[PATCH 3/9] iov_iter: overlay struct iovec and ubuf/len

[Jens Axboe <axboe@kernel.dk>]

Add an internal struct iovec that we can return as a pointer, with the
fields of the iovec overlapping with the ITER_UBUF ubuf and length
fields.

This allows doing:

struct iovec *vec = &iter->__ubuf_iovec;

interchangably with iter->iov for the first segment, enabling writing
code that deals with both without needing to check if we're dealing with
ITER_IOVEC or ITER_UBUF.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/uio.h | 29 ++++++++++++++++++++++-------
 1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 3b4403efcce1..192831775d2b 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -49,14 +49,29 @@ struct iov_iter {
 		size_t iov_offset;
 		int last_offset;
 	};
-	size_t count;
+	/*
+	 * Hack alert: overlay ubuf_iovec with iovec + count, so
+	 * that the members resolve correctly regardless of the type
+	 * of iterator used. This means that you can use:
+	 *
+	 * &iter->ubuf or iter->iov
+	 *
+	 * interchangably for the user_backed cases, hence simplifying
+	 * some of the cases that need to deal with both.
+	 */
 	union {
-		const struct iovec *iov;
-		const struct kvec *kvec;
-		const struct bio_vec *bvec;
-		struct xarray *xarray;
-		struct pipe_inode_info *pipe;
-		void __user *ubuf;
+		struct iovec __ubuf_iovec;
+		struct {
+			union {
+				const struct iovec *iov;
+				const struct kvec *kvec;
+				const struct bio_vec *bvec;
+				struct xarray *xarray;
+				struct pipe_inode_info *pipe;
+				void __user *ubuf;
+			};
+			size_t count;
+		};
 	};
 	union {
 		unsigned long nr_segs;
-- 
2.39.2