2.3 KiB
Firmware image
Firmware entry point
WolfBoot can only chain-load and execute firmware images from a specific entry point in memory, which must be specified as the origin of the FLASH memory in the linker script of the embedded application. This correspond to the first partition in the flash memory.
Multiple firmware images can be created this way, and stored in different partitions. The bootloader will take care of moving the selected firmware to the first boot partition before chain-loading the image.
Due to the presence of an image header, the entry point of the application has a fixed additional offset of 256B from the beginning of the flash partition.
Firmware image header
Each (signed) firmware image is pre-pended with a fixed-size image header, containing useful information about the firmware. The image header is padded to fit in 256B, in order to guarantee that the entry point of the actual firmware is stored on the flash starting from a 256-Bytes aligned address. This ensures that the bootloader can relocate the vector table before chain-loading the firmware the interrupt continue to work properly after the boot is complete.
The image header is stored at the beginning of the slot and the actual firmware image starts 256 Bytes after it
Firmware trailer
At the end of the actual firmware image, the signing tool stores three trailer "TLV" (type-length-value) records, respectively containing: - A hash digest of the firmware, including its firmware header, obtained using SHA-256 - A hash digest of the public key that can be used by the bootloader to verify the authenticity of the firmware. The key must already be stored with the bootloader, and this field is only used as sanity check. - The signature obtained by signing the hash digest of the firmware with the factory private key
These three fields are required by the bootloader to verify the integrity and the origin of the firmware image.
The trailer of a signed firmware contains a TLV header and three TLV records that are used by the bootloader to verify the image
Image signing tool
The image signing tool generates the header and trailers for the compiled image, and add them to the output file that can be then stored on the primary slot on the device.