Kernel-and-Types-of-kernels

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Kernel-and-Types-of-kernels

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# Kernel-and-Types-of-kernels
![image](https://github.com/nu11secur1ty/Kernel-and-Types-of-kernels/blob/master/wall/gV8hn.png)

- Last update from nu11secur1ty: https://github.com/nu11secur1ty/kernel-4.19.0V.Varbanovski_lp150.12.22_default

# Importance_of_Linux_partitions
[Read...](https://github.com/nu11secur1ty/Linux_Deployment_Administration_Hacks/tree/master/Importance_of_Linux_partitions)

# Kernel Space, User Space Interfaces
[Read...](https://www.nu11secur1ty.com/2019/01/kernel-space-user-space-interfaces.html)

-----------------------------------------------------------------------------------------------------------------
# Reference

![](https://github.com/nu11secur1ty/Kernel-and-Types-of-kernels/blob/master/scheme/kernels.png)

The kernel is the core of an operating system. It is the software responsible for running programs and providing secure access to the machine's hardware. Since there are many programs, and resources are limited, the kernel also decides when and how long a program should run. This is called scheduling. Accessing the hardware directly can be very complex, since there are many different hardware designs for the same type of component. Kernels usually implement some level of hardware abstraction (a set of instructions universal to all devices of a certain type) to hide the underlying complexity from applications and provide a clean and uniform interface. This helps application programmers to develop programs without having to know how to program for specific devices. The kernel relies upon software drivers that translate the generic command into instructions specific to that device.

An operating system kernel is not strictly needed to run a computer. Programs can be directly loaded and executed on the "bare metal" machine, provided that the authors of those programs are willing to do without any hardware abstraction or operating system support. This was the normal operating method of many early computers, which were reset and reloaded between the running of different programs. Eventually, small ancillary programs such as program loaders and debuggers were typically left in-core between runs, or loaded from read-only memory. As these were developed, they formed the basis of what became early operating system kernels. The "bare metal" approach is still used today on many video game consoles and embedded systems, but in general, newer systems use kernels and operating systems.

Four broad categories of kernels:

- Monolithic kernels provide rich and powerful abstractions of the underlying hardware.
   - Microkernels provide a small set of simple hardware abstractions and use applications called servers to provide more functionality.
   - Exokernels provide minimal abstractions, allowing low-level hardware access. In exokernel systems, library operating systems provide the abstractions typically present in monolithic kernels.
   - Hybrid (modified microkernels) are much like pure microkernels, except that they include some additional code in kernelspace to increase performance.
   
   
   
  -------------------------------------------------------------------------------------------------------------
  # Monolitic
  
  The monolithic approach is to define a high-level virtual interface over the hardware, with a set of primitives or system calls to implement operating system services such as process management, concurrency, and memory management in several modules that run in supervisor mode.

Even if every module servicing these operations is separate from the whole, the code integration is very tight and difficult to do correctly, and, since all the modules run in the same address space, a bug in one module can bring down the whole system. However, when the implementation is complete and trustworthy, the tight internal integration of components allows the low-level features of the underlying system to be effectively utilized, making a good monolithic kernel highly efficient. Proponents of the monolithic kernel approach make the case that if code is incorrect, it does not belong in a kernel, and if it is, there is little advantage in the microkernel approach. More modern monolithic kernels such as Linux, FreeBSD and Solaris can load executable modules at runtime, allowing easy extension of the kernel's capabilities as required, while helping to keep the amount of code running in kernel-space to a minimum. It is alone in supervise mode.

-----------------------------------------------------------------------------------------------------------------

# Microkernels
The microkernel approach is to define a very simple abstraction over the hardware, with a set of primitives or system calls to implement minimal OS services such as thread management, address spaces and interprocess communication. All other services, those normally provided by the kernel such as networking, are implemented in user-space programs referred to as servers. Servers are programs like any others, allowing the operating system to be modified simply by starting and stopping programs. For a small machine without networking support, for instance, the networking server simply isn't started. Under a traditional system this would require the kernel to be recompiled, something well beyond the capabilities of the average end-user. In theory the system is also more stable, because a failing server simply stops a single program, rather than causing the kernel itself to crash.

However, part of the system state is lost with the failing server, and it is generally difficult to continue execution of applications, or even of other servers with a fresh copy. For example, if a (theoretic) server responsible for TCP/IP connections is restarted, applications could be told the connection was "lost" and reconnect, going through the new instance of the server. However, other system objects, like files, do not have these convenient semantics, are supposed to be reliable, not become unavailable randomly and keep all the information written to them previously. So, database techniques like transactions, replication and checkpointing need to be used between servers in order to preserve essential state across single server restarts.

Microkernels generally underperform traditional designs, sometimes dramatically. This is due in large part to the overhead of moving in and out of the kernel, a context switch, in order to move data between the various applications and servers. It was originally believed that careful tuning could reduce this overhead dramatically, but by the mid-90s most researchers had given up. In more recent times newer microkernels, designed for performance first, have addressed these problems to a very large degree. Nevertheless the market for existing operating systems is so entrenched that little work continues on microkernel design.

------------------------------------------------------------------------------------------------------------------

# Exokernel

- General
An exokernel is a type of operating system where the kernel is limited to extending resources to sub operating systems called LibOS's. Resulting in a very small, fast kernel environment. The theory behind this method is that by providing as few abstractions as possible programs are able to do exactly what they want in a controlled environment. Such as MS-DOS achieved through real mode, except with paging and other modern programming techniques.
- LibOS
LibOS's provide a way for the programmer of an exokernel type system to easily program cross-platform programs using familiar interfaces, instead of having to write his\her own. Moreover, they provide an additional advantage over monolithic kernels in that by having multiple LibOS's running at the same time, one can theoretically run programs from Linux, Windows, and Mac (Provided that there is a LibOS for that system) all at the same time, on the same OS, and with no performance issues.

--------------------------------------------------------------------------------------------------------------------

# Hybrid

A hybrid kernel is one that combines aspects of both micro and monolithic kernels, but there is no exact definition. Often, "hybrid kernel" means that the kernel is highly modular, but all runs in the same address space. This allows the kernel avoid the overhead of a complicated message passing system within the kernel, while still retaining some microkernel-like features.

--------------------------------------------------------------------------------------------------------------------

# INIT and zombie
[see the rule](https://github.com/nu11secur1ty/Kernel-and-Types-of-kernels/blob/master/Linux/exit/exit.c#L652)

- Or all code:
```c
// SPDX-License-Identifier: GPL-2.0-only
/*
 *  linux/kernel/exit.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 */

#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/sched/autogroup.h>
#include <linux/sched/mm.h>
#include <linux/sched/stat.h>
#include <linux/sched/task.h>
#include <linux/sched/task_stack.h>
#include <linux/sched/cputime.h>
#include <linux/interrupt.h>
#include <linux/module.h>
#include <linux/capability.h>
#include <linux/completion.h>
#include <linux/personality.h>
#include <linux/tty.h>
#include <linux/iocontext.h>
#include <linux/key.h>
#include <linux/cpu.h>
#include <linux/acct.h>
#include <linux/tsacct_kern.h>
#include <linux/file.h>
#include <linux/fdtable.h>
#include <linux/freezer.h>
#include <linux/binfmts.h>
#include <linux/nsproxy.h>
#include <linux/pid_namespace.h>
#include <linux/ptrace.h>
#include <linux/profile.h>
#include <linux/mount.h>
#include <linux/proc_fs.h>
#include <linux/kthread.h>
#include <linux/mempolicy.h>
#include <linux/taskstats_kern.h>
#include <linux/delayacct.h>
#include <linux/cgroup.h>
#include <linux/syscalls.h>
#include <linux/signal.h>
#include <linux/posix-timers.h>
#include <linux/cn_proc.h>
#include <linux/mutex.h>
#include <linux/futex.h>
#include <linux/pipe_fs_i.h>
#include <linux/audit.h> /* for audit_free() */
#include <linux/resource.h>
#include <linux/blkdev.h>
#include <linux/task_io_accounting_ops.h>
#include <linux/tracehook.h>
#include <linux/fs_struct.h>
#include <linux/init_task.h>
#include <linux/perf_event.h>
#include <trace/events/sched.h>
#include <linux/hw_breakpoint.h>
#include <linux/oom.h>
#include <linux/writeback.h>
#include <linux/shm.h>
#include <linux/kcov.h>
#include <linux/random.h>
#include <linux/rcuwait.h>
#include <linux/compat.h>

#include <linux/uaccess.h>
#include <asm/unistd.h>
#include <asm/pgtable.h>
#include <asm/mmu_context.h>

static void __unhash_process(struct task_struct *p, bool group_dead)
{
	nr_threads--;
	detach_pid(p, PIDTYPE_PID);
	if (group_dead) {
		detach_pid(p, PIDTYPE_TGID);
		detach_pid(p, PIDTYPE_PGID);
		detach_pid(p, PIDTYPE_SID);

list_del_rcu(&p->tasks);
		list_del_init(&p->sibling);
		__this_cpu_dec(process_counts);
	}
	list_del_rcu(&p->thread_group);
	list_del_rcu(&p->thread_node);
}

/*
 * This function expects the tasklist_lock write-locked.
 */
static void __exit_signal(struct task_struct *tsk)
{
	struct signal_struct *sig = tsk->signal;
	bool group_dead = thread_group_leader(tsk);
	struct sighand_struct *sighand;
	struct tty_struct *uninitialized_var(tty);
	u64 utime, stime;

sighand = rcu_dereference_check(tsk->sighand,
					lockdep_tasklist_lock_is_held());
	spin_lock(&sighand->siglock);

#ifdef CONFIG_POSIX_TIMERS
	posix_cpu_timers_exit(tsk);
	if (group_dead) {
		posix_cpu_timers_exit_group(tsk);
	} else {
		/*
		 * This can only happen if the caller is de_thread().
		 * FIXME: this is the temporary hack, we should teach
		 * posix-cpu-timers to handle this case correctly.
		 */
		if (unlikely(has_group_leader_pid(tsk)))
			posix_cpu_timers_exit_group(tsk);
	}
#endif

if (group_dead) {
		tty = sig->tty;
		sig->tty = NULL;
	} else {
		/*
		 * If there is any task waiting for the group exit
		 * then notify it:
		 */
		if (sig->notify_count > 0 && !--sig->notify_count)
			wake_up_process(sig->group_exit_task);

if (tsk == sig->curr_target)
			sig->curr_target = next_thread(tsk);
	}

add_device_randomness((const void*) &tsk->se.sum_exec_runtime,
			      sizeof(unsigned long long));

/*
	 * Accumulate here the counters for all threads as they die. We could
	 * skip the group leader because it is the last user of signal_struct,
	 * but we want to avoid the race with thread_group_cputime() which can
	 * see the empty ->thread_head list.
	 */
	task_cputime(tsk, &utime, &stime);
	write_seqlock(&sig->stats_lock);
	sig->utime += utime;
	sig->stime += stime;
	sig->gtime += task_gtime(tsk);
	sig->min_flt += tsk->min_flt;
	sig->maj_flt += tsk->maj_flt;
	sig->nvcsw += tsk->nvcsw;
	sig->nivcsw += tsk->nivcsw;
	sig->inblock += task_io_get_inblock(tsk);
	sig->oublock += task_io_get_oublock(tsk);
	task_io_accounting_add(&sig->ioac, &tsk->ioac);
	sig->sum_sched_runtime += tsk->se.sum_exec_runtime;
	sig->nr_threads--;
	__unhash_process(tsk, group_dead);
	write_sequnlock(&sig->stats_lock);

/*
	 * Do this under ->siglock, we can race with another thread
	 * doing sigqueue_free() if we have SIGQUEUE_PREALLOC signals.
	 */
	flush_sigqueue(&tsk->pending);
	tsk->sighand = NULL;
	spin_unlock(&sighand->siglock);

__cleanup_sighand(sighand);
	clear_tsk_thread_flag(tsk, TIF_SIGPENDING);
	if (group_dead) {
		flush_sigqueue(&sig->shared_pending);
		tty_kref_put(tty);
	}
}

static void delayed_put_task_struct(struct rcu_head *rhp)
{
	struct task_struct *tsk = container_of(rhp, struct task_struct, rcu);

perf_event_delayed_put(tsk);
	trace_sched_process_free(tsk);
	put_task_struct(tsk);
}

void release_task(struct task_struct *p)
{
	struct task_struct *leader;
	int zap_leader;
repeat:
	/* don't need to get the RCU readlock here - the process is dead and
	 * can't be modifying its own credentials. But shut RCU-lockdep up */
	rcu_read_lock();
	atomic_dec(&__task_cred(p)->user->processes);
	rcu_read_unlock();

proc_flush_task(p);
	cgroup_release(p);

write_lock_irq(&tasklist_lock);
	ptrace_release_task(p);
	__exit_signal(p);

/*
	 * If we are the last non-leader member of the thread
	 * group, and the leader is zombie, then notify the
	 * group leader's parent process. (if it wants notification.)
	 */
	zap_leader = 0;
	leader = p->group_leader;
	if (leader != p && thread_group_empty(leader)
			&& leader->exit_state == EXIT_ZOMBIE) {
		/*
		 * If we were the last child thread and the leader has
		 * exited already, and the leader's parent ignores SIGCHLD,
		 * then we are the one who should release the leader.
		 */
		zap_leader = do_notify_parent(leader, leader->exit_signal);
		if (zap_leader)
			leader->exit_state = EXIT_DEAD;
	}

write_unlock_irq(&tasklist_lock);
	release_thread(p);
	call_rcu(&p->rcu, delayed_put_task_struct);

p = leader;
	if (unlikely(zap_leader))
		goto repeat;
}

/*
 * Note that if this function returns a valid task_struct pointer (!NULL)
 * task->usage must remain >0 for the duration of the RCU critical section.
 */
struct task_struct *task_rcu_dereference(struct task_struct **ptask)
{
	struct sighand_struct *sighand;
	struct task_struct *task;

/*
	 * We need to verify that release_task() was not called and thus
	 * delayed_put_task_struct() can't run and drop the last reference
	 * before rcu_read_unlock(). We check task->sighand != NULL,
	 * but we can read the already freed and reused memory.
	 */
retry:
	task = rcu_dereference(*ptask);
	if (!task)
		return NULL;

probe_kernel_address(&task->sighand, sighand);

/*
	 * Pairs with atomic_dec_and_test() in put_task_struct(). If this task
	 * was already freed we can not miss the preceding update of this
	 * pointer.
	 */
	smp_rmb();
	if (unlikely(task != READ_ONCE(*ptask)))
		goto retry;

/*
	 * We've re-checked that "task == *ptask", now we have two different
	 * cases:
	 *
	 * 1. This is actually the same task/task_struct. In this case
	 *    sighand != NULL tells us it is still alive.
	 *
	 * 2. This is another task which got the same memory for task_struct.
	 *    We can't know this of course, and we can not trust
	 *    sighand != NULL.
	 *
	 *    In this case we actually return a random value, but this is
	 *    correct.
	 *
	 *    If we return NULL - we can pretend that we actually noticed that
	 *    *ptask was updated when the previous task has exited. Or pretend
	 *    that probe_slab_address(&sighand) reads NULL.
	 *
	 *    If we return the new task (because sighand is not NULL for any
	 *    reason) - this is fine too. This (new) task can't go away before
	 *    another gp pass.
	 *
	 *    And note: We could even eliminate the false positive if re-read
	 *    task->sighand once again to avoid the falsely NULL. But this case
	 *    is very unlikely so we don't care.
	 */
	if (!sighand)
		return NULL;

return task;
}

void rcuwait_wake_up(struct rcuwait *w)
{
	struct task_struct *task;

rcu_read_lock();

/*
	 * Order condition vs @task, such that everything prior to the load
	 * of @task is visible. This is the condition as to why the user called
	 * rcuwait_trywake() in the first place. Pairs with set_current_state()
	 * barrier (A) in rcuwait_wait_event().
	 *
	 *    WAIT                WAKE
	 *    [S] tsk = current	  [S] cond = true
	 *        MB (A)	      MB (B)
	 *    [L] cond		  [L] tsk
	 */
	smp_mb(); /* (B) */

/*
	 * Avoid using task_rcu_dereference() magic as long as we are careful,
	 * see comment in rcuwait_wait_event() regarding ->exit_state.
	 */
	task = rcu_dereference(w->task);
	if (task)
		wake_up_process(task);
	rcu_read_unlock();
}

/*
 * Determine if a process group is "orphaned", according to the POSIX
 * definition in 2.2.2.52.  Orphaned process groups are not to be affected
 * by terminal-generated stop signals.  Newly orphaned process groups are
 * to receive a SIGHUP and a SIGCONT.
 *
 * "I ask you, have you ever known what it is to be an orphan?"
 */
static int will_become_orphaned_pgrp(struct pid *pgrp,
					struct task_struct *ignored_task)
{
	struct task_struct *p;

do_each_pid_task(pgrp, PIDTYPE_PGID, p) {
		if ((p == ignored_task) ||
		    (p->exit_state && thread_group_empty(p)) ||
		    is_global_init(p->real_parent))
			continue;

if (task_pgrp(p->real_parent) != pgrp &&
		    task_session(p->real_parent) == task_session(p))
			return 0;
	} while_each_pid_task(pgrp, PIDTYPE_PGID, p);

return 1;
}

int is_current_pgrp_orphaned(void)
{
	int retval;

read_lock(&tasklist_lock);
	retval = will_become_orphaned_pgrp(task_pgrp(current), NULL);
	read_unlock(&tasklist_lock);

return retval;
}

static bool has_stopped_jobs(struct pid *pgrp)
{
	struct task_struct *p;

do_each_pid_task(pgrp, PIDTYPE_PGID, p) {
		if (p->signal->flags & SIGNAL_STOP_STOPPED)
			return true;
	} while_each_pid_task(pgrp, PIDTYPE_PGID, p);

return false;
}

/*
 * Check to see if any process groups have become orphaned as
 * a result of our exiting, and if they have any stopped jobs,
 * send them a SIGHUP and then a SIGCONT. (POSIX 3.2.2.2)
 */
static void
kill_orphaned_pgrp(struct task_struct *tsk, struct task_struct *parent)
{
	struct pid *pgrp = task_pgrp(tsk);
	struct task_struct *ignored_task = tsk;

if (!parent)
		/* exit: our father is in a different pgrp than
		 * we are and we were the only connection outside.
		 */
		parent = tsk->real_parent;
	else
		/* reparent: our child is in a different pgrp than
		 * we are, and it was the only connection outside.
		 */
		ignored_task = NULL;

if (task_pgrp(parent) != pgrp &&
	    task_session(parent) == task_session(tsk) &&
	    will_become_orphaned_pgrp(pgrp, ignored_task) &&
	    has_stopped_jobs(pgrp)) {
		__kill_pgrp_info(SIGHUP, SEND_SIG_PRIV, pgrp);
		__kill_pgrp_info(SIGCONT, SEND_SIG_PRIV, pgrp);
	}
}

#ifdef CONFIG_MEMCG
/*
 * A task is exiting.   If it owned this mm, find a new owner for the mm.
 */
void mm_update_next_owner(struct mm_struct *mm)
{
	struct task_struct *c, *g, *p = current;

retry:
	/*
	 * If the exiting or execing task is not the owner, it's
	 * someone else's problem.
	 */
	if (mm->owner != p)
		return;
	/*
	 * The current owner is exiting/execing and there are no other
	 * candidates.  Do not leave the mm pointing to a possibly
	 * freed task structure.
	 */
	if (atomic_read(&mm->mm_users) <= 1) {
		WRITE_ONCE(mm->owner, NULL);
		return;
	}

read_lock(&tasklist_lock);
	/*
	 * Search in the children
	 */
	list_for_each_entry(c, &p->children, sibling) {
		if (c->mm == mm)
			goto assign_new_owner;
	}

/*
	 * Search in the siblings
	 */
	list_for_each_entry(c, &p->real_parent->children, sibling) {
		if (c->mm == mm)
			goto assign_new_owner;
	}

/*
	 * Search through everything else, we should not get here often.
	 */
	for_each_process(g) {
		if (g->flags & PF_KTHREAD)
			continue;
		for_each_thread(g, c) {
			if (c->mm == mm)
				goto assign_new_owner;
			if (c->mm)
				break;
		}
	}
	read_unlock(&tasklist_lock);
	/*
	 * We found no owner yet mm_users > 1: this implies that we are
	 * most likely racing with swapoff (try_to_unuse()) or /proc or
	 * ptrace or page migration (get_task_mm()).  Mark owner as NULL.
	 */
	WRITE_ONCE(mm->owner, NULL);
	return;

assign_new_owner:
	BUG_ON(c == p);
	get_task_struct(c);
	/*
	 * The task_lock protects c->mm from changing.
	 * We always want mm->owner->mm == mm
	 */
	task_lock(c);
	/*
	 * Delay read_unlock() till we have the task_lock()
	 * to ensure that c does not slip away underneath us
	 */
	read_unlock(&tasklist_lock);
	if (c->mm != mm) {
		task_unlock(c);
		put_task_struct(c);
		goto retry;
	}
	WRITE_ONCE(mm->owner, c);
	task_unlock(c);
	put_task_struct(c);
}
#endif /* CONFIG_MEMCG */

/*
 * Turn us into a lazy TLB process if we
 * aren't already..
 */
static void exit_mm(void)
{
	struct mm_struct *mm = current->mm;
	struct core_state *core_state;

mm_release(current, mm);
	if (!mm)
		return;
	sync_mm_rss(mm);
	/*
	 * Serialize with any possible pending coredump.
	 * We must hold mmap_sem around checking core_state
	 * and clearing tsk->mm.  The core-inducing thread
	 * will increment ->nr_threads for each thread in the
	 * group with ->mm != NULL.
	 */
	down_read(&mm->mmap_sem);
	core_state = mm->core_state;
	if (core_state) {
		struct core_thread self;

up_read(&mm->mmap_sem);

self.task = current;
		self.next = xchg(&core_state->dumper.next, &self);
		/*
		 * Implies mb(), the result of xchg() must be visible
		 * to core_state->dumper.
		 */
		if (atomic_dec_and_test(&core_state->nr_threads))
			complete(&core_state->startup);

for (;;) {
			set_current_state(TASK_UNINTERRUPTIBLE);
			if (!self.task) /* see coredump_finish() */
				break;
			freezable_schedule();
		}
		__set_current_state(TASK_RUNNING);
		down_read(&mm->mmap_sem);
	}
	mmgrab(mm);
	BUG_ON(mm != current->active_mm);
	/* more a memory barrier than a real lock */
	task_lock(current);
	current->mm = NULL;
	up_read(&mm->mmap_sem);
	enter_lazy_tlb(mm, current);
	task_unlock(current);
	mm_update_next_owner(mm);
	mmput(mm);
	if (test_thread_flag(TIF_MEMDIE))
		exit_oom_victim();
}

static struct task_struct *find_alive_thread(struct task_struct *p)
{
	struct task_struct *t;

for_each_thread(p, t) {
		if (!(t->flags & PF_EXITING))
			return t;
	}
	return NULL;
}

static struct task_struct *find_child_reaper(struct task_struct *father,
						struct list_head *dead)
	__releases(&tasklist_lock)
	__acquires(&tasklist_lock)
{
	struct pid_namespace *pid_ns = task_active_pid_ns(father);
	struct task_struct *reaper = pid_ns->child_reaper;
	struct task_struct *p, *n;

if (likely(reaper != father))
		return reaper;

reaper = find_alive_thread(father);
	if (reaper) {
		pid_ns->child_reaper = reaper;
		return reaper;
	}

write_unlock_irq(&tasklist_lock);
	if (unlikely(pid_ns == &init_pid_ns)) {
		panic("Attempted to kill init! exitcode=0x%08x\n",
			father->signal->group_exit_code ?: father->exit_code);
	}

list_for_each_entry_safe(p, n, dead, ptrace_entry) {
		list_del_init(&p->ptrace_entry);
		release_task(p);
	}

zap_pid_ns_processes(pid_ns);
	write_lock_irq(&tasklist_lock);

return father;
}

/*
 * When we die, we re-parent all our children, and try to:
 * 1. give them to another thread in our thread group, if such a member exists
 * 2. give it to the first ancestor process which prctl'd itself as a
 *    child_subreaper for its children (like a service manager)
 * 3. give it to the init process (PID 1) in our pid namespace
 */
static struct task_struct *find_new_reaper(struct task_struct *father,
					   struct task_struct *child_reaper)
{
	struct task_struct *thread, *reaper;

thread = find_alive_thread(father);
	if (thread)
		return thread;

if (father->signal->has_child_subreaper) {
		unsigned int ns_level = task_pid(father)->level;
		/*
		 * Find the first ->is_child_subreaper ancestor in our pid_ns.
		 * We can't check reaper != child_reaper to ensure we do not
		 * cross the namespaces, the exiting parent could be injected
		 * by setns() + fork().
		 * We check pid->level, this is slightly more efficient than
		 * task_active_pid_ns(reaper) != task_active_pid_ns(father).
		 */
		for (reaper = father->real_parent;
		     task_pid(reaper)->level == ns_level;
		     reaper = reaper->real_parent) {
			if (reaper == &init_task)
				break;
			if (!reaper->signal->is_child_subreaper)
				continue;
			thread = find_alive_thread(reaper);
			if (thread)
				return thread;
		}
	}

return child_reaper;
}

/*
* Any that need to be release_task'd are put on the @dead list.
 */
static void reparent_leader(struct task_struct *father, struct task_struct *p,
				struct list_head *dead)
{
	if (unlikely(p->exit_state == EXIT_DEAD))
		return;

/* We don't want people slaying init. */
	p->exit_signal = SIGCHLD;

/* If it has exited notify the new parent about this child's death. */
	if (!p->ptrace &&
	    p->exit_state == EXIT_ZOMBIE && thread_group_empty(p)) {
		if (do_notify_parent(p, p->exit_signal)) {
			p->exit_state = EXIT_DEAD;
			list_add(&p->ptrace_entry, dead);
		}
	}

kill_orphaned_pgrp(p, father);
}

/*
 * This does two things:
 *
 * A.  Make init inherit all the child processes
 * B.  Check to see if any process groups have become orphaned
 *	as a result of our exiting, and if they have any stopped
 *	jobs, send them a SIGHUP and then a SIGCONT.  (POSIX 3.2.2.2)
 */
static void forget_original_parent(struct task_struct *father,
					struct list_head *dead)
{
	struct task_struct *p, *t, *reaper;

if (unlikely(!list_empty(&father->ptraced)))
		exit_ptrace(father, dead);

/* Can drop and reacquire tasklist_lock */
	reaper = find_child_reaper(father, dead);
	if (list_empty(&father->children))
		return;

reaper = find_new_reaper(father, reaper);
	list_for_each_entry(p, &father->children, sibling) {
		for_each_thread(p, t) {
			t->real_parent = reaper;
			BUG_ON((!t->ptrace) != (t->parent == father));
			if (likely(!t->ptrace))
				t->parent = t->real_parent;
			if (t->pdeath_signal)
				group_send_sig_info(t->pdeath_signal,
						    SEND_SIG_NOINFO, t,
						    PIDTYPE_TGID);
		}
		/*
		 * If this is a threaded reparent there is no need to
		 * notify anyone anything has happened.
		 */
		if (!same_thread_group(reaper, father))
			reparent_leader(father, p, dead);
	}
	list_splice_tail_init(&father->children, &reaper->children);
}

/*
 * Send signals to all our closest relatives so that they know
 * to properly mourn us..
 */
static void exit_notify(struct task_struct *tsk, int group_dead)
{
	bool autoreap;
	struct task_struct *p, *n;
	LIST_HEAD(dead);

write_lock_irq(&tasklist_lock);
	forget_original_parent(tsk, &dead);

if (group_dead)
		kill_orphaned_pgrp(tsk->group_leader, NULL);

if (unlikely(tsk->ptrace)) {
		int sig = thread_group_leader(tsk) &&
				thread_group_empty(tsk) &&
				!ptrace_reparented(tsk) ?
			tsk->exit_signal : SIGCHLD;
		autoreap = do_notify_parent(tsk, sig);
	} else if (thread_group_leader(tsk)) {
		autoreap = thread_group_empty(tsk) &&
			do_notify_parent(tsk, tsk->exit_signal);
	} else {
		autoreap = true;
	}

tsk->exit_state = autoreap ? EXIT_DEAD : EXIT_ZOMBIE;
	if (tsk->exit_state == EXIT_DEAD)
		list_add(&tsk->ptrace_entry, &dead);

/* mt-exec, de_thread() is waiting for group leader */
	if (unlikely(tsk->signal->notify_count < 0))
		wake_up_process(tsk->signal->group_exit_task);
	write_unlock_irq(&tasklist_lock);

list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
		list_del_init(&p->ptrace_entry);
		release_task(p);
	}
}

#ifdef CONFIG_DEBUG_STACK_USAGE
static void check_stack_usage(void)
{
	static DEFINE_SPINLOCK(low_water_lock);
	static int lowest_to_date = THREAD_SIZE;
	unsigned long free;

free = stack_not_used(current);

if (free >= lowest_to_date)
		return;

spin_lock(&low_water_lock);
	if (free < lowest_to_date) {
		pr_info("%s (%d) used greatest stack depth: %lu bytes left\n",
			current->comm, task_pid_nr(current), free);
		lowest_to_date = free;
	}
	spin_unlock(&low_water_lock);
}
#else
static inline void check_stack_usage(void) {}
#endif

void __noreturn do_exit(long code)
{
	struct task_struct *tsk = current;
	int group_dead;

profile_task_exit(tsk);
	kcov_task_exit(tsk);

WARN_ON(blk_needs_flush_plug(tsk));

if (unlikely(in_interrupt()))
		panic("Aiee, killing interrupt handler!");
	if (unlikely(!tsk->pid))
		panic("Attempted to kill the idle task!");

/*
	 * If do_exit is called because this processes oopsed, it's possible
	 * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before
	 * continuing. Amongst other possible reasons, this is to prevent
	 * mm_release()->clear_child_tid() from writing to a user-controlled
	 * kernel address.
	 */
	set_fs(USER_DS);

ptrace_event(PTRACE_EVENT_EXIT, code);

validate_creds_for_do_exit(tsk);

/*
	 * We're taking recursive faults here in do_exit. Safest is to just
	 * leave this task alone and wait for reboot.
	 */
	if (unlikely(tsk->flags & PF_EXITING)) {
		pr_alert("Fixing recursive fault but reboot is needed!\n");
		/*
		 * We can do this unlocked here. The futex code uses
		 * this flag just to verify whether the pi state
		 * cleanup has been done or not. In the worst case it
		 * loops once more. We pretend that the cleanup was
		 * done as there is no way to return. Either the
		 * OWNER_DIED bit is set by now or we push the blocked
		 * task into the wait for ever nirwana as well.
		 */
		tsk->flags |= PF_EXITPIDONE;
		set_current_state(TASK_UNINTERRUPTIBLE);
		schedule();
	}

exit_signals(tsk);  /* sets PF_EXITING */
	/*
	 * Ensure that all new tsk->pi_lock acquisitions must observe
	 * PF_EXITING. Serializes against futex.c:attach_to_pi_owner().
	 */
	smp_mb();
	/*
	 * Ensure that we must observe the pi_state in exit_mm() ->
	 * mm_release() -> exit_pi_state_list().
	 */
	raw_spin_lock_irq(&tsk->pi_lock);
	raw_spin_unlock_irq(&tsk->pi_lock);

if (unlikely(in_atomic())) {
		pr_info("note: %s[%d] exited with preempt_count %d\n",
			current->comm, task_pid_nr(current),
			preempt_count());
		preempt_count_set(PREEMPT_ENABLED);
	}

/* sync mm's RSS info before statistics gathering */
	if (tsk->mm)
		sync_mm_rss(tsk->mm);
	acct_update_integrals(tsk);
	group_dead = atomic_dec_and_test(&tsk->signal->live);
	if (group_dead) {
#ifdef CONFIG_POSIX_TIMERS
		hrtimer_cancel(&tsk->signal->real_timer);
		exit_itimers(tsk->signal);
#endif
		if (tsk->mm)
			setmax_mm_hiwater_rss(&tsk->signal->maxrss, tsk->mm);
	}
	acct_collect(code, group_dead);
	if (group_dead)
		tty_audit_exit();
	audit_free(tsk);

tsk->exit_code = code;
	taskstats_exit(tsk, group_dead);

exit_mm();

if (group_dead)
		acct_process();
	trace_sched_process_exit(tsk);

exit_sem(tsk);
	exit_shm(tsk);
	exit_files(tsk);
	exit_fs(tsk);
	if (group_dead)
		disassociate_ctty(1);
	exit_task_namespaces(tsk);
	exit_task_work(tsk);
	exit_thread(tsk);
	exit_umh(tsk);

/*
	 * Flush inherited counters to the parent - before the parent
	 * gets woken up by child-exit notifications.
	 *
	 * because of cgroup mode, must be called before cgroup_exit()
	 */
	perf_event_exit_task(tsk);

sched_autogroup_exit_task(tsk);
	cgroup_exit(tsk);

/*
	 * FIXME: do that only when needed, using sched_exit tracepoint
	 */
	flush_ptrace_hw_breakpoint(tsk);

exit_tasks_rcu_start();
	exit_notify(tsk, group_dead);
	proc_exit_connector(tsk);
	mpol_put_task_policy(tsk);
#ifdef CONFIG_FUTEX
	if (unlikely(current->pi_state_cache))
		kfree(current->pi_state_cache);
#endif
	/*
	 * Make sure we are holding no locks:
	 */
	debug_check_no_locks_held();
	/*
	 * We can do this unlocked here. The futex code uses this flag
	 * just to verify whether the pi state cleanup has been done
	 * or not. In the worst case it loops once more.
	 */
	tsk->flags |= PF_EXITPIDONE;

if (tsk->io_context)
		exit_io_context(tsk);

if (tsk->splice_pipe)
		free_pipe_info(tsk->splice_pipe);

if (tsk->task_frag.page)
		put_page(tsk->task_frag.page);

validate_creds_for_do_exit(tsk);

check_stack_usage();
	preempt_disable();
	if (tsk->nr_dirtied)
		__this_cpu_add(dirty_throttle_leaks, tsk->nr_dirtied);
	exit_rcu();
	exit_tasks_rcu_finish();

lockdep_free_task(tsk);
	do_task_dead();
}
EXPORT_SYMBOL_GPL(do_exit);

void complete_and_exit(struct completion *comp, long code)
{
	if (comp)
		complete(comp);

do_exit(code);
}
EXPORT_SYMBOL(complete_and_exit);

SYSCALL_DEFINE1(exit, int, error_code)
{
	do_exit((error_code&0xff)<<8);
}

/*
 * Take down every thread in the group.  This is called by fatal signals
 * as well as by sys_exit_group (below).
 */
void
do_group_exit(int exit_code)
{
	struct signal_struct *sig = current->signal;

BUG_ON(exit_code & 0x80); /* core dumps don't get here */

if (signal_group_exit(sig))
		exit_code = sig->group_exit_code;
	else if (!thread_group_empty(current)) {
		struct sighand_struct *const sighand = current->sighand;

spin_lock_irq(&sighand->siglock);
		if (signal_group_exit(sig))
			/* Another thread got here before we took the lock.  */
			exit_code = sig->group_exit_code;
		else {
			sig->group_exit_code = exit_code;
			sig->flags = SIGNAL_GROUP_EXIT;
			zap_other_threads(current);
		}
		spin_unlock_irq(&sighand->siglock);
	}

do_exit(exit_code);
	/* NOTREACHED */
}

/*
 * this kills every thread in the thread group. Note that any externally
 * wait4()-ing process will get the correct exit code - even if this
 * thread is not the thread group leader.
 */
SYSCALL_DEFINE1(exit_group, int, error_code)
{
	do_group_exit((error_code & 0xff) << 8);
	/* NOTREACHED */
	return 0;
}

struct waitid_info {
	pid_t pid;
	uid_t uid;
	int status;
	int cause;
};

struct wait_opts {
	enum pid_type		wo_type;
	int			wo_flags;
	struct pid		*wo_pid;

struct waitid_info	*wo_info;
	int			wo_stat;
	struct rusage		*wo_rusage;

wait_queue_entry_t		child_wait;
	int			notask_error;
};

static int eligible_pid(struct wait_opts *wo, struct task_struct *p)
{
	return	wo->wo_type == PIDTYPE_MAX ||
		task_pid_type(p, wo->wo_type) == wo->wo_pid;
}

static int
eligible_child(struct wait_opts *wo, bool ptrace, struct task_struct *p)
{
	if (!eligible_pid(wo, p))
		return 0;

/*
	 * Wait for all children (clone and not) if __WALL is set or
	 * if it is traced by us.
	 */
	if (ptrace || (wo->wo_flags & __WALL))
		return 1;

/*
	 * Otherwise, wait for clone children *only* if __WCLONE is set;
	 * otherwise, wait for non-clone children *only*.
	 *
	 * Note: a "clone" child here is one that reports to its parent
	 * using a signal other than SIGCHLD, or a non-leader thread which
	 * we can only see if it is traced by us.
	 */
	if ((p->exit_signal != SIGCHLD) ^ !!(wo->wo_flags & __WCLONE))
		return 0;

return 1;
}

/*
 * Handle sys_wait4 work for one task in state EXIT_ZOMBIE.  We hold
 * read_lock(&tasklist_lock) on entry.  If we return zero, we still hold
 * the lock and this task is uninteresting.  If we return nonzero, we have
 * released the lock and the system call should return.
 */
static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
{
	int state, status;
	pid_t pid = task_pid_vnr(p);
	uid_t uid = from_kuid_munged(current_user_ns(), task_uid(p));
	struct waitid_info *infop;

if (!likely(wo->wo_flags & WEXITED))
		return 0;

if (unlikely(wo->wo_flags & WNOWAIT)) {
		status = p->exit_code;
		get_task_struct(p);
		read_unlock(&tasklist_lock);
		sched_annotate_sleep();
		if (wo->wo_rusage)
			getrusage(p, RUSAGE_BOTH, wo->wo_rusage);
		put_task_struct(p);
		goto out_info;
	}
	/*
	 * Move the task's state to DEAD/TRACE, only one thread can do this.
	 */
	state = (ptrace_reparented(p) && thread_group_leader(p)) ?
		EXIT_TRACE : EXIT_DEAD;
	if (cmpxchg(&p->exit_state, EXIT_ZOMBIE, state) != EXIT_ZOMBIE)
		return 0;
	/*
	 * We own this thread, nobody else can reap it.
	 */
	read_unlock(&tasklist_lock);
	sched_annotate_sleep();

/*
	 * Check thread_group_leader() to exclude the traced sub-threads.
	 */
	if (state == EXIT_DEAD && thread_group_leader(p)) {
		struct signal_struct *sig = p->signal;
		struct signal_struct *psig = current->signal;
		unsigned long maxrss;
		u64 tgutime, tgstime;

/*
		 * The resource counters for the group leader are in its
		 * own task_struct.  Those for dead threads in the group
		 * are in its signal_struct, as are those for the child
		 * processes it has previously reaped.  All these
		 * accumulate in the parent's signal_struct c* fields.
		 *
		 * We don't bother to take a lock here to protect these
		 * p->signal fields because the whole thread group is dead
		 * and nobody can change them.
		 *
		 * psig->stats_lock also protects us from our sub-theads
		 * which can reap other children at the same time. Until
		 * we change k_getrusage()-like users to rely on this lock
		 * we have to take ->siglock as well.
		 *
		 * We use thread_group_cputime_adjusted() to get times for
		 * the thread group, which consolidates times for all threads
		 * in the group including the group leader.
		 */
		thread_group_cputime_adjusted(p, &tgutime, &tgstime);
		spin_lock_irq(&current->sighand->siglock);
		write_seqlock(&psig->stats_lock);
		psig->cutime += tgutime + sig->cutime;
		psig->cstime += tgstime + sig->cstime;
		psig->cgtime += task_gtime(p) + sig->gtime + sig->cgtime;
		psig->cmin_flt +=
			p->min_flt + sig->min_flt + sig->cmin_flt;
		psig->cmaj_flt +=
			p->maj_flt + sig->maj_flt + sig->cmaj_flt;
		psig->cnvcsw +=
			p->nvcsw + sig->nvcsw + sig->cnvcsw;
		psig->cnivcsw +=
			p->nivcsw + sig->nivcsw + sig->cnivcsw;
		psig->cinblock +=
			task_io_get_inblock(p) +
			sig->inblock + sig->cinblock;
		psig->coublock +=
			task_io_get_oublock(p) +
			sig->oublock + sig->coublock;
		maxrss = max(sig->maxrss, sig->cmaxrss);
		if (psig->cmaxrss < maxrss)
			psig->cmaxrss = maxrss;
		task_io_accounting_add(&psig->ioac, &p->ioac);
		task_io_accounting_add(&psig->ioac, &sig->ioac);
		write_sequnlock(&psig->stats_lock);
		spin_unlock_irq(&current->sighand->siglock);
	}

if (wo->wo_rusage)
		getrusage(p, RUSAGE_BOTH, wo->wo_rusage);
	status = (p->signal->flags & SIGNAL_GROUP_EXIT)
		? p->signal->group_exit_code : p->exit_code;
	wo->wo_stat = status;

if (state == EXIT_TRACE) {
		write_lock_irq(&tasklist_lock);
		/* We dropped tasklist, ptracer could die and untrace */
		ptrace_unlink(p);

/* If parent wants a zombie, don't release it now */
		state = EXIT_ZOMBIE;
		if (do_notify_parent(p, p->exit_signal))
			state = EXIT_DEAD;
		p->exit_state = state;
		write_unlock_irq(&tasklist_lock);
	}
	if (state == EXIT_DEAD)
		release_task(p);

out_info:
	infop = wo->wo_info;
	if (infop) {
		if ((status & 0x7f) == 0) {
			infop->cause = CLD_EXITED;
			infop->status = status >> 8;
		} else {
			infop->cause = (status & 0x80) ? CLD_DUMPED : CLD_KILLED;
			infop->status = status & 0x7f;
		}
		infop->pid = pid;
		infop->uid = uid;
	}

return pid;
}

static int *task_stopped_code(struct task_struct *p, bool ptrace)
{
	if (ptrace) {
		if (task_is_traced(p) && !(p->jobctl & JOBCTL_LISTENING))
			return &p->exit_code;
	} else {
		if (p->signal->flags & SIGNAL_STOP_STOPPED)
			return &p->signal->group_exit_code;
	}
	return NULL;
}

/**
 * wait_task_stopped - Wait for %TASK_STOPPED or %TASK_TRACED
 * @wo: wait options
 * @ptrace: is the wait for ptrace
 * @p: task to wait for
 *
 * Handle sys_wait4() work for %p in state %TASK_STOPPED or %TASK_TRACED.
 *
 * CONTEXT:
 * read_lock(&tasklist_lock), which is released if return value is
 * non-zero.  Also, grabs and releases @p->sighand->siglock.
 *
 * RETURNS:
 * 0 if wait condition didn't exist and search for other wait conditions
 * should continue.  Non-zero return, -errno on failure and @p's pid on
 * success, implies that tasklist_lock is released and wait condition
 * search should terminate.
 */
static int wait_task_stopped(struct wait_opts *wo,
				int ptrace, struct task_struct *p)
{
	struct waitid_info *infop;
	int exit_code, *p_code, why;
	uid_t uid = 0; /* unneeded, required by compiler */
	pid_t pid;

/*
	 * Traditionally we see ptrace'd stopped tasks regardless of options.
	 */
	if (!ptrace && !(wo->wo_flags & WUNTRACED))
		return 0;

if (!task_stopped_code(p, ptrace))
		return 0;

exit_code = 0;
	spin_lock_irq(&p->sighand->siglock);

p_code = task_stopped_code(p, ptrace);
	if (unlikely(!p_code))
		goto unlock_sig;

exit_code = *p_code;
	if (!exit_code)
		goto unlock_sig;

if (!unlikely(wo->wo_flags & WNOWAIT))
		*p_code = 0;

uid = from_kuid_munged(current_user_ns(), task_uid(p));
unlock_sig:
	spin_unlock_irq(&p->sighand->siglock);
	if (!exit_code)
		return 0;

/*
	 * Now we are pretty sure this task is interesting.
	 * Make sure it doesn't get reaped out from under us while we
	 * give up the lock and then examine it below.  We don't want to
	 * keep holding onto the tasklist_lock while we call getrusage and
	 * possibly take page faults for user memory.
	 */
	get_task_struct(p);
	pid = task_pid_vnr(p);
	why = ptrace ? CLD_TRAPPED : CLD_STOPPED;
	read_unlock(&tasklist_lock);
	sched_annotate_sleep();
	if (wo->wo_rusage)
		getrusage(p, RUSAGE_BOTH, wo->wo_rusage);
	put_task_struct(p);

if (likely(!(wo->wo_flags & WNOWAIT)))
		wo->wo_stat = (exit_code << 8) | 0x7f;

infop = wo->wo_info;
	if (infop) {
		infop->cause = why;
		infop->status = exit_code;
		infop->pid = pid;
		infop->uid = uid;
	}
	return pid;
}

/*
 * Handle do_wait work for one task in a live, non-stopped state.
 * read_lock(&tasklist_lock) on entry.  If we return zero, we still hold
 * the lock and this task is uninteresting.  If we return nonzero, we have
 * released the lock and the system call should return.
 */
static int wait_task_continued(struct wait_opts *wo, struct task_struct *p)
{
	struct waitid_info *infop;
	pid_t pid;
	uid_t uid;

if (!unlikely(wo->wo_flags & WCONTINUED))
		return 0;

if (!(p->signal->flags & SIGNAL_STOP_CONTINUED))
		return 0;

spin_lock_irq(&p->sighand->siglock);
	/* Re-check with the lock held.  */
	if (!(p->signal->flags & SIGNAL_STOP_CONTINUED)) {
		spin_unlock_irq(&p->sighand->siglock);
		return 0;
	}
	if (!unlikely(wo->wo_flags & WNOWAIT))
		p->signal->flags &= ~SIGNAL_STOP_CONTINUED;
	uid = from_kuid_munged(current_user_ns(), task_uid(p));
	spin_unlock_irq(&p->sighand->siglock);

pid = task_pid_vnr(p);
	get_task_struct(p);
	read_unlock(&tasklist_lock);
	sched_annotate_sleep();
	if (wo->wo_rusage)
		getrusage(p, RUSAGE_BOTH, wo->wo_rusage);
	put_task_struct(p);

infop = wo->wo_info;
	if (!infop) {
		wo->wo_stat = 0xffff;
	} else {
		infop->cause = CLD_CONTINUED;
		infop->pid = pid;
		infop->uid = uid;
		infop->status = SIGCONT;
	}
	return pid;
}

/*
 * Consider @p for a wait by @parent.
 *
 * -ECHILD should be in ->notask_error before the first call.
 * Returns nonzero for a final return, when we have unlocked tasklist_lock.
 * Returns zero if the search for a child should continue;
 * then ->notask_error is 0 if @p is an eligible child,
 * or still -ECHILD.
 */
static int wait_consider_task(struct wait_opts *wo, int ptrace,
				struct task_struct *p)
{
	/*
	 * We can race with wait_task_zombie() from another thread.
	 * Ensure that EXIT_ZOMBIE -> EXIT_DEAD/EXIT_TRACE transition
	 * can't confuse the checks below.
	 */
	int exit_state = READ_ONCE(p->exit_state);
	int ret;

if (unlikely(exit_state == EXIT_DEAD))
		return 0;

ret = eligible_child(wo, ptrace, p);
	if (!ret)
		return ret;

if (unlikely(exit_state == EXIT_TRACE)) {
		/*
		 * ptrace == 0 means we are the natural parent. In this case
		 * we should clear notask_error, debugger will notify us.
		 */
		if (likely(!ptrace))
			wo->notask_error = 0;
		return 0;
	}

if (likely(!ptrace) && unlikely(p->ptrace)) {
		/*
		 * If it is traced by its real parent's group, just pretend
		 * the caller is ptrace_do_wait() and reap this child if it
		 * is zombie.
		 *
		 * This also hides group stop state from real parent; otherwise
		 * a single stop can be reported twice as group and ptrace stop.
		 * If a ptracer wants to distinguish these two events for its
		 * own children it should create a separate process which takes
		 * the role of real parent.
		 */
		if (!ptrace_reparented(p))
			ptrace = 1;
	}

/* slay zombie? */
	if (exit_state == EXIT_ZOMBIE) {
		/* we don't reap group leaders with subthreads */
		if (!delay_group_leader(p)) {
			/*
			 * A zombie ptracee is only visible to its ptracer.
			 * Notification and reaping will be cascaded to the
			 * real parent when the ptracer detaches.
			 */
			if (unlikely(ptrace) || likely(!p->ptrace))
				return wait_task_zombie(wo, p);
		}

/*
		 * Allow access to stopped/continued state via zombie by
		 * falling through.  Clearing of notask_error is complex.
		 *
		 * When !@ptrace:
		 *
		 * If WEXITED is set, notask_error should naturally be
		 * cleared.  If not, subset of WSTOPPED|WCONTINUED is set,
		 * so, if there are live subthreads, there are events to
		 * wait for.  If all subthreads are dead, it's still safe
		 * to clear - this function will be called again in finite
		 * amount time once all the subthreads are released and
		 * will then return without clearing.
		 *
		 * When @ptrace:
		 *
		 * Stopped state is per-task and thus can't change once the
		 * target task dies.  Only continued and exited can happen.
		 * Clear notask_error if WCONTINUED | WEXITED.
		 */
		if (likely(!ptrace) || (wo->wo_flags & (WCONTINUED | WEXITED)))
			wo->notask_error = 0;
	} else {
		/*
		 * @p is alive and it's gonna stop, continue or exit, so
		 * there always is something to wait for.
		 */
		wo->notask_error = 0;
	}

/*
	 * Wait for stopped.  Depending on @ptrace, different stopped state
	 * is used and the two don't interact with each other.
	 */
	ret = wait_task_stopped(wo, ptrace, p);
	if (ret)
		return ret;

/*
	 * Wait for continued.  There's only one continued state and the
	 * ptracer can consume it which can confuse the real parent.  Don't
	 * use WCONTINUED from ptracer.  You don't need or want it.
	 */
	return wait_task_continued(wo, p);
}

/*
 * Do the work of do_wait() for one thread in the group, @tsk.
 *
 * -ECHILD should be in ->notask_error before the first call.
 * Returns nonzero for a final return, when we have unlocked tasklist_lock.
 * Returns zero if the search for a child should continue; then
 * ->notask_error is 0 if there were any eligible children,
 * or still -ECHILD.
 */
static int do_wait_thread(struct wait_opts *wo, struct task_struct *tsk)
{
	struct task_struct *p;

list_for_each_entry(p, &tsk->children, sibling) {
		int ret = wait_consider_task(wo, 0, p);

if (ret)
			return ret;
	}

return 0;
}

static int ptrace_do_wait(struct wait_opts *wo, struct task_struct *tsk)
{
	struct task_struct *p;

list_for_each_entry(p, &tsk->ptraced, ptrace_entry) {
		int ret = wait_consider_task(wo, 1, p);

if (ret)
			return ret;
	}

return 0;
}

static int child_wait_callback(wait_queue_entry_t *wait, unsigned mode,
				int sync, void *key)
{
	struct wait_opts *wo = container_of(wait, struct wait_opts,
						child_wait);
	struct task_struct *p = key;

if (!eligible_pid(wo, p))
		return 0;

if ((wo->wo_flags & __WNOTHREAD) && wait->private != p->parent)
		return 0;

return default_wake_function(wait, mode, sync, key);
}

void __wake_up_parent(struct task_struct *p, struct task_struct *parent)
{
	__wake_up_sync_key(&parent->signal->wait_chldexit,
				TASK_INTERRUPTIBLE, 1, p);
}

static long do_wait(struct wait_opts *wo)
{
	struct task_struct *tsk;
	int retval;

trace_sched_process_wait(wo->wo_pid);

init_waitqueue_func_entry(&wo->child_wait, child_wait_callback);
	wo->child_wait.private = current;
	add_wait_queue(&current->signal->wait_chldexit, &wo->child_wait);
repeat:
	/*
	 * If there is nothing that can match our criteria, just get out.
	 * We will clear ->notask_error to zero if we see any child that
	 * might later match our criteria, even if we are not able to reap
	 * it yet.
	 */
	wo->notask_error = -ECHILD;
	if ((wo->wo_type < PIDTYPE_MAX) &&
	   (!wo->wo_pid || hlist_empty(&wo->wo_pid->tasks[wo->wo_type])))
		goto notask;

set_current_state(TASK_INTERRUPTIBLE);
	read_lock(&tasklist_lock);
	tsk = current;
	do {
		retval = do_wait_thread(wo, tsk);
		if (retval)
			goto end;

retval = ptrace_do_wait(wo, tsk);
		if (retval)
			goto end;

if (wo->wo_flags & __WNOTHREAD)
			break;
	} while_each_thread(current, tsk);
	read_unlock(&tasklist_lock);

notask:
	retval = wo->notask_error;
	if (!retval && !(wo->wo_flags & WNOHANG)) {
		retval = -ERESTARTSYS;
		if (!signal_pending(current)) {
			schedule();
			goto repeat;
		}
	}
end:
	__set_current_state(TASK_RUNNING);
	remove_wait_queue(&current->signal->wait_chldexit, &wo->child_wait);
	return retval;
}

static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop,
			  int options, struct rusage *ru)
{
	struct wait_opts wo;
	struct pid *pid = NULL;
	enum pid_type type;
	long ret;

switch (which) {
	case P_ALL:
		type = PIDTYPE_MAX;
		break;
	case P_PID:
		type = PIDTYPE_PID;
		if (upid <= 0)
			return -EINVAL;
		break;
	case P_PGID:
		type = PIDTYPE_PGID;
		if (upid <= 0)
			return -EINVAL;
		break;
	default:
		return -EINVAL;
	}

if (type < PIDTYPE_MAX)
		pid = find_get_pid(upid);

wo.wo_type	= type;
	wo.wo_pid	= pid;
	wo.wo_flags	= options;
	wo.wo_info	= infop;
	wo.wo_rusage	= ru;
	ret = do_wait(&wo);

put_pid(pid);
	return ret;
}

SYSCALL_DEFINE5(waitid, int, which, pid_t, upid, struct siginfo __user *,
		infop, int, options, struct rusage __user *, ru)
{
	struct rusage r;
	struct waitid_info info = {.status = 0};
	long err = kernel_waitid(which, upid, &info, options, ru ? &r : NULL);
	int signo = 0;

if (err > 0) {
		signo = SIGCHLD;
		err = 0;
		if (ru && copy_to_user(ru, &r, sizeof(struct rusage)))
			return -EFAULT;
	}
	if (!infop)
		return err;

if (!user_access_begin(infop, sizeof(*infop)))
		return -EFAULT;

unsafe_put_user(signo, &infop->si_signo, Efault);
	unsafe_put_user(0, &infop->si_errno, Efault);
	unsafe_put_user(info.cause, &infop->si_code, Efault);
	unsafe_put_user(info.pid, &infop->si_pid, Efault);
	unsafe_put_user(info.uid, &infop->si_uid, Efault);
	unsafe_put_user(info.status, &infop->si_status, Efault);
	user_access_end();
	return err;
Efault:
	user_access_end();
	return -EFAULT;
}

long kernel_wait4(pid_t upid, int __user *stat_addr, int options,
		  struct rusage *ru)
{
	struct wait_opts wo;
	struct pid *pid = NULL;
	enum pid_type type;
	long ret;

/* -INT_MIN is not defined */
	if (upid == INT_MIN)
		return -ESRCH;

if (upid == -1)
		type = PIDTYPE_MAX;
	else if (upid < 0) {
		type = PIDTYPE_PGID;
		pid = find_get_pid(-upid);
	} else if (upid == 0) {
		type = PIDTYPE_PGID;
		pid = get_task_pid(current, PIDTYPE_PGID);
	} else /* upid > 0 */ {
		type = PIDTYPE_PID;
		pid = find_get_pid(upid);
	}

wo.wo_type	= type;
	wo.wo_pid	= pid;
	wo.wo_flags	= options | WEXITED;
	wo.wo_info	= NULL;
	wo.wo_stat	= 0;
	wo.wo_rusage	= ru;
	ret = do_wait(&wo);
	put_pid(pid);
	if (ret > 0 && stat_addr && put_user(wo.wo_stat, stat_addr))
		ret = -EFAULT;

return ret;
}

SYSCALL_DEFINE4(wait4, pid_t, upid, int __user *, stat_addr,
		int, options, struct rusage __user *, ru)
{
	struct rusage r;
	long err = kernel_wait4(upid, stat_addr, options, ru ? &r : NULL);

if (err > 0) {
		if (ru && copy_to_user(ru, &r, sizeof(struct rusage)))
			return -EFAULT;
	}
	return err;
}

#ifdef __ARCH_WANT_SYS_WAITPID

/*
 * sys_waitpid() remains for compatibility. waitpid() should be
 * implemented by calling sys_wait4() from libc.a.
 */
SYSCALL_DEFINE3(waitpid, pid_t, pid, int __user *, stat_addr, int, options)
{
	return kernel_wait4(pid, stat_addr, options, NULL);
}

#endif

#ifdef CONFIG_COMPAT
COMPAT_SYSCALL_DEFINE4(wait4,
	compat_pid_t, pid,
	compat_uint_t __user *, stat_addr,
	int, options,
	struct compat_rusage __user *, ru)
{
	struct rusage r;
	long err = kernel_wait4(pid, stat_addr, options, ru ? &r : NULL);
	if (err > 0) {
		if (ru && put_compat_rusage(&r, ru))
			return -EFAULT;
	}
	return err;
}

COMPAT_SYSCALL_DEFINE5(waitid,
		int, which, compat_pid_t, pid,
		struct compat_siginfo __user *, infop, int, options,
		struct compat_rusage __user *, uru)
{
	struct rusage ru;
	struct waitid_info info = {.status = 0};
	long err = kernel_waitid(which, pid, &info, options, uru ? &ru : NULL);
	int signo = 0;
	if (err > 0) {
		signo = SIGCHLD;
		err = 0;
		if (uru) {
			/* kernel_waitid() overwrites everything in ru */
			if (COMPAT_USE_64BIT_TIME)
				err = copy_to_user(uru, &ru, sizeof(ru));
			else
				err = put_compat_rusage(&ru, uru);
			if (err)
				return -EFAULT;
		}
	}

if (!infop)
		return err;

if (!user_access_begin(infop, sizeof(*infop)))
		return -EFAULT;

__weak void abort(void)
{
	BUG();

/* if that doesn't kill us, halt */
	panic("Oops failed to kill thread");
}
EXPORT_SYMBOL(abort);
```
-----------------------------------------------------------------------------------------
- Creator
https://github.com/torvalds/linux.git

- Kernel doc 
https://www.kernel.org/doc/html/v4.15/index.html#

- How to patching your Kernel
https://www.kernel.org/doc/html/v4.10/process/applying-patches.html

- Source:
https://www.kernel.org/

- - - Online:

[![](https://github.com/nu11secur1ty/Kernel-and-Types-of-kernels/blob/master/logo/Linux.png)](https://elixir.bootlin.com/linux/latest/source)

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