iov_iter: saner checks for attempt to copy to/from iterator

instead of "don't do it to ITER_PIPE" check for ->data_source being
false on copying from iterator.  Check for !->data_source for
copying to iterator, while we are at it.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

@@ -520,6 +520,8 @@ static size_t csum_and_copy_to_pipe_iter(const void *addr, size_t bytes,
 
 size_t _copy_to_iter(const void *addr, size_t bytes, struct iov_iter *i)
 {
+	if (WARN_ON_ONCE(i->data_source))
+		return 0;
 	if (unlikely(iov_iter_is_pipe(i)))
 		return copy_pipe_to_iter(addr, bytes, i);
 	if (user_backed_iter(i))
@@ -606,6 +608,8 @@ static size_t copy_mc_pipe_to_iter(const void *addr, size_t bytes,
  */
 size_t _copy_mc_to_iter(const void *addr, size_t bytes, struct iov_iter *i)
 {
+	if (WARN_ON_ONCE(i->data_source))
+		return 0;
 	if (unlikely(iov_iter_is_pipe(i)))
 		return copy_mc_pipe_to_iter(addr, bytes, i);
 	if (user_backed_iter(i))
@@ -622,10 +626,9 @@ EXPORT_SYMBOL_GPL(_copy_mc_to_iter);
 
 size_t _copy_from_iter(void *addr, size_t bytes, struct iov_iter *i)
 {
-	if (unlikely(iov_iter_is_pipe(i))) {
-		WARN_ON(1);
+	if (WARN_ON_ONCE(!i->data_source))
 		return 0;
-	}
+
 	if (user_backed_iter(i))
 		might_fault();
 	iterate_and_advance(i, bytes, base, len, off,
@@ -639,10 +642,9 @@ EXPORT_SYMBOL(_copy_from_iter);
 
 size_t _copy_from_iter_nocache(void *addr, size_t bytes, struct iov_iter *i)
 {
-	if (unlikely(iov_iter_is_pipe(i))) {
-		WARN_ON(1);
+	if (WARN_ON_ONCE(!i->data_source))
 		return 0;
-	}
+
 	iterate_and_advance(i, bytes, base, len, off,
 		__copy_from_user_inatomic_nocache(addr + off, base, len),
 		memcpy(addr + off, base, len)
@@ -671,10 +673,9 @@ EXPORT_SYMBOL(_copy_from_iter_nocache);
  */
 size_t _copy_from_iter_flushcache(void *addr, size_t bytes, struct iov_iter *i)
 {
-	if (unlikely(iov_iter_is_pipe(i))) {
-		WARN_ON(1);
+	if (WARN_ON_ONCE(!i->data_source))
 		return 0;
-	}
+
 	iterate_and_advance(i, bytes, base, len, off,
 		__copy_from_user_flushcache(addr + off, base, len),
 		memcpy_flushcache(addr + off, base, len)
@@ -714,6 +715,8 @@ size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
 	size_t res = 0;
 	if (!page_copy_sane(page, offset, bytes))
 		return 0;
+	if (WARN_ON_ONCE(i->data_source))
+		return 0;
 	if (unlikely(iov_iter_is_pipe(i)))
 		return copy_page_to_iter_pipe(page, offset, bytes, i);
 	page += offset / PAGE_SIZE; // first subpage
@@ -811,9 +814,8 @@ size_t copy_page_from_iter_atomic(struct page *page, unsigned offset, size_t byt
 		kunmap_atomic(kaddr);
 		return 0;
 	}
-	if (unlikely(iov_iter_is_pipe(i) || iov_iter_is_discard(i))) {
+	if (WARN_ON_ONCE(!i->data_source)) {
 		kunmap_atomic(kaddr);
-		WARN_ON(1);
 		return 0;
 	}
 	iterate_and_advance(i, bytes, base, len, off,
@@ -1525,10 +1527,9 @@ size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum,
 {
 	__wsum sum, next;
 	sum = *csum;
-	if (unlikely(iov_iter_is_pipe(i) || iov_iter_is_discard(i))) {
-		WARN_ON(1);
+	if (WARN_ON_ONCE(!i->data_source))
 		return 0;
-	}
+
 	iterate_and_advance(i, bytes, base, len, off, ({
 		next = csum_and_copy_from_user(base, addr + off, len);
 		sum = csum_block_add(sum, next, off);
@@ -1548,6 +1549,8 @@ size_t csum_and_copy_to_iter(const void *addr, size_t bytes, void *_csstate,
 	struct csum_state *csstate = _csstate;
 	__wsum sum, next;
 
+	if (WARN_ON_ONCE(i->data_source))
+		return 0;
 	if (unlikely(iov_iter_is_discard(i))) {
 		// can't use csum_memcpy() for that one - data is not copied
 		csstate->csum = csum_block_add(csstate->csum,