X.509: Add an ASN.1 decoder

Add an ASN.1 BER/DER/CER decoder.  This uses the bytecode from the ASN.1
compiler in the previous patch to inform it as to what to expect to find in the
encoded byte stream.  The output from the compiler also tells it what functions
to call on what tags, thus allowing the caller to retrieve information.

The decoder is called as follows:

	int asn1_decoder(const struct asn1_decoder *decoder,
			 void *context,
			 const unsigned char *data,
			 size_t datalen);

The decoder argument points to the bytecode from the ASN.1 compiler.  context
is the caller's context and is passed to the action functions.  data and
datalen define the byte stream to be decoded.

Note that the decoder is currently limited to datalen being less than 64K.
This reduces the amount of stack space used by the decoder because ASN.1 is a
nested construct.  Similarly, the decoder is limited to a maximum of 10 levels
of constructed data outside of a leaf node also in an effort to keep stack
usage down.

These restrictions can be raised if necessary.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/asn1_decoder.h

@@ -0,0 +1,24 @@
+/* ASN.1 decoder
+ *
+ * Copyright (C) 2012 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public Licence
+ * as published by the Free Software Foundation; either version
+ * 2 of the Licence, or (at your option) any later version.
+ */
+
+#ifndef _LINUX_ASN1_DECODER_H
+#define _LINUX_ASN1_DECODER_H
+
+#include <linux/asn1.h>
+
+struct asn1_decoder;
+
+extern int asn1_ber_decoder(const struct asn1_decoder *decoder,
+			    void *context,
+			    const unsigned char *data,
+			    size_t datalen);
+
+#endif /* _LINUX_ASN1_DECODER_H */