RetailOS

The stock operating system running on non-iOS iPods. It runs everything from device drivers to the clickwheel user interface.

Naming

The only 'official' name seems to be 'RetailOS', found in the Nano 3G WTF. It is also referred to as 'osos' per the file name in the resource partition of the firmware bundle.

Architecture

RetailOS is a small, embedded, single-user, single-binary, real time operating system. With time it acquire more and more complex functionality, like PowerVR drivers and being able to load external applications ('eApps') which are used for games.

The core of the system is based on RTXC 3.2, with the end-user interface based on intellectual property from a company called Pixo.

Security

As evidenced by the success of the Notes vulnerability, at least up to Nano 4G there was no kind of security hardening, and in fact all processes, including games, seem to be running in ARM system mode. This should make exploitation of newer RetailOS bugs trivial.

Boot chain

RetailOS is loaded by the second-stage bootloader (stored on NOR/NAND depending on the device generation), from NAND into DRAM.

While other stages of the boot chain (eg. the bootloader, WTF mode in newer devices, the diagnostics tool) are based around EFI firmware volumes and an EFI runtime, RetailOS is a single binary blob without any built-in modularity.

eApp Signing

Not yet documented fully. Each game seems to ship with a Manifest.plist.p7p which is a PKCS#7 signature for the main Manifest.plist.

Options

We have found some 'secret' options that can be set by creating specially named files. See Options.

RTXC

Documentation

This seems to be the best public document available about RTXC 3.2: DSAAXSA0003458.pdf. It contains example code for most services, but unfortunately is still missing any structure definitions.

There's also some training slides available: 5752851.pdf. These introduce the general architecture and concept of RTXC 3.2.

Services / Syscalls

This table comes from cross-referencing RetailOS, publicly available RTXC PDFs and publicly availble RTXC binaries with debug symbols.

Name	Number	Description
void KS_pend(SEMA sema)	0x03	Semaphore DONE -> PENDING.
RTXCMSG *KS_receive(MBOX mailbox, TASK task)	0x05	Receive from mailbox.
KSRC KS_enqueue[w](QUEUE queue, void *entry)	0x0c	Push into FIFO (and block if full with 'w' variant).
void KS_dequeue[w](QUEUE queue, void *dest)	0x0d	Pop from FIFO (and block if empty with 'w' variant).
KSRC KS_lock(RESOURCE resource)	0x0e	Lock a resource.
KSRC KS_lockt(RESOURCE resource, TICKS timoeut)	0x0e	Lock a resource with timeout.
KSRC KS_unlock(RESOURCE resource)	0x0f	Unlock an owned resource.
CLKBLK *KS_alloc_timer(void)	0x10	Allocate next free timer from pool.
CLKBLK *KS_start_timer(CLKBLK *timer, TICKS initial_period, TICKS cycle_time, SEMA sema)	0x12	Start timer.
KSRC KS_stop_timer(CLKBLK *timer)	0x13	Stop timer.
void KS_delay(TASK task, TICKS period)	0x14	Block specified task for a period of time.
void KS_execute(TASK task)	0x15	Start a task from its beginning address.
KSRC KS_deftask(TASK task, PRIORITY priority, char *stack, size_t stacksize, void (*entry)(void))	0x16	Define the attributes of an inactive task.
TASK KS_alloc_task(void)	0x17	Allocate the next available Task Control Block from the pool of free TCBs.
void KS_terminate(TASK task)	0x18	Stop a task by setting it to INACTIVE.
void KS_suspend(TASK task)	0x19	Suspend a task until resumed or re-executed.
void KS_defpriority(TASK task, PRIORITY priority)	0x1b	Define or set priority of task.
void KS_yield(void)	0x1c	Voluntary release of control to any other task of the same priority.
SEMA KS_waitm(SEMA *semalist)	0x22	Wait on multiple semaphores.
time_t KS_inqtime(void)	0x24	Get current time-of-day.
void KS_deftime(time_t time)	0x25	Set current time-of-day.
TASK KS_inqres(RESOURCE resource)	0x26	Get owner of resource.
KSRC KS_defres(RESOURCE resource, RESATTR condition)	0x27	Define priority inversion on resource.
void *KS_inqtask_arg(TASK task)	0x28	Get environment arguments of task.
void KS_deftask_arg(TASK task, void *arg)	0x29	Set environment arguments for task.
KSRC KS_defqueue(QUEUE queue, size_t width, int depth, void *body, int currsize)	0x2e	Define queue.
int KS_user(int (*func) (void *), void *arg)	0x30	Execute function as if it were kernel service.

The RTXC memory allocation facilities (KS_alloc/free/create_part/alloc_part/defpart/free_part) are not used by RetailOS and not built into the service dispatcher, at least on Nano 5G.

External links

• RTXC Kernel Services (1999)

https://twitter.com/johnwhitley/status/1451952369248264201