Add U-mode task isolation and fix self-termination

U-mode tasks could previously control other tasks and had no way to
properly terminate themselves. This adds permission checks to restrict
task control syscalls to self-only operations and enables safe
self-termination through the existing zombie task mechanism.

Author: HeatCrab

kernel/syscall.c

@@ -278,8 +278,10 @@ int sys_task_spawn(void *task, int stack_size)
 
 static int _tcancel(int id)
 {
-    if (unlikely(id <= 0))
+    if (unlikely((unsigned int) id - 1 >= UINT16_MAX))
         return -EINVAL;
+    if ((uint16_t) id != mo_task_id())
+        return -EPERM;
 
     return mo_task_cancel(id);
 }
@@ -316,8 +318,10 @@ int sys_tdelay(int ticks)
 
 static int _tsuspend(int id)
 {
-    if (unlikely(id <= 0))
+    if (unlikely((unsigned int) id - 1 >= UINT16_MAX))
         return -EINVAL;
+    if ((uint16_t) id != mo_task_id())
+        return -EPERM;
 
     return mo_task_suspend(id);
 }
@@ -329,10 +333,9 @@ int sys_tsuspend(int id)
 
 static int _tresume(int id)
 {
-    if (unlikely(id <= 0))
-        return -EINVAL;
-
-    return mo_task_resume(id);
+    (void) id;
+    /* U-mode cannot resume any task; suspended task cannot call syscall */
+    return -EPERM;
 }
 
 int sys_tresume(int id)
@@ -342,8 +345,10 @@ int sys_tresume(int id)
 
 static int _tpriority(int id, int priority)
 {
-    if (unlikely(id <= 0))
+    if (unlikely((unsigned int) id - 1 >= UINT16_MAX))
         return -EINVAL;
+    if ((uint16_t) id != mo_task_id())
+        return -EPERM;
 
     return mo_task_priority(id, priority);
 }

kernel/task.c

@@ -395,7 +395,7 @@ static void task_cleanup_zombies(void)
         list_node_t *next = list_next(node);
         tcb_t *tcb = node->data;
 
-        if (tcb && tcb->state == TASK_ZOMBIE) {
+        if (tcb && tcb->state == TASK_ZOMBIE && node != kcb->task_current) {
             /* Remove from task list */
             list_remove(kcb->tasks, node);
             kcb->task_count--;
@@ -992,9 +992,22 @@ int32_t mo_task_spawn_user(void *task_entry, uint16_t stack_size)
 
 int32_t mo_task_cancel(uint16_t id)
 {
-    if (id == 0 || id == mo_task_id())
+    if (id == 0)
         return ERR_TASK_CANT_REMOVE;
 
+    /* Self-termination marks the task as zombie and yields to the scheduler.
+     * The dispatcher will reclaim resources after context switch completes.
+     */
+    if (id == mo_task_id()) {
+        tcb_t *self = kcb->task_current->data;
+        CRITICAL_ENTER();
+        self->state = TASK_ZOMBIE;
+        CRITICAL_LEAVE();
+        _yield();
+        while (1)
+            ;
+    }
+
     CRITICAL_ENTER();
     list_node_t *node = find_task_node_by_id(id);
     if (!node) {