MCUboot: images always confirmed

[real-tintin]

Hi!

I'm wondering if some can explain to me why it seems like all my compiled MCUboot images are confirmed?

When building I get three binaries (I have enable both CONFIG_MCUBOOT_GENERATE_UNSIGNED_IMAGE, CONFIG_MCUBOOT_GENERATE_CONFIRMED_IMAGE - is that correct?):

1. zephyr.bin
2. zephyr.signed.bin
3. zephyr.signed.confirmed.bin

My assumption is that they are what the name suggests: 1. not signed nor confirmed, 2. only signed, 3. signed and confirmed.

However, when using MCUboot's method boot_is_img_confirmed() they all returns true. I have also tried updating the image using Hawkbit, which work fine, however it's still confirmed.

Note, I saw that in this PR: https://github.com/zephyrproject-rtos/zephyr/pull/51743/files it is mentioned that all pristine/initial image are confirmed?

[real-tintin]

I believe I figured it out why the image was confirmed (RTFM!).

There exist a "trailer", metadata in the end of the image. Amongst other things, this contains information about image confirmation. On default, this is missing/not defined (?), but can be 0xff padded with an image signing tool e.g., adding --pad flag. When padded, the image is "initialised" such that the magic is not unset and hence not confirmed. So, any subsequent (after initial flash) needs to be 0xff padded, to unconfirmed, and set via SW.

Ref: https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/mcuboot/design.html#new-swaps-non-resumes

Also helped me: mcu-tools/mcuboot#1066