Subject: [PATCH v2 12/29] crypto: scatterwalk - add new functions for skipping data

From: Eric Biggers <[email-masked]>
Date: Sun, 29 Dec 2024 16:14:01 -0800

From: Eric Biggers <[email-masked]>

Add scatterwalk_skip() to skip the given number of bytes in a
scatter_walk.  Previously support for skipping was provided through
scatterwalk_copychunks(..., 2) followed by scatterwalk_done(), which was
confusing and less efficient.

Also add scatterwalk_start_at_pos() which starts a scatter_walk at the
given position, equivalent to scatterwalk_start() + scatterwalk_skip().
This addresses another common need in a more streamlined way.

Later patches will convert various users to use these functions.

Signed-off-by: Eric Biggers <[email-masked]>
---
 crypto/scatterwalk.c         | 15 +++++++++++++++
 include/crypto/scatterwalk.h | 18 ++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/crypto/scatterwalk.c b/crypto/scatterwalk.c
index 16f6ba896fb6..af436ad02e3f 100644
--- a/crypto/scatterwalk.c
+++ b/crypto/scatterwalk.c
@@ -13,10 +13,25 @@
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/scatterlist.h>
 
+void scatterwalk_skip(struct scatter_walk *walk, unsigned int nbytes)
+{
+	struct scatterlist *sg = walk->sg;
+
+	nbytes += walk->offset - sg->offset;
+
+	while (nbytes > sg->length) {
+		nbytes -= sg->length;
+		sg = sg_next(sg);
+	}
+	walk->sg = sg;
+	walk->offset = sg->offset + nbytes;
+}
+EXPORT_SYMBOL_GPL(scatterwalk_skip);
+
 static inline void memcpy_dir(void *buf, void *sgdata, size_t nbytes, int out)
 {
 	void *src = out ? buf : sgdata;
 	void *dst = out ? sgdata : buf;
 
diff --git a/include/crypto/scatterwalk.h b/include/crypto/scatterwalk.h
index 924efbaefe67..5c7765f601e0 100644
--- a/include/crypto/scatterwalk.h
+++ b/include/crypto/scatterwalk.h
@@ -31,10 +31,26 @@ static inline void scatterwalk_start(struct scatter_walk *walk,
 {
 	walk->sg = sg;
 	walk->offset = sg->offset;
 }
 
+/*
+ * This is equivalent to scatterwalk_start(walk, sg) followed by
+ * scatterwalk_skip(walk, pos).
+ */
+static inline void scatterwalk_start_at_pos(struct scatter_walk *walk,
+					    struct scatterlist *sg,
+					    unsigned int pos)
+{
+	while (pos > sg->length) {
+		pos -= sg->length;
+		sg = sg_next(sg);
+	}
+	walk->sg = sg;
+	walk->offset = sg->offset + pos;
+}
+
 static inline unsigned int scatterwalk_pagelen(struct scatter_walk *walk)
 {
 	unsigned int len = walk->sg->offset + walk->sg->length - walk->offset;
 	unsigned int len_this_page = offset_in_page(~walk->offset) + 1;
 	return len_this_page > len ? len : len_this_page;
@@ -90,10 +106,12 @@ static inline void scatterwalk_done(struct scatter_walk *walk, int out,
 	if (!more || walk->offset >= walk->sg->offset + walk->sg->length ||
 	    !(walk->offset & (PAGE_SIZE - 1)))
 		scatterwalk_pagedone(walk, out, more);
 }
 
+void scatterwalk_skip(struct scatter_walk *walk, unsigned int nbytes);
+
 void scatterwalk_copychunks(void *buf, struct scatter_walk *walk,
 			    size_t nbytes, int out);
 
 void scatterwalk_map_and_copy(void *buf, struct scatterlist *sg,
 			      unsigned int start, unsigned int nbytes, int out);
-- 
2.47.1