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apply return guard and stack canary monitoring #39

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Merged
JGoard merged 3 commits into from
Aug 5, 2025
Merged

apply return guard and stack canary monitoring #39

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4676db9
apply return guard and stack canary monitoring
nick-a-schneider Aug 5, 2025
afaeaa9
include os header changes
nick-a-schneider Aug 5, 2025
ea24aa8
include tcb header changes
nick-a-schneider Aug 5, 2025
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7 changes: 4 additions & 3 deletions jocktos/inc/os.h
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Original file line number Diff line number Diff line change
Expand Up @@ -83,9 +83,10 @@
typedef struct {
volatile bool pending;
volatile uint32_t tick_count;
volatile TaskControlBlock* running; ///< Currently running task
volatile TaskControlBlock* ready; ///< Singly linked list of tasks ready to run, in decending order of priority
volatile TaskControlBlock* suspended; ///< Singly linked list of suspended tasks, in decending order of priority
volatile TaskControlBlock* terminated; ///< Currently running task
volatile TaskControlBlock* running; ///< Currently running task
volatile TaskControlBlock* ready; ///< Singly linked list of tasks ready to run, in decending order of priority
volatile TaskControlBlock* suspended; ///< Singly linked list of suspended tasks, in decending order of priority
} Scheduler;

/**
Expand Down
3 changes: 2 additions & 1 deletion jocktos/inc/tcb.h
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Expand Up @@ -88,7 +88,8 @@ typedef enum {
RUNNING = 0, ///< The task is currently executing on the CPU with priority over others.
READY = 1, ///< The task is ready and waiting for execution by the scheduler.
BLOCKED = 2, ///< The task is waiting for a resource (e.g., semaphore, mutex) to become available.
SUSPENDED = 3 ///< The task is temporarily inactive and can be reactivated by an event.
SUSPENDED = 3, ///< The task is temporarily inactive and can be reactivated by an event.
TERMINATED = 4 ///< The task has been terminated and is no longer executing.
} TaskState;

/**
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40 changes: 32 additions & 8 deletions jocktos/src/os.c
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Original file line number Diff line number Diff line change
Expand Up @@ -18,11 +18,20 @@

/* -- Defines ------------------------------------------------------------- */

#define FILL 0xBABEFACE // for task allocation debugging
#define CANARY 0xDEADBEEF
#define CANARY_SIZE 16
#define TRIGGER_PendSV *(uintptr_t volatile *)0xE000ED04 = (1U << 28)

/* -- Types --------------------------------------------------------------- */

/* -- Private Function Declarations --------------------------------------- */
/**
* \details Safely catches an unexpected return from a task.
* Any task that returns is marked as 'terminated' and moved accordingly.
*
*/
void task_exit_guard(void);

/**
* \brief Updates the task control blocks stack_usage
Expand Down Expand Up @@ -182,8 +191,7 @@ void initializeStack(TaskControlBlock* tcb) {
// - non-critical registers are initialized to their index
*(--stack_ptr) = (1U << 24); ///< Set thumb state bit in EPSR
*(--stack_ptr) = (uintptr_t)tcb->task_handle; ///< Set PC to task function handle
*(--stack_ptr) = 0xFFFFFFF9U; ///< Set LR register for MSP thread mode
// *(--stack_ptr) = 0xFFFFFFFDU; ///< Set LR register for PSP thread mode
*(--stack_ptr) = (uintptr_t)task_exit_guard; // LR: Return trap
*(--stack_ptr) = 0x0000000CU; ///< Set R12 register deafult to its index
*(--stack_ptr) = 0x00000003U; ///< Set R3 register deafult to its index
*(--stack_ptr) = 0x00000002U; ///< Set R2 register deafult to its index
Expand All @@ -201,11 +209,11 @@ void initializeStack(TaskControlBlock* tcb) {
*(--stack_ptr) = 0x00000004U; ///< Set R4 register deafult to its index
tcb->stack_pointer = stack_ptr;
// Fill unused process stack with known value
while (stack_ptr > tcb->stack_overflow + 8) {
*(--stack_ptr) = 0xBABEFACEU;
while (stack_ptr > tcb->stack_overflow + CANARY_SIZE) {
*(--stack_ptr) = FILL;
}
while (stack_ptr > tcb->stack_overflow) {
*(--stack_ptr) = 0xDEADBEEFU; ///< set top 8 bytes to something else
*(--stack_ptr) = CANARY; ///< set top 8 bytes to something else
}
}

Expand All @@ -224,7 +232,17 @@ void SysTick_Handler(void) {
uint32_t primask;
primask = jock_os_enter_critical_section();
JOCKTOSScheduler.tick_count++;
jock_os_switch_running_task(&JOCKTOSScheduler.ready);
TaskControlBlock** dest = &JOCKTOSScheduler.ready;
TaskControlBlock* tcb = JOCKTOSScheduler.running;
uint32_t* ptr = (uint32_t*)tcb->stack_overflow + CANARY_SIZE;
for (int i = 0; i < CANARY_SIZE; i++) {
if (*(--ptr) != CANARY) {
if (tcb == &usr_main_tcb) break; // ignore stack canary for main [TODO]
dest = &JOCKTOSScheduler.terminated;
break;
}
}
jock_os_switch_running_task(dest);
jock_os_leave_critical_section(primask);

}
Expand Down Expand Up @@ -294,7 +312,7 @@ void monitorJOCKTOS(void* arg) {
}


void idleJOCKTOS(void* arg) {
void idleJOCKTOS(void* arg __attribute__((unused))) {
/* Disable interrupts to make sure that the busy flag does not get
modified between the check in the while condition and the system
sleep */
Expand Down Expand Up @@ -351,4 +369,10 @@ void idleJOCKTOS(void* arg) {
// TODO: Figure out how to low power
}


__attribute__((noreturn))
void task_exit_guard(void) {
uint32_t primask = jock_os_enter_critical_section();
JOCKTOSScheduler.running->state = TERMINATED;
jock_os_switch_running_task(&JOCKTOSScheduler.terminated);
jock_os_leave_critical_section(primask);
}