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Subject: [PATCH 2/7] iov_iter: Add a general purpose iteration function
From:   David Howells <dhowells@redhat.com>
To:     Steve French <smfrench@gmail.com>
Cc:     dhowells@redhat.com, Shyam Prasad N <nspmangalore@gmail.com>,
        Rohith Surabattula <rohiths.msft@gmail.com>,
        Jeff Layton <jlayton@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>, linux-cifs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Date:   Wed, 25 May 2022 15:26:22 +0100
Message-ID: <165348878240.2106726.14850277156941003064.stgit@warthog.procyon.org.uk>
In-Reply-To: <165348876794.2106726.9240233279581920208.stgit@warthog.procyon.org.uk>
References: <165348876794.2106726.9240233279581920208.stgit@warthog.procyon.org.uk>
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Precedence: bulk

Add a function, iov_iter_scan(), to iterate over the buffers described by
an I/O iterator, kmapping and passing each contiguous chunk 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 |    4 ++++
 lib/iov_iter.c      |   40 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 5a3c6f296b96..8d89203064e7 100644
--- a/include/linux/uio.h
+++ b/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)
 {
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 8db34ddd23be..834e1e268eb6 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1789,6 +1789,46 @@ 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)
 {