137 lines
5.0 KiB
Markdown
137 lines
5.0 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 a `flash_map` to be able to know how the flash is
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partitioned. A `flash_map` consists of `struct flash_area` entries
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specifying the partitions, where a `flash_area` defined as follows:
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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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`fa_id` is can be one of the following options:
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```c
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#define FLASH_AREA_IMAGE_0 1
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#define FLASH_AREA_IMAGE_1 2
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#define FLASH_AREA_IMAGE_SCRATCH 3
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```
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The functions that must be defined for working with the `flash_area`s are:
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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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uint8_t flash_area_align(const struct flash_area *);
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/*< Initializes an array of flash_area elements for the slot's sectors */
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int flash_area_to_sectors(int idx, int *cnt, struct flash_area *ret);
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/*< Returns the `fa_id` for slot, where slot is 0 or 1 */
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int flash_area_id_from_image_slot(int slot);
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/*< Returns the slot, for the `fa_id` supplied */
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int flash_area_id_to_image_slot(int area_id);
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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 [^cite1]:
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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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If your system already provides functions with compatible signatures, those
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can be used directly here, otherwise create new functions that glue to
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your `calloc/free` implementations.
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[^cite1]```https://tls.mbed.org/api/platform_8h.html```
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