[CRYPTO] scatterwalk: Add scatterwalk_map_and_copy

This patch adds the function scatterwalk_map_and_copy which reads or
writes a chunk of data from a scatterlist at a given offset.  It will
be used by authenc which would read/write the authentication data at
the end of the cipher/plain text.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 22 insertions, 0 deletions

diff --git a/crypto/scatterwalk.c b/crypto/scatterwalk.c
index 81afd1790a1d..e93a8f6f35d4 100644
--- a/crypto/scatterwalk.c
+++ b/crypto/scatterwalk.c
@@ -107,3 +107,25 @@ void scatterwalk_copychunks(void *buf, struct scatter_walk *walk,
         }
 }
 EXPORT_SYMBOL_GPL(scatterwalk_copychunks);
+
+void scatterwalk_map_and_copy(void *buf, struct scatterlist *sg,
+                              unsigned int start, unsigned int nbytes, int out)
+{
+        struct scatter_walk walk;
+        unsigned int offset = 0;
+
+        for (;;) {
+                scatterwalk_start(&walk, sg);
+
+                if (start < offset + sg->length)
+                        break;
+
+                offset += sg->length;
+                sg = sg_next(sg);
+        }
+
+        scatterwalk_advance(&walk, start - offset);
+        scatterwalk_copychunks(buf, &walk, nbytes, out);
+        scatterwalk_done(&walk, out, 0);
+}
+EXPORT_SYMBOL_GPL(scatterwalk_map_and_copy);