196 lines
8.4 KiB
Markdown
196 lines
8.4 KiB
Markdown
# Porting how-to
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This document describes the requirements and necessary steps required to port
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`MCUboot` to a new target `OS`.
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# Requirements
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* `MCUboot` requires a configuration file, which can be included as
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mcuboot_config/mcuboot_config.h, which configures various options
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(that begin with MCUBOOT_).
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* `MCUboot` requires that the target provides a `flash` API with ability to
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get the flash's minimum write size, and read/write/erase individual sectors.
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* `MCUboot` doesn't bundle a cryptographic library, which means the target
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OS must already have it bundled. The supported libraries at the moment are
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either `Mbed TLS` or the set `tinycrypt` + `Mbed TLS` (where `Mbed TLS` is
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used to provide functionality not existing in `tinycrypt`).
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# Steps to port
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## Main app and calling the bootloader
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From the perspective of the target OS, the bootloader can be seen as a library,
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so an entry point must be provided. This is likely a typical `app` for the
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target OS, and it must call the following function to run the bootloader:
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```c
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int boot_go(struct boot_rsp *rsp);
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```
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This function is located at `boot/bootutil/loader.c` and receives a `struct
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boot_rsp` pointer. The `struct boot_rsp` is defined as:
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```c
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struct boot_rsp {
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/** A pointer to the header of the image to be executed. */
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const struct image_header *br_hdr;
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/**
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* The flash offset of the image to execute. Indicates the position of
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* the image header.
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*/
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uint8_t br_flash_id;
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uint32_t br_image_addr;
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};
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```
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After running the management functions of the bootloader, `boot_go` returns
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an initialized `boot_rsp` which has pointers to the location of the image
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where the target firmware is located which can be used to jump to.
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## Configuration file
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You must provide a file, mcuboot_config/mcuboot_config.h. This is
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included by several files in the "library" portion of MCUboot; it
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provides preprocessor definitions that configure the library's
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build.
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See the file samples/mcuboot_config/mcuboot_config.template.h for a
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starting point and more information. This is a good place to convert
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settings in your environment's configuration system to those required
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by MCUboot. For example, Mynewt uses MYNEWT_VAL() and Zephyr uses
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Kconfig; these configuration systems are converted to MCUBOOT_ options
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in the following files:
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- boot/zephyr/include/mcuboot_config/mcuboot_config.h
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- boot/mynewt/mcuboot_config/include/mcuboot_config/mcuboot_config.h
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## Flash Map
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The bootloader requires to be able to address flash regions where the code
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for MCUboot and images of applications are stored, in system-agnostic way.
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For that purpose the MCUboot uses ID, which is integer (uint8_t) number
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that should uniquely identify each flash region.
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Such flash regions are served by object of `const struct flash_area` type while
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layout of these objects is gathered under `flash_map`.
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The common code of MCUboot, that is non-system specific, does not directly
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access contents of that object and never modifies it, instead it calls
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`flash_area_` API to perform any actions on that object.
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This way systems are free to implement internal logic of flash map or define
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`struct flash_area` as they wish; the only restriction is that ID should be
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uniquely tied to region characterized by device, offset and size.
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Changes to common MCUboot code should not affect system specific internals
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of flash map, on the other side system specific code, within MCUboot, is
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is not restricted from directly accessing `struct flash_area` elements.
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An implementation of `struct flash_area` may take form of:
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```c
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struct flash_area {
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uint8_t fa_id; /** The slot/scratch identification */
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uint8_t fa_device_id; /** The device id (usually there's only one) */
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uint16_t pad16;
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uint32_t fa_off; /** The flash offset from the beginning */
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uint32_t fa_size; /** The size of this sector */
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};
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```
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The above example of structure hold all information that is currently required
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by MCUboot, although the MCUboot will not be trying to access them directly,
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instead a system is required to provide following mandatory getter functions:
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```c
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/*< Obtains ID of the flash area characterized by `fa` */
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int flash_area_get_id(const struct flash_area *fa);
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/*< Obtains ID of a device the flash area `fa` described region resides on */
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int flash_area_get_device_id(const struct flash_area *fa)
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/*< Obtains offset, from the beginning of a device, the flash area described
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* region starts at */
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uint32_t flash_area_get_off(const struct flash_area *fa)
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/*< Obtains size, from the offset, of the flash area `fa` characterized region */
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uint32_t flash_area_get_size(const struct flash_area *fa)
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```
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The MCUboot common code uses following defines that should be defined by system
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specific header files and are used to identify destination of flash area by ID:
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```c
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/* Independent from multiple image boot */
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#define FLASH_AREA_BOOTLOADER 0
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#define FLASH_AREA_IMAGE_SCRATCH 3
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```
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```c
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/* Flash area IDs of the first image in case of multiple images */
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#define FLASH_AREA_IMAGE_PRIMARY 1
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#define FLASH_AREA_IMAGE_SECONDARY 2
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```
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```c
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/* Flash area IDs of the second image in case of multiple images */
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#define FLASH_AREA_IMAGE_PRIMARY 5
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#define FLASH_AREA_IMAGE_SECONDARY 6
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```
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The numbers, given above, are provided as an example and depend on system
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implementation.
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The main, also required, set of API functions that perform operations on
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flash characterized by `struct flash_area` objects is as follows:
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```c
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/*< Opens the area for use. id is one of the `fa_id`s */
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int flash_area_open(uint8_t id, const struct flash_area **);
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void flash_area_close(const struct flash_area *);
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/*< Reads `len` bytes of flash memory at `off` to the buffer at `dst` */
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int flash_area_read(const struct flash_area *, uint32_t off, void *dst,
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uint32_t len);
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/*< Writes `len` bytes of flash memory at `off` from the buffer at `src` */
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int flash_area_write(const struct flash_area *, uint32_t off,
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const void *src, uint32_t len);
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/*< Erases `len` bytes of flash memory at `off` */
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int flash_area_erase(const struct flash_area *, uint32_t off, uint32_t len);
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/*< Returns this `flash_area`s alignment */
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uint32_t flash_area_align(const struct flash_area *);
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/*< What is value is read from erased flash bytes. */
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uint8_t flash_area_erased_val(const struct flash_area *);
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/*< Given flash area ID, return info about sectors within the area. */
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int flash_area_get_sectors(int fa_id, uint32_t *count,
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struct flash_sector *sectors);
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/*< Returns the `fa_id` for slot, where slot is 0 (primary) or 1 (secondary).
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`image_index` (0 or 1) is the index of the image. Image index is
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relevant only when multi-image support support is enabled */
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int flash_area_id_from_multi_image_slot(int image_index, int slot);
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/*< Returns the slot (0 for primary or 1 for secondary), for the supplied
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`image_index` and `area_id`. `area_id` is unique and is represented by
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`fa_id` in the `flash_area` struct. */
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int flash_area_id_to_multi_image_slot(int image_index, int area_id);
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```
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---
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***Note***
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*As of writing, it is possible that MCUboot will open a flash area multiple times simultaneously (through nested calls to `flash_area_open`). As a result, MCUboot may call `flash_area_close` on a flash area that is still opened by another part of MCUboot. As a workaround when porting, it may be necessary to implement a counter of the number of times a given flash area has been opened by MCUboot. The `flash_area_close` implementation should only fully deinitialize the underlying flash area when the open counter is decremented to 0. See [this GitHub PR](https://github.com/mcu-tools/mcuboot/pull/894/) for a more detailed discussion.*
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---
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## Memory management for Mbed TLS
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`Mbed TLS` employs dynamic allocation of memory, making use of the pair
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`calloc/free`. If `Mbed TLS` is to be used for crypto, your target RTOS
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needs to provide this pair of function.
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To configure the what functions are called when allocating/deallocating
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memory `Mbed TLS` uses the following call:
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```
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int mbedtls_platform_set_calloc_free (void *(*calloc_func)(size_t, size_t),
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void (*free_func)(void *));
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```
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For reference see [Mbed TLS platform.h](https://tls.mbed.org/api/platform_8h.html).
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If your system already provides functions with compatible signatures, those can
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be used directly here, otherwise create new functions that glue to your
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`calloc/free` implementations.
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