feat: establish bootable new architecture checkpoint

Replace the compressed boot path with a relocatable higher-half RISC-V entry, expand memory and SMP support, and add the driver, TTY, VFS, exec, signal, syscall, initramfs, and self-test infrastructure needed for a bootable kernel checkpoint.

1 files changed, 431 insertions, 50 deletions

diff --git a/na/kernel/task.cc b/na/kernel/task.cc
index 9f09d62..9f9b043 100644
--- a/na/kernel/task.cc
+++ b/na/kernel/task.cc
@@ -20,8 +20,18 @@
 #include <lume/vmm.h>
 #include <lume/errno.h>
 #include <lume/pid.h>
+#include <lume/wait.h>
+#include <lume/clone.h>
+#include <lume/signal_nr.h>
 #include <arch/cpu.h>
 #include <arch/context.h>
+#include <lume/uaccess.h>
+#include <lume/fs_struct.h>
+#include <lume/fdtable.h>
+#include <lume/signal.h>
+#include <lume/signal_types.h>
+#include <lume/pgrp.h>
+#include <arch/pmap.h>
 #include <kernel/sync/spinlock.h>
 
 /* ======================================================================
@@ -57,11 +67,16 @@ void task_init()
 }
 
 /* ======================================================================
- * Internal: allocate TCB + kernel stack
+ * alloc_task — allocate TCB + kernel stack
+ *
+ * This is the SINGLE canonical TCB initializer. Both kthread_create
+ * (task.cc) and sched_init (sched_core.cc) must use this function.
+ * NEVER duplicate this logic — adding fields to TaskControlBlock
+ * requires updating only this function.
  * ====================================================================== */
 
-static TaskControlBlock* alloc_task(const char* name, uint8 priority,
-                                    uint8 flags)
+TaskControlBlock* alloc_task(const char* name, uint8 priority,
+                             uint8 flags, uint8 cpu)
 {
     auto* task = new TaskControlBlock;
     if (!task) return nullptr;
@@ -80,23 +95,45 @@ static TaskControlBlock* alloc_task(const char* name, uint8 priority,
     /* Write stack canary at bottom */
     *reinterpret_cast<uint64*>(task->kstack_va) = kStackCanary;
 
+    /* Place TrapFrame at the top of the kernel stack (like Linux pt_regs).
+     * ALL tasks — kernel threads and user processes — have a TrapFrame
+     * here. kthread_create sets the initial SP below the TrapFrame so
+     * the kthread function never touches this reserved area.
+     * kernel_execve / do_execve can later fill it in for user-mode return. */
+    uint64 tf_addr = task->kstack_va + kKernelStackSize
+                     - sizeof(arch::TrapFrame);
+    task->trapframe = reinterpret_cast<arch::TrapFrame*>(tf_addr);
+    __builtin_memset(task->trapframe, 0, sizeof(arch::TrapFrame));
+
     /* Basic fields */
     task->name = name;
     task->priority = priority;
     task->time_slice = priority_to_timeslice(priority);
     task->state = TaskState::Runnable;
-    task->cpu_id = static_cast<uint8>(arch::cpu::id());
+    task->cpu_id = cpu;
     task->flags = flags;
     task->exit_code = 0;
+    task->exit_signal = 0;
+    task->preempt_count = 0;
+    task->clear_child_tid = 0;
+    task->stopped_code = 0;
+    task->notify_continued = false;
     task->parent = nullptr;
     task->mm = nullptr;
-    task->trapframe = nullptr;
+    task->fpstate = {};
+    task->files = nullptr;
+    task->fs = nullptr;
+    task->pending.init();
+    task->blocked.clear();
+    task->sighand = nullptr;
+    task->signal  = nullptr;
 
     /* Init all list nodes */
     task->run_link.init();
     task->all_link.init();
     task->children.init();
     task->sibling.init();
+    task->lock.init("task");
     task->wait_chld.init("wait_chld");
 
     /* Default sched class */
@@ -143,7 +180,8 @@ TaskControlBlock* kthread_create(const char* name,
                                  void* arg,
                                  uint8 priority)
 {
-    TaskControlBlock* task = alloc_task(name, priority, TF_KTHREAD);
+    TaskControlBlock* task = alloc_task(name, priority, TF_KTHREAD,
+                                         static_cast<uint8>(arch::cpu::id()));
     if (!task) return nullptr;
 
     /* Allocate PID */
@@ -156,19 +194,30 @@ TaskControlBlock* kthread_create(const char* name,
     }
     task->tgid = task->pid;
 
-    /* Set up context for kthread_entry_trampoline */
-    uint64 stack_top = task->kstack_va + kKernelStackSize;
+    /* Kernel threads: exit_signal = -1 → auto-reap on exit.
+     * Linux: kernel_thread() passes (flags & ~CSIGNAL) → exit_signal = 0,
+     * but kthreads' parent (kthreadd) reaps them.  We have no kthreadd,
+     * so we use -1 which makes exit_notify() call release_task() directly,
+     * identical to Linux's CLONE_THREAD autoreap path. */
+    task->exit_signal = -1;
+
+    /* Set up context for kthread_entry_trampoline.
+     * SP starts BELOW the TrapFrame reserved at the stack top.
+     * This ensures the kthread function never clobbers the TrapFrame area. */
+    uint64 stack_top = task->kstack_va + kKernelStackSize
+                       - sizeof(arch::TrapFrame);
     task->context = {};
     task->context.ra = reinterpret_cast<uint64>(kthread_entry_trampoline);
     task->context.sp = stack_top;
     task->context.s[0] = reinterpret_cast<uint64>(entry);
     task->context.s[1] = reinterpret_cast<uint64>(arg);
 
-    /* Parent is kernel_task (PID 0) */
-    task->parent = g_kernel_task;
-    if (g_kernel_task) {
-        LockGuard guard(g_task_list_lock);
-        list_node* head = &g_kernel_task->children;
+    /* Parent is kernel_task (PID 0) or current caller */
+    TaskControlBlock* parent = g_kernel_task ? g_kernel_task : current_task();
+    task->parent = parent;
+    if (parent) {
+        LockGuard guard(parent->lock);
+        list_node* head = &parent->children;
         task->sibling.prev = head->prev;
         task->sibling.next = head;
         head->prev->next = &task->sibling;
@@ -191,18 +240,32 @@ void task_exit(int code)
     TaskControlBlock* curr = current_task();
     if (!curr) kernel_panic("task_exit: no current task");
 
-    curr->exit_code = code;
+    /* Linux ABI: normal exit encodes status as (code & 0xFF) << 8 */
+    curr->exit_code = (code & 0xFF) << 8;
 
-    /* Release user address space if we're the last reference */
-    if (curr->mm) {
-        curr->mm->destroy();
+    /* CLONE_CHILD_CLEARTID / set_tid_address: write 0 to user tidptr.
+     * This is how pthread_join detects thread termination.
+     * TODO: futex_wake(clear_child_tid, 1) after writing 0. */
+    if (curr->clear_child_tid != 0 && curr->mm != nullptr) {
+        int32 zero = 0;
+        copy_to_user(curr->clear_child_tid, &zero, sizeof(zero));
+        curr->clear_child_tid = 0;
+    }
+
+    /* Release user address space (refcount-safe).
+     * MUST switch to kernel page table FIRST — otherwise we're running
+     * on a freed page table after put() deallocates it. */
+    if (curr->mm != nullptr) {
+        extern uint64 g_kernel_pgtbl;
+        pmap::activate(g_kernel_pgtbl);
+        curr->mm->put();
         curr->mm = nullptr;
     }
 
     /* Reparent children to init (or kernel_task if no init yet) */
     TaskControlBlock* foster = g_init_task ? g_init_task : g_kernel_task;
     if (foster && !curr->children.is_empty()) {
-        LockGuard guard(g_task_list_lock);
+        LockGuard guard(curr->lock);
 
         list_node* child_node = curr->children.next;
         while (child_node != &curr->children) {
@@ -226,7 +289,46 @@ void task_exit(int code)
         curr->children.init();
     }
 
-    /* Transition to Zombie — TCB + kstack preserved for parent wait() */
+
+    /* ---- exit_notify: Linux kernel/exit.c:exit_notify() ----
+     * Determine whether this task should be auto-reaped or become zombie.
+     *
+     * Linux logic (simplified):
+     *   - exit_signal == -1 (kthreads, sub-threads): autoreap = true
+     *   - exit_signal >= 1 (user processes): notify parent, become zombie
+     *
+     * If autoreap, we call release_task() equivalent inline:
+     * free PID, remove from task list, then deschedule to Dead. */
+    bool autoreap = (curr->exit_signal <= 0);
+
+    if (autoreap) {
+        /* Auto-reap: release resources immediately (no zombie state).
+         * Linux: exit_notify() → tsk->exit_state = EXIT_DEAD → release_task() */
+        task_list_remove(curr);
+        g_pid_alloc.free(curr->pid);
+
+        /* Remove from parent's children list */
+        curr->sibling.prev->next = curr->sibling.next;
+        curr->sibling.next->prev = curr->sibling.prev;
+        curr->sibling.init();
+
+        /* Drop signal_struct ref: last task in the thread group also
+         * detaches us from the pgrp hash. */
+        if (curr->signal) {
+            put_signal_struct(curr->signal);
+            curr->signal = nullptr;
+        }
+        if (curr->sighand) {
+            put_sighand(curr->sighand);
+            curr->sighand = nullptr;
+        }
+
+        curr->state = TaskState::Dead;
+        schedule();
+        __builtin_unreachable();
+    }
+
+    /* User process path: become Zombie for parent wait() */
     curr->state = TaskState::Zombie;
 
     /* Wake parent->wait_chld queue */
@@ -242,8 +344,55 @@ void task_exit(int code)
 
 void task_kill_current(int sig)
 {
-    int code = (sig & 0x7F) | kExitCodeSignalFlag;
-    task_exit(code);
+    TaskControlBlock* curr = current_task();
+    if (!curr) kernel_panic("task_kill_current: no current task");
+
+    /* Linux ABI: signal-killed encodes status as (signo & 0x7F) */
+    curr->exit_code = sig & 0x7F;
+
+    /* Release user address space (refcount-safe).
+     * Switch to kernel page table first (same as task_exit). */
+    if (curr->mm) {
+        extern uint64 g_kernel_pgtbl;
+        pmap::activate(g_kernel_pgtbl);
+        curr->mm->put();
+        curr->mm = nullptr;
+    }
+
+    /* Reparent children (same as task_exit) */
+    TaskControlBlock* foster = g_init_task ? g_init_task : g_kernel_task;
+    if (foster && !curr->children.is_empty()) {
+        LockGuard guard(curr->lock);
+
+        list_node* child_node = curr->children.next;
+        while (child_node != &curr->children) {
+            list_node* next_sib = child_node->next;
+            auto* child = list_entry<TaskControlBlock,
+                                     &TaskControlBlock::sibling>(child_node);
+            child->parent = foster;
+
+            child_node->prev->next = child_node->next;
+            child_node->next->prev = child_node->prev;
+
+            list_node* fhead = &foster->children;
+            child_node->prev = fhead->prev;
+            child_node->next = fhead;
+            fhead->prev->next = child_node;
+            fhead->prev = child_node;
+
+            child_node = next_sib;
+        }
+        curr->children.init();
+    }
+
+    curr->state = TaskState::Zombie;
+
+    if (curr->parent) {
+        curr->parent->wait_chld.wake_all();
+    }
+
+    schedule();
+    __builtin_unreachable();
 }
 
 /* ======================================================================
@@ -263,6 +412,19 @@ void task_destroy(TaskControlBlock* task)
     /* Free PID */
     g_pid_alloc.free(task->pid);
 
+    /* Drop signal_struct + sighand refs.  For zombies these are delayed
+     * until reap (here) so a waitpid() walker can still inspect
+     * pgid/sid; Linux defers __exit_signal/__cleanup_sighand to
+     * release_task() for the same reason. */
+    if (task->signal) {
+        put_signal_struct(task->signal);
+        task->signal = nullptr;
+    }
+    if (task->sighand) {
+        put_sighand(task->sighand);
+        task->sighand = nullptr;
+    }
+
     /* Free kernel stack */
     if (task->kstack_pa != 0) {
            pmm_free_pages(pa_to_page(phys_addr(task->kstack_pa)), 1);
@@ -273,61 +435,155 @@ void task_destroy(TaskControlBlock* task)
 }
 
 /* ======================================================================
- * task_wait — sleep until a child exits, then reclaim it
- * Returns PID of exited child, or -ECHILD if no children exist.
+ * do_wait4 — wait for a child to exit (Linux-compatible)
+ *
+ * @wait_pid: >0  = specific child PID
+ *            -1  = any child
+ *            0   = any child in same process group (TODO)
+ *            <-1 = any child in specific process group (TODO)
+ * @options:  WNOHANG = return immediately if no zombie
+ * @exit_code: output parameter (already in Linux wstatus format)
+ *
+ * Returns: PID of exited child, -ECHILD, or -EAGAIN (WNOHANG).
  * ====================================================================== */
 
-int32 task_wait(int32* exit_code)
+int32 do_wait4(int32 wait_pid, int options, int32* exit_code)
 {
     TaskControlBlock* curr = current_task();
     if (!curr) return -EINVAL;
 
     while (true) {
         TaskControlBlock* zombie = nullptr;
+        TaskControlBlock* report = nullptr;   /* Stopped/Continued report */
+        int32 report_status = 0;
+        bool has_children = false;
+
+        /*
+         * Two-phase wait: put ourselves on the wait queue BEFORE
+         * checking the condition. This closes the race window where
+         * a child exits between our check and schedule().
+         *
+         * If a child exits and calls wake_all() while we're checking,
+         * our state will be flipped back to Runnable, and schedule()
+         * will simply return immediately.
+         */
+        curr->wait_chld.prepare_to_wait(curr, TaskState::Sleeping);
+
+        /* Scan children */
+        curr->lock.acquire();
 
-        /* Look for a Zombie child */
-        g_task_list_lock.acquire();
         if (curr->children.is_empty()) {
-            g_task_list_lock.release();
+            curr->lock.release();
+            curr->wait_chld.finish_wait(curr);
             return -ECHILD;
         }
+        has_children = true;
 
         list_node* node = curr->children.next;
         while (node != &curr->children) {
             auto* child = list_entry<TaskControlBlock, &TaskControlBlock::sibling>(node);
+            bool matches = (wait_pid == -1) || (wait_pid == child->pid);
+            if (!matches) {
+                node = node->next;
+                continue;
+            }
+
+            /* Zombie reaping has priority — it frees resources. */
             if (child->state == TaskState::Zombie) {
                 zombie = child;
                 break;
             }
+
+            /* WUNTRACED: report Stopped child once per stop event. */
+            if ((options & WUNTRACED) && child->stopped_code != 0) {
+                report = child;
+                report_status = child->stopped_code;
+                break;
+            }
+
+            /* WCONTINUED: report a one-shot SIGCONT resume. */
+            if ((options & WCONTINUED) && child->notify_continued) {
+                report = child;
+                report_status = W_CONTINUED();
+                break;
+            }
+
             node = node->next;
         }
-        g_task_list_lock.release();
+        curr->lock.release();
 
         if (zombie) {
+            curr->wait_chld.finish_wait(curr);
             int32 pid = zombie->pid;
-            if (exit_code) {
-                /* In a real kernel, we copy_to_user */
-                *exit_code = zombie->exit_code;
-            }
+            if (exit_code) *exit_code = zombie->exit_code;
             task_destroy(zombie);
             return pid;
         }
 
-        /* Sleep if no child has exited yet */
-        curr->wait_chld.sleep(&g_task_list_lock);
+        if (report) {
+            /* Status-only report: clear the one-shot flag, leave the
+             * child alive.  WNOWAIT (peek) would skip the clear, but
+             * we don't yet honour it. */
+            curr->wait_chld.finish_wait(curr);
+            int32 pid = report->pid;
+            if (exit_code) *exit_code = report_status;
+            if (W_CONTINUED() == report_status) {
+                report->notify_continued = false;
+            } else {
+                report->stopped_code = 0;
+            }
+            return pid;
+        }
+
+        /* No event to report */
+        if (!has_children) {
+            curr->wait_chld.finish_wait(curr);
+            return -ECHILD;
+        }
+        if (options & WNOHANG) {
+            curr->wait_chld.finish_wait(curr);
+            return 0;
+        }
+
+        /* Condition still false — deschedule.
+         * We are already on the wait queue and marked Sleeping. */
+        schedule();
+
+        /* Woken up — finish_wait removes us from queue, restores Runnable */
+        curr->wait_chld.finish_wait(curr);
+
+        /* Loop back to re-check the condition */
     }
 }
 
+/* Legacy wrapper for internal callers */
+int32 task_wait(int32* exit_code)
+{
+    return do_wait4(-1, 0, exit_code);
+}
+
 /* ======================================================================
- * task_fork — clone current task
+ * task_clone — clone current task with Linux-compatible flags
+ *
+ * @tf:          trap frame to copy to child
+ * @clone_flags: CLONE_VM | CLONE_THREAD | CSIGNAL etc.
+ * @child_stack: if non-zero, set as child's user SP (for pthread)
+ * @parent_tidptr: write child tid here in parent (CLONE_PARENT_SETTID)
+ * @child_tidptr:  write child tid here in child  (CLONE_CHILD_SETTID)
+ * @tls:         TLS pointer (CLONE_SETTLS → tp register)
+ *
+ * Returns: child PID in parent, 0 in child, or negative errno.
  * ====================================================================== */
 
-int32 task_fork(arch::TrapFrame* tf)
+int32 task_clone(arch::TrapFrame* tf, uint64 clone_flags,
+                 uint64 child_stack, uint64 parent_tidptr,
+                 uint64 child_tidptr, uint64 tls)
 {
     TaskControlBlock* curr = current_task();
     if (!curr) return -EINVAL;
 
-    TaskControlBlock* child = alloc_task("forked", curr->priority, 0);
+    TaskControlBlock* child = alloc_task("cloned", curr->priority, 0,
+                                          static_cast<uint8>(arch::cpu::id()));
     if (!child) return -ENOMEM;
 
     /* Allocate PID */
@@ -337,16 +593,35 @@ int32 task_fork(arch::TrapFrame* tf)
         delete child;
         return -ENOMEM;
     }
-    child->tgid = child->pid;
 
-    /* Copy VmSpace */
-    if (curr->mm) {
-        child->mm = vmm_clone(curr->mm);
-        if (!child->mm) {
-            g_pid_alloc.free(child->pid);
-            pmm_free_pages(pa_to_page(phys_addr(child->kstack_pa)), 1);
-            delete child;
-            return -ENOMEM;
+    /* exit_signal + tgid: from clone_flags.
+     * Linux: fork.c line 2418/2424 */
+    if (clone_flags & CLONE_THREAD) {
+        child->tgid = curr->tgid;
+        child->exit_signal = -1;     /* Linux: sub-threads auto-reap */
+    } else {
+        child->tgid = child->pid;
+        child->exit_signal = static_cast<int32>(clone_flags & 0xFF);  /* CSIGNAL */
+        if (child->exit_signal == 0) {
+            child->exit_signal = SIGCHLD;  /* default for fork() */
+        }
+    }
+
+    /* Address space: CLONE_VM shares, otherwise deep copy */
+    if (clone_flags & CLONE_VM) {
+        if (curr->mm) {
+            curr->mm->incref();
+            child->mm = curr->mm;
+        }
+    } else {
+        if (curr->mm) {
+            child->mm = vmm_clone(curr->mm);
+            if (!child->mm) {
+                g_pid_alloc.free(child->pid);
+                pmm_free_pages(pa_to_page(phys_addr(child->kstack_pa)), 1);
+                delete child;
+                return -ENOMEM;
+            }
         }
     }
 
@@ -355,20 +630,94 @@ int32 task_fork(arch::TrapFrame* tf)
     child->trapframe = reinterpret_cast<arch::TrapFrame*>(tf_addr);
     *child->trapframe = *tf;
 
-    /* Child returns 0 from fork */
+    /* Set Kernel SP */
+    child->trapframe->kernel_sp = reinterpret_cast<uint64>(child->trapframe);
+    child->trapframe->kernel_cpu_id = child->cpu_id;
+
+    /* Child returns 0 from fork/clone */
     child->trapframe->a[0] = 0;
 
+    /* If caller specified a child stack, use it (pthread_create) */
+    if (child_stack != 0) {
+        child->trapframe->sp = child_stack;
+    }
+
+    /* CLONE_SETTLS: set thread pointer register */
+    if (clone_flags & CLONE_SETTLS) {
+        child->trapframe->tp = tls;
+    }
+
     /* Set up context for ret_from_fork */
-    uint64 stack_top = tf_addr; // Context goes below TrapFrame conceptually, but we just set SP
+    uint64 stack_top = tf_addr;
     child->context = {};
     child->context.ra = reinterpret_cast<uint64>(ret_from_fork);
     child->context.sp = stack_top;
-    child->context.s[0] = tf_addr; /* Passed to a0 in ret_from_fork */
+    child->context.s[0] = tf_addr;
+
+    /* ---- Copy fs_struct (Linux: copy_fs in fork.c) ----
+     * CLONE_FS: share the same fs_struct (increment refcount).
+     * Otherwise: allocate a new fs_struct copying root/pwd. */
+    if (curr->fs != nullptr) {
+        if (clone_flags & CLONE_FS) {
+            curr->fs->count.fetch_add(1, __ATOMIC_RELAXED);
+            child->fs = curr->fs;
+        } else {
+            child->fs = copy_fs_struct(curr->fs);
+        }
+    }
+
+    /* ---- Copy files_struct (Linux: copy_files in fork.c) ----
+     * CLONE_FILES: share the same fd table.
+     * Otherwise: duplicate the fd table. */
+    if (curr->files != nullptr) {
+        if (clone_flags & CLONE_FILES) {
+            curr->files->count.fetch_add(1, __ATOMIC_RELAXED);
+            child->files = curr->files;
+        } else {
+            child->files = dup_fd(curr->files);
+        }
+    }
+
+    /* ---- Copy sighand (Linux kernel/fork.c:copy_sighand) ----
+     * CLONE_SIGHAND: share signal handlers (incref parent's).
+     * Otherwise: deep copy of the action table; child gets its own
+     * sighand_struct (and its own siglock).  Both branches MUST yield
+     * a non-null sighand — every user task needs one so signal paths
+     * can take the siglock without lazy-allocation races. */
+    if (clone_flags & CLONE_SIGHAND) {
+        if (!curr->sighand) {
+            /* CLONE_SIGHAND requires the parent to have a sighand. */
+            return -EINVAL;
+        }
+        get_sighand(curr->sighand);
+        child->sighand = curr->sighand;
+    } else {
+        child->sighand = copy_sighand(curr->sighand);
+        if (!child->sighand) {
+            /* OOM — same best-effort policy as fs/files/signal above.
+             * A separate cleanup-on-OOM pass is on the to-do list. */
+        }
+    }
+
+    /* ---- Copy signal_struct (Linux: copy_signal in fork.c) ----
+     * CLONE_THREAD shares the parent's signal_struct (incref).
+     * Otherwise allocate a new one that inherits pgid/sid/tty from the
+     * parent so the child stays in the same process group and session —
+     * this is what makes fork() participate in the existing job-control
+     * identity without explicit setpgid() calls. */
+    if (curr->signal != nullptr) {
+        if (clone_flags & CLONE_THREAD) {
+            get_signal_struct(curr->signal);
+            child->signal = curr->signal;
+        } else {
+            child->signal = fork_signal_struct(curr->signal, child);
+        }
+    }
 
     /* Add to task tree */
     child->parent = curr;
     {
-        LockGuard guard(g_task_list_lock);
+        LockGuard guard(curr->lock);
         list_node* head = &curr->children;
         child->sibling.prev = head->prev;
         child->sibling.next = head;
@@ -380,9 +729,41 @@ int32 task_fork(arch::TrapFrame* tf)
     task_list_add(child);
     sched_enqueue(child);
 
+    /* CLONE_PARENT_SETTID: write child tid to parent's user address */
+    if ((clone_flags & CLONE_PARENT_SETTID) && parent_tidptr != 0) {
+        copy_to_user(parent_tidptr, &child->pid, sizeof(child->pid));
+    }
+
+    /* CLONE_CHILD_SETTID: write child tid into child's user address.
+     * Note: this runs in parent context, but the address is in the
+     * child's mm. For CLONE_VM (threads), parent and child share mm
+     * so this works directly. For fork, the page was just CoW-copied. */
+    if ((clone_flags & CLONE_CHILD_SETTID) && child_tidptr != 0) {
+        if (clone_flags & CLONE_VM) {
+            /* Same address space — write directly */
+            copy_to_user(child_tidptr, &child->pid, sizeof(child->pid));
+        }
+        /* For non-CLONE_VM fork, the child's mm is separate.
+         * The write would need to go into the child's pagetable.
+         * Linux handles this in the child's first schedule via
+         * a special flag. For now, threads (CLONE_VM) are the
+         * primary consumer. */
+    }
+
+    /* CLONE_CHILD_CLEARTID: kernel writes 0 to child_tidptr on exit */
+    if (clone_flags & CLONE_CHILD_CLEARTID) {
+        child->clear_child_tid = child_tidptr;
+    }
+
     return child->pid;
 }
 
+/* Legacy wrapper — plain fork() semantics */
+int32 task_fork(arch::TrapFrame* tf)
+{
+    return task_clone(tf, SIGCHLD, 0, 0, 0, 0);
+}
+
 
 /* ======================================================================
  * task_check_stack_canary