[crypto] Allow for parsing of DER data separate from DER images

We currently provide pem_asn1() to allow for parsing of PEM data that
is not necessarily contained in an image.  Provide an equivalent
function der_asn1() to allow for similar parsing of DER data.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/image/der.c

@@ -38,32 +38,41 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  */
 
 /**
- * Extract ASN.1 object from image
+ * Extract ASN.1 object from DER data
  *
- * @v image		DER image
- * @v offset		Offset within image
+ * @v data		DER data
+ * @v len		Length of DER data
+ * @v offset		Offset within data
  * @v cursor		ASN.1 cursor to fill in
- * @ret next		Offset to next image, or negative error
+ * @ret next		Offset to next object, or negative error
  *
  * The caller is responsible for eventually calling free() on the
  * allocated ASN.1 cursor.
  */
-static int der_asn1 ( struct image *image, size_t offset __unused,
-		      struct asn1_cursor **cursor ) {
-	void *data;
+int der_asn1 ( userptr_t data, size_t len, size_t offset,
+	       struct asn1_cursor **cursor ) {
+	size_t remaining;
+	void *raw;
+
+	/* Sanity check */
+	assert ( offset <= len );
+	remaining = ( len - offset );
 
 	/* Allocate cursor and data buffer */
-	*cursor = malloc ( sizeof ( **cursor ) + image->len );
+	*cursor = malloc ( sizeof ( **cursor ) + remaining );
 	if ( ! *cursor )
 		return -ENOMEM;
-	data = ( ( ( void * ) *cursor ) + sizeof ( **cursor ) );
+	raw = ( ( ( void * ) *cursor ) + sizeof ( **cursor ) );
 
 	/* Populate cursor and data buffer */
-	(*cursor)->data = data;
-	(*cursor)->len = image->len;
-	copy_from_user ( data, image->data, 0, image->len );
+	(*cursor)->data = raw;
+	(*cursor)->len = remaining;
+	copy_from_user ( raw, data, offset, remaining );
 
-	return image->len;
+	/* Shrink cursor */
+	asn1_shrink_any ( *cursor );
+
+	return ( offset + (*cursor)->len );
 }
 
 /**
@@ -72,7 +81,7 @@ static int der_asn1 ( struct image *image, size_t offset __unused,
  * @v image		DER image
  * @ret rc		Return status code
  */
-static int der_probe ( struct image *image ) {
+static int der_image_probe ( struct image *image ) {
 	struct asn1_cursor cursor;
 	uint8_t buf[8];
 	size_t extra;
@@ -105,9 +114,37 @@ static int der_probe ( struct image *image ) {
 	return 0;
 }
 
+/**
+ * Extract ASN.1 object from DER image
+ *
+ * @v image		DER image
+ * @v offset		Offset within image
+ * @v cursor		ASN.1 cursor to fill in
+ * @ret next		Offset to next image, or negative error
+ *
+ * The caller is responsible for eventually calling free() on the
+ * allocated ASN.1 cursor.
+ */
+static int der_image_asn1 ( struct image *image, size_t offset,
+			    struct asn1_cursor **cursor ) {
+	int next;
+	int rc;
+
+	/* Extract ASN.1 object */
+	if ( ( next = der_asn1 ( image->data, image->len, offset,
+				 cursor ) ) < 0 ) {
+		rc = next;
+		DBGC ( image, "DER %s could not extract ASN.1: %s\n",
+		       image->name, strerror ( rc ) );
+		return rc;
+	}
+
+	return next;
+}
+
 /** DER image type */
 struct image_type der_image_type __image_type ( PROBE_NORMAL ) = {
 	.name = "DER",
-	.probe = der_probe,
-	.asn1 = der_asn1,
+	.probe = der_image_probe,
+	.asn1 = der_image_asn1,
 };

src/include/ipxe/der.h

@@ -9,8 +9,13 @@
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
+#include <stdint.h>
+#include <ipxe/asn1.h>
 #include <ipxe/image.h>
 
+extern int der_asn1 ( userptr_t data, size_t len, size_t offset,
+		      struct asn1_cursor **cursor );
+
 extern struct image_type der_image_type __image_type ( PROBE_NORMAL );
 
 #endif /* _IPXE_DER_H */