.43.7