Re: [PATCH v6 1/6] lib: add ASN.1 encoder

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Tue, Mar 03, 2020 at 03:49:30PM -0500, James Bottomley wrote:
> On Tue, 2020-03-03 at 21:22 +0200, Jarkko Sakkinen wrote:
> > On Mon, Mar 02, 2020 at 07:27:54AM -0500, James Bottomley wrote:
> [...]
> > > diff --git a/lib/asn1_encoder.c b/lib/asn1_encoder.c
> > > new file mode 100644
> > > index 000000000000..c7493667656e
> > > --- /dev/null
> > > +++ b/lib/asn1_encoder.c
> > > @@ -0,0 +1,431 @@
> > > +// SPDX-License-Identifier: GPL-2.0-only
> > > +/*
> > > + * Simple encoder primitives for ASN.1 BER/DER/CER
> > > + *
> > > + * Copyright (C) 2019 James.Bottomley@HansenPartnership.com
> > > + */
> > > +
> > > +#include <linux/asn1_encoder.h>
> > > +#include <linux/bug.h>
> > > +#include <linux/string.h>
> > > +#include <linux/module.h>
> > > +
> > > +/**
> > > + * asn1_encode_integer() - encode positive integer to ASN.1
> > > + * @data:	pointer to the pointer to the data
> > > + * @end_data:	end of data pointer, points one beyond last
> > > usable byte in @data
> > > + * @integer:	integer to be encoded
> > > + *
> > > + * This is a simplified encoder: it only currently does
> > > + * positive integers, but it should be simple enough to add the
> > > + * negative case if a use comes along.
> > > + */
> > > +unsigned char *
> > > +asn1_encode_integer(unsigned char *data, const unsigned char
> > > *end_data,
> > > +		    s64 integer)
> > > +{
> > > +	unsigned char *d = &data[2];
> > 
> > So what magic does index 2 contain?
> 
> ASN.1 has the form <tag> <length> <content> so a small integer has one
> byte for the _tag(UNIV, PRIM, INT), one byte for the length, which must
> be between 1 and 4, so the actual integer itself starts at 2.

OK cool.

> > > +	int i;
> > > +	bool found = false;
> > > +	int data_len = end_data - data;
> > 
> > I'd reorder these:
> > 
> > int data_len = end_data - data;
> > unsigned char *d = &data[2];
> > bool found = false;
> > int i;
> 
> Ah, reverse Christmas tree ... I can do that.

Always prefer it.

/Jarkko