iov_iter: overlay struct iovec and ubuf/len

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.

Then we can have iter_iov() check for the appropriate type, and return
&iter->__ubuf_iovec for ITER_UBUF and iter->__iov for ITER_IOVEC and
things will magically work out for a single segment request regardless
of either type.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 5dbd2dc..ed35f44 100644 (file)
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -49,15 +49,35 @@ 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_iovec or iter->__iov
+        *
+        * interchangably for the user_backed cases, hence simplifying
+        * some of the cases that need to deal with both.
+        */
        union {
-               /* use iter_iov() to get the current vec */
-               const struct iovec *__iov;
-               const struct kvec *kvec;
-               const struct bio_vec *bvec;
-               struct xarray *xarray;
-               struct pipe_inode_info *pipe;
-               void __user *ubuf;
+               /*
+                * This really should be a const, but we cannot do that without
+                * also modifying any of the zero-filling iter init functions.
+                * Leave it non-const for now, but it should be treated as such.
+                */
+               struct iovec __ubuf_iovec;
+               struct {
+                       union {
+                               /* use iter_iov() to get the current vec */
+                               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;
@@ -69,7 +89,13 @@ struct iov_iter {
        };
 };
 
-#define iter_iov(iter) (iter)->__iov
+static inline const struct iovec *iter_iov(const struct iov_iter *iter)
+{
+       if (iter->iter_type == ITER_UBUF)
+               return (const struct iovec *) &iter->__ubuf_iovec;
+       return iter->__iov;
+}
+
 #define iter_iov_addr(iter)    (iter_iov(iter)->iov_base + (iter)->iov_offset)
 #define iter_iov_len(iter)     (iter_iov(iter)->iov_len - (iter)->iov_offset)