linux/uio: remove "skip" offset for UIO_ITER

For UIO_ITER, we are just wrapping a kernel iterator. It will take care
of its own offsets if necessary. We don't need to do anything, and if we
do try to do anything with it (like advancing the iterator by the skip
in zfs_uio_advance) we're just confusing the kernel iterator, ending up
at the wrong position or worse, off the end of the memory region.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Tino Reichardt <[email protected]>
Reviewed-by: Brian Atkinson <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Closes openzfs#17298
(cherry picked from commit 2ee5b51)

Author: robn

include/os/linux/spl/sys/uio.h

@@ -151,7 +151,7 @@ zfs_uio_bvec_init(zfs_uio_t *uio, struct bio *bio, struct request *rq)
 #if defined(HAVE_VFS_IOV_ITER)
 static inline void
 zfs_uio_iov_iter_init(zfs_uio_t *uio, struct iov_iter *iter, offset_t offset,
-    ssize_t resid, size_t skip)
+    ssize_t resid)
 {
 	uio->uio_iter = iter;
 	uio->uio_iovcnt = iter->nr_segs;
@@ -161,7 +161,7 @@ zfs_uio_iov_iter_init(zfs_uio_t *uio, struct iov_iter *iter, offset_t offset,
 	uio->uio_fmode = 0;
 	uio->uio_extflg = 0;
 	uio->uio_resid = resid;
-	uio->uio_skip = skip;
+	uio->uio_skip = 0;
 }
 #endif
 

module/os/linux/zfs/zfs_uio.c

@@ -268,9 +268,6 @@ zfs_uiomove_iter(void *p, size_t n, zfs_uio_rw_t rw, zfs_uio_t *uio,
 {
 	size_t cnt = MIN(n, uio->uio_resid);
 
-	if (uio->uio_skip)
-		iov_iter_advance(uio->uio_iter, uio->uio_skip);
-
 	if (rw == UIO_READ)
 		cnt = copy_to_iter(p, cnt, uio->uio_iter);
 	else

module/os/linux/zfs/zpl_file.c

@@ -227,7 +227,8 @@ zpl_uio_init(zfs_uio_t *uio, struct kiocb *kiocb, struct iov_iter *to,
     loff_t pos, ssize_t count, size_t skip)
 {
 #if defined(HAVE_VFS_IOV_ITER)
-	zfs_uio_iov_iter_init(uio, to, pos, count, skip);
+	(void) skip;
+	zfs_uio_iov_iter_init(uio, to, pos, count);
 #else
 	zfs_uio_iovec_init(uio, zfs_uio_iter_iov(to), to->nr_segs, pos,
 	    zfs_uio_iov_iter_type(to) & ITER_KVEC ?