[PATCH 1/3] iov_iter: teach iov_iter_iovec() to deal with ITER_UBUF

[Jens Axboe <axboe@kernel.dk>]

Even if we're returning an iovec, we can trivially fill it in with the
details from an ITER_UBUF as well. This enables loops that assume
ITER_IOVEC to deal with ITER_UBUF transparently.

This is done in preparation for automatically importing single segment
iovecs as ITER_UBUF.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/uio.h | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 27e3fd942960..3b4403efcce1 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -143,13 +143,29 @@ static inline size_t iov_length(const struct iovec *iov, unsigned long nr_segs)
 	return ret;
 }
 
+/*
+ * Don't assume we're called with ITER_IOVEC, enable usage of ITER_UBUF
+ * as well by simply filling in the iovec.
+ */
 static inline struct iovec iov_iter_iovec(const struct iov_iter *iter)
 {
-	return (struct iovec) {
-		.iov_base = iter->iov->iov_base + iter->iov_offset,
-		.iov_len = min(iter->count,
-			       iter->iov->iov_len - iter->iov_offset),
-	};
+	if (WARN_ON_ONCE(!iter->user_backed)) {
+		return (struct iovec) {
+			.iov_base = NULL,
+			.iov_len = 0
+		};
+	} else if (iter_is_ubuf(iter)) {
+		return (struct iovec) {
+			.iov_base = iter->ubuf + iter->iov_offset,
+			.iov_len = iter->count
+		};
+	} else {
+		return (struct iovec) {
+			.iov_base = iter->iov->iov_base + iter->iov_offset,
+			.iov_len = min(iter->count,
+				       iter->iov->iov_len - iter->iov_offset),
+		};
+	}
 }
 
 size_t copy_page_from_iter_atomic(struct page *page, unsigned offset,
-- 
2.39.2