mcuboot in single app mode sysbuild, imgtool.py signs for wrong partition

[spookles]

Hello, question about mcuboot, particularly the imgtool.py.

When I run west build, the build eventually fails on:

Error: Image size (0xbac8) + trailer (0xc30) exceeds requested size 0x1000
The builder runs the script with --slot-size 4096
scripts/imgtool.py sign --version 0.0.0+0 --header-size 0x200 --slot-size 4096 --align 8 --key
Which is way too low.

Before that it reports after building the application that my application is about 47480 bytes.

Memory region Used Size Region Size %age Used
FLASH: 47480 B 67248 B 70.60%
RAM: 10752 B 31 KB 33.87%
RetainedMem: 0 GB 4 B 0.00%
IDT_LIST: 0 GB 32 KB 0.00%

My pm_static.yml looks like:

mcuboot:
  address: 0x00000000
  end_address: 0x0000C800
  size: 0xC800   # 50 KB
  region: flash_primary

app:
  address: 0x0000C800
  end_address: 0x00012800
  size: 0x12800  # 74 KB
  region: flash_primary

app_pad:
  address: 0x0001F000
  end_address: 0x00020000
  size: 0x1000   # 4 KB
  region: flash_primary

And my dts:

/ {

    chosen {

        zephyr,sram = &sram0;

        zephyr,flash = &flash0;

        zephyr,code-partition = &slot0_partition;

        zephyr,boot-mode = &retention0;

    };

};

&flash0 {

    partitions {

        compatible = "fixed-partitions";

        #address-cells = <1>;

        #size-cells = <1>;

        boot_partition: partition@0 {

            label = "mcuboot";

            reg = <0x00000000 DT_SIZE_K(50)>; // 50 KB

            read-only;

        };

        slot0_partition: partition@C800 {

            label = "image-0";

            reg = <0x0000C800 DT_SIZE_K(74)>; // 74 KB

        };

        slot1_partition: partition@1F000 {

            label = "image-1";

            reg = <0x0001F000 DT_SIZE_K(4)>; // 4 KB dummy partition

        };

    };

};

My root prj.conf has the following lines for the bootloader

CONFIG_BOOTLOADER_MCUBOOT=y

CONFIG_REBOOT=y

CONFIG_RETENTION_BOOT_MODE=y

CONFIG_RETAINED_MEM=y

CONFIG_RETENTION=y

CONFIG_MCUBOOT_BOOTLOADER_MODE_SINGLE_APP=y

CONFIG_MCUBOOT_SIGNATURE_KEY_FILE="root-rsa-2048.pem"

The sysbuild/mcuboot.conf has the following lines

CONFIG_BOOT_SIGNATURE_KEY_FILE="./root-rsa-2048.pem"

CONFIG_BOOT_SERIAL_BOOT_MODE=y

CONFIG_RETENTION_BOOT_MODE=y

CONFIG_RETENTION=y

CONFIG_RETAINED_MEM=y

CONFIG_MCUBOOT_BOOTLOADER_MODE_SINGLE_APP=y

CONFIG_CONSOLE=y

CONFIG_FLASH=y

Because of my limited flash size (128kb) I want to run mcuboot as single app as specified in my prj.conf.
The linker appears to think it should sign the appliction for the slot1_partition which I confirmed when I increased the slot1 dummy partition size. Ideally I do not need to define a slot1, but that seems to be a requirement as I get CMake errors that it expects a slot1 when I remove it entirely.

Any help/guidance would be welcome!

[spookles]

Digging into the mcuboot.cmake I found

if(NOT CONFIG_MCUBOOT_BOOTLOADER_MODE_SINGLE_APP AND (NOT CONFIG_MCUBOOT_BOOTLOADER_MODE_FIRMWARE_UPDATER OR CONFIG_MCUBOOT_APPLICATION_FIRMWARE_UPDATER)
AND NOT CONFIG_MCUBOOT_BOOTLOADER_MODE_SINGLE_APP_RAM_LOAD)
# Slot 1 size is used instead of slot 0 size
set(slot_size)
dt_nodelabel(slot1_flash NODELABEL "slot1_partition" REQUIRED)
dt_prop(slot_size PATH "${slot1_flash}" PROPERTY "reg" INDEX 1 REQUIRED)
endif()

So I assume CONFIG_MCUBOOT_BOOTLOADER_MODE_SINGLE_APP=y is never processed.

[spookles]

Modified the mcuboot.cmake and added

message(STATUS "CONFIG_MCUBOOT_BOOTLOADER_MODE_SINGLE_APP = ${CONFIG_MCUBOOT_BOOTLOADER_MODE_SINGLE_APP}")

Which resulted in an empty string, so the kconfig variable is not set correctly.