Re: [Suit] CBOR and Firmware Word Alignment

[David Brown <[e-mail]>]

On Tue, Jun 26, 2018 at 06:20:53PM +0000, Paul Lambert wrote:

>Has any thought gone into word alignment issues?

>Many platforms require firmware or other data objects to be word
>aligned. It waould be especially desirable to have large objects
>(like the firmware) be packaged such that the data does not have to
>be copied to a new location just to ensure word alignment. Word
>alignment would also facilitate in-place decryption of the firmeare.

What we do today in MCUboot is something like:

     +--------------------+
     | Header             |
     +--------------------+
     | Padding            |
     +--------------------+
     | XIP firmware image |
     +--------------------+
     | Manifest           |
     +--------------------+

where the header contains a length field to make finding the manifest
easy.

Because the images are execute in place (XIP), they have to be
compiled and linked at a specific address.  This precludes placing
them after the manifest, since the information there is variable (even
some signature encodings aren't deterministic in length).

The constraints on this image are fairly specific:

  - It is XIP, and therefore must be placed at an address known at
    compile/link time.

  - The start address must be aligned to a particular alignment,
    depending on the particular hardware (for Cortex M, it is a power
    of two greater or equal to the size of the interrupt vector table,
    typically 256 or 512 bytes).

It would be possible to insert 1 or more tags of some type into a CBOR
stream in order to pad data, and this could possibly be standardized
to one of the small values, so that this tag is only a single byte.

We haven't determine how the image and manifest will be delivered,
currently it seems like the manifest will be a separate file, but the
image file that is then downloaded, will also already have the
manifest appended to it.

David