btrfs: fix iomap_begin length for nocow writes

can_nocow_extent can reduce the len passed in, which needs to be
propagated to btrfs_dio_iomap_begin so that iomap does not submit
more data then is mapped.

This problems exists since the btrfs_get_blocks_direct helper was added
in commit c5794e51 ("btrfs: Factor out write portion of
btrfs_get_blocks_direct"), but the ordered_extent splitting added in
commit b73a6fd1 ("btrfs: split partial dio bios before submit")
added a WARN_ON that made a syzkaller test fail.

Reported-by: syzbot+ee90502d5c8fd1d0dd93@syzkaller.appspotmail.com
Fixes: c5794e51 ("btrfs: Factor out write portion of btrfs_get_blocks_direct")
CC: stable@vger.kernel.org # 6.1+
Tested-by: syzbot+ee90502d5c8fd1d0dd93@syzkaller.appspotmail.com
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -7264,7 +7264,7 @@ static struct extent_map *create_io_em(struct btrfs_inode *inode, u64 start,
 static int btrfs_get_blocks_direct_write(struct extent_map **map,
 					 struct inode *inode,
 					 struct btrfs_dio_data *dio_data,
-					 u64 start, u64 len,
+					 u64 start, u64 *lenp,
 					 unsigned int iomap_flags)
 {
 	const bool nowait = (iomap_flags & IOMAP_NOWAIT);
@@ -7275,6 +7275,7 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map,
 	struct btrfs_block_group *bg;
 	bool can_nocow = false;
 	bool space_reserved = false;
+	u64 len = *lenp;
 	u64 prev_len;
 	int ret = 0;
 
@@ -7345,15 +7346,19 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map,
 		free_extent_map(em);
 		*map = NULL;
 
-		if (nowait)
-			return -EAGAIN;
+		if (nowait) {
+			ret = -EAGAIN;
+			goto out;
+		}
 
 		/*
 		 * If we could not allocate data space before locking the file
 		 * range and we can't do a NOCOW write, then we have to fail.
 		 */
-		if (!dio_data->data_space_reserved)
-			return -ENOSPC;
+		if (!dio_data->data_space_reserved) {
+			ret = -ENOSPC;
+			goto out;
+		}
 
 		/*
 		 * We have to COW and we have already reserved data space before,
@@ -7394,6 +7399,7 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map,
 		btrfs_delalloc_release_extents(BTRFS_I(inode), len);
 		btrfs_delalloc_release_metadata(BTRFS_I(inode), len, true);
 	}
+	*lenp = len;
 	return ret;
 }
 
@@ -7570,7 +7576,7 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start,
 
 	if (write) {
 		ret = btrfs_get_blocks_direct_write(&em, inode, dio_data,
-						    start, len, flags);
+						    start, &len, flags);
 		if (ret < 0)
 			goto unlock_err;
 		unlock_extents = true;