iov_iter: Add a general purpose iteration function

Add a function, iov_iter_scan(), to iterate over the buffers described by
an I/O iterator, kmapping and passing each contiguous chunk the the
supplied scanner function in turn, up to the requested amount of data or
until the scanner function returns an error.

This can be used, for example, to hash all the data in an iterator by
having the scanner function call the appropriate crypto update function.

Signed-off-by: David Howells <dhowells@redhat.com>

include/linux/uio.h

@@ -244,6 +244,10 @@ int iov_iter_npages(const struct iov_iter *i, int maxpages);
 void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state);
 
 const void *dup_iter(struct iov_iter *new, struct iov_iter *old, gfp_t flags);
+ssize_t iov_iter_scan(struct iov_iter *i, size_t bytes,
+		      ssize_t (*scanner)(struct iov_iter *i, const void *p,
+					 size_t len, size_t off, void *priv),
+		      void *priv);
 
 static inline size_t iov_iter_count(const struct iov_iter *i)
 {

lib/iov_iter.c

@@ -1777,6 +1777,46 @@ ssize_t extract_iter_to_iter(struct iov_iter *orig,
 }
 EXPORT_SYMBOL(extract_iter_to_iter);
 
+/**
+ * iov_iter_scan - Scan a source iter
+ * @i: The iterator to scan
+ * @bytes: The amount of buffer/data to scan
+ * @scanner: The function to call for each bit
+ * @priv: Private data to pass to the scanner function
+ *
+ * Scan an iterator, passing each segment to the scanner function.  If the
+ * scanner returns an error at any time, scanning stops and the error is
+ * returned, otherwise the sum of the scanner results is returned.
+ */
+ssize_t iov_iter_scan(struct iov_iter *i, size_t bytes,
+		      ssize_t (*scanner)(struct iov_iter *i, const void *p,
+					 size_t len, size_t off, void *priv),
+		      void *priv)
+{
+	ssize_t ret = 0, scanned = 0;
+
+	if (!bytes)
+		return 0;
+	if (iter_is_iovec(i))
+		might_fault();
+
+	iterate_and_advance(
+		i, bytes, base, len, off, ({
+				ret = scanner(i, base, len, off, priv);
+				if (ret < 0)
+					break;
+				scanned += ret;
+			}), ({
+				ret = scanner(i, base, len, off, priv);
+				if (ret < 0)
+					break;
+				scanned += ret;
+			})
+	);
+	return ret < 0 ? ret : scanned;
+}
+EXPORT_SYMBOL(iov_iter_scan);
+
 size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum,
 			       struct iov_iter *i)
 {