signal Linux 信号

Linux Signal

Posted by Peng Weilin on 2016-07-04

信号的基本使用场景：使用 ctrl+c 中止一个程序，或者使用 kill pid 命令杀掉一个进程。Linux 信号机制基本上每个同学都用过，但是信号的具体实现机制还是有很多人不清楚的。在很多人的概念中信号是一种异步机制，像中断一样。但是除了硬中断，信号也是由中断实现的吗？如果不是中断，系统又怎么样来利用软件机制模拟类似如异步中断的动作？

本文的代码分析基于 Linux Kernel 3.18.22，最好的学习方法还是 “read the fucking source code”

1.信号的响应时机

理解信号异步机制的关键是信号的响应时机，我们对一个进程发送一个信号以后，其实并没有硬中断发生，只是简单把信号挂载到目标进程的信号 pending 队列上去，信号真正得到执行的时机是进程执行完异常/中断返回到用户态的时刻。

让信号看起来是一个异步中断的关键就是，正常的用户进程是会频繁的在用户态和内核态之间切换的（这种切换包括：系统调用、缺页异常、系统中断…），所以信号能很快的能得到执行。但这也带来了一点问题，内核进程是不响应信号的，除非它刻意的去查询。所以通常情况下我们无法通过kill命令去杀死一个内核进程。

信号响应时机

  • arch/arm64/kernel/entry.s:
  • el0_sync()/el0_irq() -> ret_to_user() -> work_pending() -> do_notify_resume()
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	// (1) 在arm64架构中，kernel运行在el1，用户态运行在el0。
	// el0_sync是用户态发生异常的入口，el0_irq是用户态发生中断的的入口。
	// 异常包括几种：系统调用el0_svc、数据异常el0_da、指令异常el0_ia等等几种。
	.align	11
ENTRY(vectors)
	ventry	el0_sync			// Synchronous 64-bit EL0
	ventry	el0_irq				// IRQ 64-bit EL0


	// (2) 用户态异常el0_sync
	.align	6
el0_sync:
	kernel_entry 0
	mrs	x25, esr_el1			// read the syndrome register
	lsr	x24, x25, #ESR_EL1_EC_SHIFT	// exception class
	cmp	x24, #ESR_EL1_EC_SVC64		// SVC in 64-bit state
	b.eq	el0_svc
	cmp	x24, #ESR_EL1_EC_DABT_EL0	// data abort in EL0
	b.eq	el0_da
	cmp	x24, #ESR_EL1_EC_IABT_EL0	// instruction abort in EL0
	b.eq	el0_ia
	cmp	x24, #ESR_EL1_EC_FP_ASIMD	// FP/ASIMD access
	b.eq	el0_fpsimd_acc
	cmp	x24, #ESR_EL1_EC_FP_EXC64	// FP/ASIMD exception
	b.eq	el0_fpsimd_exc
	cmp	x24, #ESR_EL1_EC_SYS64		// configurable trap
	b.eq	el0_undef
	cmp	x24, #ESR_EL1_EC_SP_ALIGN	// stack alignment exception
	b.eq	el0_sp_pc
	cmp	x24, #ESR_EL1_EC_PC_ALIGN	// pc alignment exception
	b.eq	el0_sp_pc
	cmp	x24, #ESR_EL1_EC_UNKNOWN	// unknown exception in EL0
	b.eq	el0_undef
	cmp	x24, #ESR_EL1_EC_BREAKPT_EL0	// debug exception in EL0
	b.ge	el0_dbg
	b	el0_inv

	// (2.1) 用户态数据访问el0_da
el0_da:
	/*
	 * Data abort handling
	 */
	mrs	x26, far_el1
	// enable interrupts before calling the main handler
	enable_dbg_and_irq
	ct_user_exit
	bic	x0, x26, #(0xff << 56)
	mov	x1, x25
	mov	x2, sp
	bl	do_mem_abort
	b	ret_to_user

	// (3) 用户态中断el0_irq
	.align	6
el0_irq:
	kernel_entry 0
el0_irq_naked:
	enable_dbg
#ifdef CONFIG_TRACE_IRQFLAGS
	bl	trace_hardirqs_off
#endif

	ct_user_exit
	irq_handler

#ifdef CONFIG_TRACE_IRQFLAGS
	bl	trace_hardirqs_on
#endif
	b	ret_to_user
ENDPROC(el0_irq)

	// (4) 返回用户态的处理函数ret_to_user
	// 判断thread_info->flags与#_TIF_WORK_MASK，是否有置位，有则跳转到work_pending执行。
	// _TIF_SIGPENDING置位即代表了进程有信号需要处理
	// #define _TIF_WORK_MASK		(_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
	//			 _TIF_NOTIFY_RESUME | _TIF_FOREIGN_FPSTATE)
ret_to_user:
	disable_irq				// disable interrupts
	ldr	x1, [tsk, #TI_FLAGS]
	and	x2, x1, #_TIF_WORK_MASK
	cbnz	x2, work_pending
	enable_step_tsk x1, x2
no_work_pending:
#ifdef CONFIG_MTK_COMPAT
	kernel_exit_compat ret = 0
#else
	kernel_exit 0, ret = 0
#endif
ENDPROC(ret_to_user)

	// (5) work_pending
fast_work_pending:
	str	x0, [sp, #S_X0]			// returned x0
work_pending:
	tbnz	x1, #TIF_NEED_RESCHED, work_resched
	/* TIF_SIGPENDING, TIF_NOTIFY_RESUME or TIF_FOREIGN_FPSTATE case */
	ldr	x2, [sp, #S_PSTATE]
	mov	x0, sp				// 'regs'
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	tst	x2, #PSR_MODE_MASK		// user mode regs?
	b.ne	no_work_pending			// returning to kernel
	enable_irq				// enable interrupts for do_notify_resume()
	bl	do_notify_resume
	b	ret_to_user
work_resched:
	bl	schedule
  • arch/arm64/kernel/signal.c:
  • -> do_notify_resume() -> do_signal() -> get_signal()/handle_signal()
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asmlinkage void do_notify_resume(struct pt_regs *regs,
				 unsigned int thread_flags)
{
	// (5.1)具体的信号处理过程
	if (thread_flags & _TIF_SIGPENDING)
		do_signal(regs);

	if (thread_flags & _TIF_NOTIFY_RESUME) {
		clear_thread_flag(TIF_NOTIFY_RESUME);
		tracehook_notify_resume(regs);
	}

	if (thread_flags & _TIF_FOREIGN_FPSTATE)
		fpsimd_restore_current_state();

}

1.1 INTERRUPTIBLE/UNINTERRUPTIBLE 进程对信号的响应

上节主要描述运行状态（TASK_RUNNING）进程对信号的响应时机：信号发送后挂到目标进程的信号队列，进程返回用户态的时候在 do_notify_resume() 中处理信号。

那么对于阻塞状态的进程又怎么样来响应信号呢？

让一个进程进入阻塞状态，我们可以选择让其进入可中断（TASK_INTERRUPTIBLE）或者不可中断（TASK_UNINTERRUPTIBLE）状态，比如 mutex 操作分为 mutex_lock() 和 mutex_lock_interruptible()。所谓的可中断和不可中断就是说是否可以被中断信号打断：如果进程处于可中断（TASK_INTERRUPTIBLE）状态，信号发送函数会直接唤醒进程，让进程处理完内核态操作去返回用户态，让进程迅速去执行信号处理函数；如果进程处于不可中断（TASK_UNINTERRUPTIBLE）状态俗称为 D 进程，信号只会挂到信号队列，但是没有机会去立即执行。

  • kernel/signal.c:
  • __send_signal() -> complete_signal() -> signal_wake_up() -> signal_wake_up_state()
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void signal_wake_up_state(struct task_struct *t, unsigned int state)
{
	set_tsk_thread_flag(t, TIF_SIGPENDING);
	/*
	 * TASK_WAKEKILL also means wake it up in the stopped/traced/killable
	 * case. We don't check t->state here because there is a race with it
	 * executing another processor and just now entering stopped state.
	 * By using wake_up_state, we ensure the process will wake up and
	 * handle its death signal.
	 */
	// (1)在发送完信号后，会唤醒状态为TASK_INTERRUPTIBLE的进程。
	if (!wake_up_state(t, state | TASK_INTERRUPTIBLE))
		kick_process(t);
}

1.2内核进程响应信号

上面说到内核进程普通情况下是不会响应信号的，如果需要内核进程响应信号，可以在内核进程中加入如下代码：

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if (signal_pending(current))
{
    // 自定义信号处理函数
}
flush_signals(current);

2.信号简介

在给大家引出重点的信号响应时机以后，还是简单介绍以下信号的背景知识。信号也是一种进程间通讯的机制，它传递的信息很短，只有一个编号。

2.1 常规信号和实时信号

Linux 传统的信号 1~31 为常规信号（regular signal），POSIX 还引入了一种新的信号实时信号（real-time signal）编号为 32~64。它们的不同在于：常规信号同一个编号在 pending 队列中只存在一份，如果有重复的则直接丢弃；实时信号的多个相同信号不能丢弃，需要保证每个信号都能送达。

Linux 常用的是常规信号，以下是具体的定义ULK：

编号 信号名称 缺省操作 解释 POSIX
1 SIGHUP Terminate Hang up controlling terminal or process Yes
2 SIGINT Terminate Interrupt from keyboard Yes
3 SIGQUIT Dump Quit from keyboard Yes
4 SIGILL Dump Illegal instruction Yes
5 SIGTRAP Dump Breakpoint for debugging No
6 SIGABRT Dump Abnormal termination Yes
6 SIGIOT Dump Equivalent to SIGABRT No
7 SIGBUS Dump Bus error No
8 SIGFPE Dump Floating-point exception Yes
9 SIGKILL Terminate Forced-process termination Yes
10 SIGUSR1 Terminate Available to processes Yes
11 SIGSEGV Dump Invalid memory reference Yes
12 SIGUSR2 Terminate Available to processes Yes
13 SIGPIPE Terminate Write to pipe with no readers Yes
14 SIGALRM Terminate Real-timerclock Yes
15 SIGTERM Terminate Process termination Yes
16 SIGSTKFLT Terminate Coprocessor stack error No
17 SIGCHLD Ignore Child process stopped or terminated, or got signal if traced Yes
18 SIGCONT Continue Resume execution, if stopped Yes
19 SIGSTOP Stop Stop process execution Yes
20 SIGTSTP Stop Stop process issued from tty Yes
21 SIGTTIN Stop Background process requires input Yes
22 SIGTTOU Stop Background process requires output Yes
23 SIGURG Ignore Urgent condition on socket No
24 SIGXCPU Dump CPU time limit exceeded No
25 SIGXFSZ Dump File size limit exceeded No
26 SIGVTALRM Terminate Virtual timer clock No
27 SIGPROF Terminate Profile timer clock No
28 SIGWINCH Ignore Window resizing No
29 SIGIO Terminate I/O now possible No
29 SIGPOLL Terminate Equivalent to SIGIO No
30 SIGPWR Terminate Power supply failure No
31 SIGSYS Dump Bad system call No
31 SIGUNUSED Dump Equivalent to SIGSYS No

所谓的缺省操作：是在用户没有注册用户态的信号处理函数的情况下，默认的信号内核处理方法。在第4节中会详细的讲解。

3.信号的发送

信号的发送者可以是 user 也可以是 kernel，我们经常是通过用户态来调用 kill()、tkill() 等函数来发送信号的，我们通过分析这些系统调用来理解信号的具体发送过程。

  • 与信号相关的系统调用主要有以下函数：
系统调用 说明
kill 向线程组发送信号
tkill 向进程发送信号
tgkill 向指定线程组中的进程发送信号
signal 注册信号的用户态处理函数
sigprocmask block/unblock信号

3.1 kill()

kill() 系统调用的功能是发送一个信号给线程组，只需要线程组挑出一个线程来响应处理信号。但是对于致命信号，线程组内所有进程都会被杀死，而不仅仅是处理信号的线程。

kill 调用

  • kernel/signal.c:
  • kill() -> kill_something_info() -> kill_pid_info() -> group_send_sig_info() -> do_send_sig_info() -> send_signal() -> __send_signal()
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SYSCALL_DEFINE2(kill, pid_t, pid, int, sig)
{
	struct siginfo info;

	info.si_signo = sig;
	info.si_errno = 0;
	info.si_code = SI_USER;
	info.si_pid = task_tgid_vnr(current);
	info.si_uid = from_kuid_munged(current_user_ns(), current_uid());

	return kill_something_info(sig, &info, pid);
}
| →
static int kill_something_info(int sig, struct siginfo *info, pid_t pid)
{
	int ret;
	// (1)pid>0, 发送信号给pid进程所在的线程组
	if (pid > 0) {
		rcu_read_lock();
		ret = kill_pid_info(sig, info, find_vpid(pid));
		rcu_read_unlock();
		return ret;
	}

	read_lock(&tasklist_lock);
	// (2)(pid <= 0) && (pid != -1), 发送信号给pid进程所在进程组中的每一个线程组
	if (pid != -1) {
		ret = __kill_pgrp_info(sig, info,
				pid ? find_vpid(-pid) : task_pgrp(current));
	} else {
	// (3)pid = -1, 发送信号给所有进程的进程组，除了pid=1和当前进程自己
		int retval = 0, count = 0;
		struct task_struct * p;

		for_each_process(p) {
			if (task_pid_vnr(p) > 1 &&
					!same_thread_group(p, current)) {
				int err = group_send_sig_info(sig, info, p);
				++count;
				if (err != -EPERM)
					retval = err;
			}
		}
		ret = count ? retval : -ESRCH;
	}
	read_unlock(&tasklist_lock);

	return ret;
}
|| →
int group_send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
{
	int ret;

	rcu_read_lock();
	ret = check_kill_permission(sig, info, p);
	rcu_read_unlock();

	if (!ret && sig)
		// (1.1)参数group=ture，信号发送给线程组
		ret = do_send_sig_info(sig, info, p, true);

	return ret;
}

信号队列

接下来来到了发送信号的核心函数 __send_signal()，函数的主要目的是把信号挂到信号的 pending 队列中去。pending 队列有两种：一种是进程组共享的 task_struct->signal->shared_pending，发送给线程组的信号会挂载到该队列；另一种是进程私有队列 task_struct->pending，发送给进程的信号会挂载到该队列。

从下面的代码中，我们可以看到在创建线程时，线程组贡献信号队列 task_struct->signal->shared_pending 是怎么实现的。

  • kernel/fork.c:
  • do_fork() -> copy_process() -> copy_signal()/copy_sighand()
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static struct task_struct *copy_process(unsigned long clone_flags,
					unsigned long stack_start,
					unsigned long stack_size,
					int __user *child_tidptr,
					struct pid *pid,
					int trace)
{
    ...
	// (1)复制父进程current的task_struct结构体到新进程p；
	// 这里已经包含做了signal的复制动作:p->signal=current->signal
	p = dup_task_struct(current);
	...
	retval = copy_sighand(clone_flags, p);
	if (retval)
		goto bad_fork_cleanup_fs;
	// (2)如果是创建线程(CLONE_THREAD被置位)，那么新进程和父进程共享tsk->signal结构，
	// 不会分配新的tsk->signal结构空间
	retval = copy_signal(clone_flags, p);
	if (retval)
		goto bad_fork_cleanup_sighand;
	...
}
| →
static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
{
	struct signal_struct *sig;

	// (2.1)如果是创建线程(CLONE_THREAD被置位)，不分配新的tsk->signal空间直接返回
	if (clone_flags & CLONE_THREAD)
		return 0;

    sig = kmem_cache_zalloc(signal_cachep, GFP_KERNEL);
	tsk->signal = sig;
	...
}
| →
static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)
{
	struct sighand_struct *sig;

	// (2.2)同样，也可以用CLONE_SIGHAND标志来控制是否共享tsk->sighand
	if (clone_flags & CLONE_SIGHAND) {
		atomic_inc(&current->sighand->count);
		return 0;
	}
	sig = kmem_cache_alloc(sighand_cachep, GFP_KERNEL);
	rcu_assign_pointer(tsk->sighand, sig);
	if (!sig)
		return -ENOMEM;
	atomic_set(&sig->count, 1);
	memcpy(sig->action, current->sighand->action, sizeof(sig->action));
	return 0;
}

继续来看 __send_signal() 的具体实现：

  • kernel/signal.c:
  • -> __send_signal() -> prepare_signal()/complete_signal()
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static int __send_signal(int sig, struct siginfo *info, struct task_struct *t,
			int group, int from_ancestor_ns)
{
	struct sigpending *pending;
	struct sigqueue *q;
	int override_rlimit;
	int ret = 0, result;

	assert_spin_locked(&t->sighand->siglock);

	result = TRACE_SIGNAL_IGNORED;
	// (1)判断是否可以忽略信号
	if (!prepare_signal(sig, t,
			from_ancestor_ns || (info == SEND_SIG_FORCED)))
		goto ret;

	// (2)选择信号pending队列
	// 线程组共享队列(t->signal->shared_pending) or 进程私有队列(t->pending)
	pending = group ? &t->signal->shared_pending : &t->pending;
	/*
	 * Short-circuit ignored signals and support queuing
	 * exactly one non-rt signal, so that we can get more
	 * detailed information about the cause of the signal.
	 */
	result = TRACE_SIGNAL_ALREADY_PENDING;
	// (3)如果信号是常规信号(regular signal)，且已经在pending队列中，则忽略重复信号；
	// 另外一方面也说明了，如果是实时信号，尽管信号重复，但还是要加入pending队列；
	// 实时信号的多个信号都需要能被接收到。
	if (legacy_queue(pending, sig))
		goto ret;

	result = TRACE_SIGNAL_DELIVERED;
	/*
	 * fast-pathed signals for kernel-internal things like SIGSTOP
	 * or SIGKILL.
	 */
	// (4)如果是强制信号(SEND_SIG_FORCED)，不走挂载pending队列的流程，直接快速路径优先处理。
	if (info == SEND_SIG_FORCED)
		goto out_set;

	/*
	 * Real-time signals must be queued if sent by sigqueue, or
	 * some other real-time mechanism.  It is implementation
	 * defined whether kill() does so.  We attempt to do so, on
	 * the principle of least surprise, but since kill is not
	 * allowed to fail with EAGAIN when low on memory we just
	 * make sure at least one signal gets delivered and don't
	 * pass on the info struct.
	 */
	// (5)符合条件的特殊信号可以突破siganl pending队列的大小限制(rlimit)
	// 否则在队列满的情况下，丢弃信号
	// signal pending队列大小rlimit的值可以通过命令"ulimit -i"查看
	if (sig < SIGRTMIN)
		override_rlimit = (is_si_special(info) || info->si_code >= 0);
	else
		override_rlimit = 0;

	// (6)没有ignore的信号，加入到pending队列中。
	q = __sigqueue_alloc(sig, t, GFP_ATOMIC | __GFP_NOTRACK_FALSE_POSITIVE,
		override_rlimit);
	if (q) {
		list_add_tail(&q->list, &pending->list);
		switch ((unsigned long) info) {
		case (unsigned long) SEND_SIG_NOINFO:
			q->info.si_signo = sig;
			q->info.si_errno = 0;
			q->info.si_code = SI_USER;
			q->info.si_pid = task_tgid_nr_ns(current,
							task_active_pid_ns(t));
			q->info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
			break;
		case (unsigned long) SEND_SIG_PRIV:
			q->info.si_signo = sig;
			q->info.si_errno = 0;
			q->info.si_code = SI_KERNEL;
			q->info.si_pid = 0;
			q->info.si_uid = 0;
			break;
		default:
			copy_siginfo(&q->info, info);
			if (from_ancestor_ns)
				q->info.si_pid = 0;
			break;
		}

		userns_fixup_signal_uid(&q->info, t);

	} else if (!is_si_special(info)) {
		if (sig >= SIGRTMIN && info->si_code != SI_USER) {
			/*
			 * Queue overflow, abort.  We may abort if the
			 * signal was rt and sent by user using something
			 * other than kill().
			 */
			result = TRACE_SIGNAL_OVERFLOW_FAIL;
			ret = -EAGAIN;
			goto ret;
		} else {
			/*
			 * This is a silent loss of information.  We still
			 * send the signal, but the *info bits are lost.
			 */
			result = TRACE_SIGNAL_LOSE_INFO;
		}
	}

out_set:
	signalfd_notify(t, sig);
	// (7)更新pending->signal信号集合中对应的bit
	sigaddset(&pending->signal, sig);
	// (8)选择合适的进程来响应信号，如果需要并唤醒对应的进程
	complete_signal(sig, t, group);
ret:
	trace_signal_generate(sig, info, t, group, result);
	return ret;
}
| →
static bool prepare_signal(int sig, struct task_struct *p, bool force)
{
	struct signal_struct *signal = p->signal;
	struct task_struct *t;
	sigset_t flush;

	if (signal->flags & (SIGNAL_GROUP_EXIT | SIGNAL_GROUP_COREDUMP)) {
		// (1.1)如果进程正在处于SIGNAL_GROUP_COREDUMP，则当前信号被忽略
		if (signal->flags & SIGNAL_GROUP_COREDUMP) {
			pr_debug("[%d:%s] is in the middle of doing coredump so skip sig %d\n", p->pid, p->comm, sig);
			return 0;
		}
		/*
		 * The process is in the middle of dying, nothing to do.
		 */
	} else if (sig_kernel_stop(sig)) {
		// (1.2)如果当前是stop信号，则移除线程组所有线程pending队列中的SIGCONT信号
		/*
		 * This is a stop signal.  Remove SIGCONT from all queues.
		 */
		siginitset(&flush, sigmask(SIGCONT));
		flush_sigqueue_mask(&flush, &signal->shared_pending);
		for_each_thread(p, t)
			flush_sigqueue_mask(&flush, &t->pending);
	} else if (sig == SIGCONT) {
		unsigned int why;
		// (1.3)如果当前是SIGCONT信号，则移除线程组所有线程pending队列中的stop信号，并唤醒stop进程
		/*
		 * Remove all stop signals from all queues, wake all threads.
		 */
		siginitset(&flush, SIG_KERNEL_STOP_MASK);
		flush_sigqueue_mask(&flush, &signal->shared_pending);
		for_each_thread(p, t) {
			flush_sigqueue_mask(&flush, &t->pending);
			task_clear_jobctl_pending(t, JOBCTL_STOP_PENDING);
			if (likely(!(t->ptrace & PT_SEIZED)))
				wake_up_state(t, __TASK_STOPPED);
			else
				ptrace_trap_notify(t);
		}

		/*
		 * Notify the parent with CLD_CONTINUED if we were stopped.
		 *
		 * If we were in the middle of a group stop, we pretend it
		 * was already finished, and then continued. Since SIGCHLD
		 * doesn't queue we report only CLD_STOPPED, as if the next
		 * CLD_CONTINUED was dropped.
		 */
		why = 0;
		if (signal->flags & SIGNAL_STOP_STOPPED)
			why |= SIGNAL_CLD_CONTINUED;
		else if (signal->group_stop_count)
			why |= SIGNAL_CLD_STOPPED;

		if (why) {
			/*
			 * The first thread which returns from do_signal_stop()
			 * will take ->siglock, notice SIGNAL_CLD_MASK, and
			 * notify its parent. See get_signal_to_deliver().
			 */
			signal->flags = why | SIGNAL_STOP_CONTINUED;
			signal->group_stop_count = 0;
			signal->group_exit_code = 0;
		}
	}

	// (1.4)进一步判断信号是否会被忽略
	return !sig_ignored(p, sig, force);
}
|| →
static int sig_ignored(struct task_struct *t, int sig, bool force)
{
	/*
	 * Blocked signals are never ignored, since the
	 * signal handler may change by the time it is
	 * unblocked.
	 */
	// (1.4.1)如果信号被blocked，不会被忽略
	if (sigismember(&t->blocked, sig) || sigismember(&t->real_blocked, sig))
		return 0;

	// (1.4.2)进一步判断信号的忽略条件
	if (!sig_task_ignored(t, sig, force))
		return 0;

	/*
	 * Tracers may want to know about even ignored signals.
	 */
	// (1.4.3)信号符合忽略条件，且没有被trace，则信号被忽略
	return !t->ptrace;
}
||| →
static int sig_task_ignored(struct task_struct *t, int sig, bool force)
{
	void __user *handler;

	// (1.4.2.1)提取信号的操作函数
	handler = sig_handler(t, sig);

	// (1.4.2.2)如果符合条件，信号被忽略
	if (unlikely(t->signal->flags & SIGNAL_UNKILLABLE) &&
			handler == SIG_DFL && !force)
		return 1;

	// (1.4.2.3)
	return sig_handler_ignored(handler, sig);
}
|||| →
static int sig_handler_ignored(void __user *handler, int sig)
{
	/* Is it explicitly or implicitly ignored? */
	// (1.4.2.3.1)如果信号操作函数是忽略SIG_IGN，或者操作函数是默认SIG_DFL但是默认动作是忽略
	// 默认动作是忽略的信号包括：
	// #define SIG_KERNEL_IGNORE_MASK (\
	//    rt_sigmask(SIGCONT)   |  rt_sigmask(SIGCHLD)   | \
	//    rt_sigmask(SIGWINCH)  |  rt_sigmask(SIGURG)    )
	// 忽略这一类信号
	return handler == SIG_IGN ||
		(handler == SIG_DFL && sig_kernel_ignore(sig));
}
| →
static void complete_signal(int sig, struct task_struct *p, int group)
{
	struct signal_struct *signal = p->signal;
	struct task_struct *t;

	/*
	 * Now find a thread we can wake up to take the signal off the queue.
	 *
	 * If the main thread wants the signal, it gets first crack.
	 * Probably the least surprising to the average bear.
	 */
	// (8.1)判断当前线程是否符合响应信号的条件
	if (wants_signal(sig, p))
		t = p;
	else if (!group || thread_group_empty(p))
		// (8.2)如果信号是发给单线程的，直接返回
		/*
		 * There is just one thread and it does not need to be woken.
		 * It will dequeue unblocked signals before it runs again.
		 */
		return;
	else {
		/*
		 * Otherwise try to find a suitable thread.
		 */
		// (8.3)在当前线程组中挑出一个符合响应信号条件的线程
		// 从signal->curr_target线程开始查找
		t = signal->curr_target;
		while (!wants_signal(sig, t)) {
			t = next_thread(t);
			if (t == signal->curr_target)
				/*
				 * No thread needs to be woken.
				 * Any eligible threads will see
				 * the signal in the queue soon.
				 */
				return;
		}
		signal->curr_target = t;
	}

	/*
	 * Found a killable thread.  If the signal will be fatal,
	 * then start taking the whole group down immediately.
	 */
	if (sig_fatal(p, sig) &&
	    !(signal->flags & (SIGNAL_UNKILLABLE | SIGNAL_GROUP_EXIT)) &&
	    !sigismember(&t->real_blocked, sig) &&
	    (sig == SIGKILL || !t->ptrace)) {
		/*
		 * This signal will be fatal to the whole group.
		 */
		if (!sig_kernel_coredump(sig)) {
			/*
			 * Start a group exit and wake everybody up.
			 * This way we don't have other threads
			 * running and doing things after a slower
			 * thread has the fatal signal pending.
			 */
			signal->flags = SIGNAL_GROUP_EXIT;
			signal->group_exit_code = sig;
			signal->group_stop_count = 0;
			t = p;
			do {
				task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
				sigaddset(&t->pending.signal, SIGKILL);
				signal_wake_up(t, 1);
			} while_each_thread(p, t);
			return;
		}
	}

	/*
	 * The signal is already in the shared-pending queue.
	 * Tell the chosen thread to wake up and dequeue it.
	 */
	// (8.4)唤醒挑选出的响应信号的线程
	signal_wake_up(t, sig == SIGKILL);
	return;
}
|| →
static inline void ptrace_signal_wake_up(struct task_struct *t, bool resume)
{
	signal_wake_up_state(t, resume ? __TASK_TRACED : 0);
}
||| →
void signal_wake_up_state(struct task_struct *t, unsigned int state)
{
	// (8.4.1)设置thread_info->flags中的TIF_SIGPENDING标志
	// ret_to_user()时会根据此标志来调用do_notify_resume()
	set_tsk_thread_flag(t, TIF_SIGPENDING);
	/*
	 * TASK_WAKEKILL also means wake it up in the stopped/traced/killable
	 * case. We don't check t->state here because there is a race with it
	 * executing another processor and just now entering stopped state.
	 * By using wake_up_state, we ensure the process will wake up and
	 * handle its death signal.
	 */
	// (8.4.2)唤醒阻塞状态为TASK_INTERRUPTIBLE的信号响应线程
	if (!wake_up_state(t, state | TASK_INTERRUPTIBLE))
		kick_process(t);
}

3.2 tkill()

kill() 是向进程组发一个信号，而 tkill() 是向某一个进程发送信号。

tkill 调用

  • kernel/signal.c:
  • tkill() -> do_tkill() -> do_send_specific() -> send_signal()
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SYSCALL_DEFINE2(tkill, pid_t, pid, int, sig)
{
	/* This is only valid for single tasks */
	if (pid <= 0)
		return -EINVAL;

	return do_tkill(0, pid, sig);
}
| →
static int do_tkill(pid_t tgid, pid_t pid, int sig)
{
	struct siginfo info = {};

	info.si_signo = sig;
	info.si_errno = 0;
	info.si_code = SI_TKILL;
	info.si_pid = task_tgid_vnr(current);
	info.si_uid = from_kuid_munged(current_user_ns(), current_uid());

	return do_send_specific(tgid, pid, sig, &info);
}
|| →
static int
do_send_specific(pid_t tgid, pid_t pid, int sig, struct siginfo *info)
{
	struct task_struct *p;
	int error = -ESRCH;

	rcu_read_lock();
	p = find_task_by_vpid(pid);
	if (p && (tgid <= 0 || task_tgid_vnr(p) == tgid)) {
		// (1)tkill()符合条件1：tgid=0
		// tgkill()需要符合条件2：tgid指定的线程组 == p所在的线程组
		error = check_kill_permission(sig, info, p);
		/*
		 * The null signal is a permissions and process existence
		 * probe.  No signal is actually delivered.
		 */
		if (!error && sig) {
			// (2)参数group=false，信号发送给单个进程组
			error = do_send_sig_info(sig, info, p, false);
			/*
			 * If lock_task_sighand() failed we pretend the task
			 * dies after receiving the signal. The window is tiny,
			 * and the signal is private anyway.
			 */
			if (unlikely(error == -ESRCH))
				error = 0;
		}
	}
	rcu_read_unlock();

	return error;
}

3.3 tgkill()

tgkill() 是向特定线程组中的进程发送信号。

tgkill 调用

  • kernel/signal.c:
  • tkill() -> do_tkill() -> do_send_specific() -> send_signal()
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SYSCALL_DEFINE3(tgkill, pid_t, tgid, pid_t, pid, int, sig)
{
	/* This is only valid for single tasks */
	if (pid <= 0 || tgid <= 0)
		return -EINVAL;

	return do_tkill(tgid, pid, sig);
}

3.4 signal()

signal()/sigaction() 注册用户自定义的信号处理函数。

  • kernel/signal.c:
  • signal() -> do_sigaction()
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SYSCALL_DEFINE2(signal, int, sig, __sighandler_t, handler)
{
	struct k_sigaction new_sa, old_sa;
	int ret;

	new_sa.sa.sa_handler = handler;
	new_sa.sa.sa_flags = SA_ONESHOT | SA_NOMASK;
	sigemptyset(&new_sa.sa.sa_mask);

	ret = do_sigaction(sig, &new_sa, &old_sa);

	return ret ? ret : (unsigned long)old_sa.sa.sa_handler;
}
| →
int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
{
	struct task_struct *p = current, *t;
	struct k_sigaction *k;
	sigset_t mask;

	if (!valid_signal(sig) || sig < 1 || (act && sig_kernel_only(sig)))
		return -EINVAL;

	k = &p->sighand->action[sig-1];

	spin_lock_irq(&p->sighand->siglock);
	if (oact)
		*oact = *k;

	if (act) {
		sigdelsetmask(&act->sa.sa_mask,
			      sigmask(SIGKILL) | sigmask(SIGSTOP));
		// (1)将信号处理函数sighand->action[sig-1]替换成用户函数
		*k = *act;
		/*
		 * POSIX 3.3.1.3:
		 *  "Setting a signal action to SIG_IGN for a signal that is
		 *   pending shall cause the pending signal to be discarded,
		 *   whether or not it is blocked."
		 *
		 *  "Setting a signal action to SIG_DFL for a signal that is
		 *   pending and whose default action is to ignore the signal
		 *   (for example, SIGCHLD), shall cause the pending signal to
		 *   be discarded, whether or not it is blocked"
		 */
		if (sig_handler_ignored(sig_handler(p, sig), sig)) {
			sigemptyset(&mask);
			sigaddset(&mask, sig);
			flush_sigqueue_mask(&mask, &p->signal->shared_pending);
			for_each_thread(p, t)
				flush_sigqueue_mask(&mask, &t->pending);
		}
	}

	spin_unlock_irq(&p->sighand->siglock);
	return 0;
}

3.5 sigprocmask()

sigprocmask() 用来设置进程对信号是否阻塞。阻塞以后，信号继续挂载到信号 pending 队列，但是信号处理时不响应信号。SIG_BLOCK 命令阻塞信号，SIG_UNBLOCK 命令解除阻塞信号。

  • kernel/signal.c:
  • sigprocmask() -> set_current_blocked()
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SYSCALL_DEFINE3(sigprocmask, int, how, old_sigset_t __user *, nset,
		old_sigset_t __user *, oset)
{
	old_sigset_t old_set, new_set;
	sigset_t new_blocked;

	old_set = current->blocked.sig[0];

	if (nset) {
		if (copy_from_user(&new_set, nset, sizeof(*nset)))
			return -EFAULT;

		new_blocked = current->blocked;

		switch (how) {
		case SIG_BLOCK:
			sigaddsetmask(&new_blocked, new_set);
			break;
		case SIG_UNBLOCK:
			sigdelsetmask(&new_blocked, new_set);
			break;
		case SIG_SETMASK:
			new_blocked.sig[0] = new_set;
			break;
		default:
			return -EINVAL;
		}

		// (1)根据SIG_BLOCK/SIG_UNBLOCK命令来重新设计阻塞信号set current->blocked。
		set_current_blocked(&new_blocked);
	}

	if (oset) {
		if (copy_to_user(oset, &old_set, sizeof(*oset)))
			return -EFAULT;
	}

	return 0;
}

关于信号阻塞 current->blocked 的使用在信号处理函数 get_signal() 中使用。

  • arch/arm64/kernel/signal.c:
  • do_signal() -> get_signal()
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int get_signal(struct ksignal *ksig)
{
    ...
    signr = dequeue_signal(current, &current->blocked, &ksig->info);
    ...
}
| →
int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
{
	int signr;

	/* We only dequeue private signals from ourselves, we don't let
	 * signalfd steal them
	 */
	signr = __dequeue_signal(&tsk->pending, mask, info);
	if (!signr) {
		signr = __dequeue_signal(&tsk->signal->shared_pending,
					 mask, info);
    ...
}
|| →
static int __dequeue_signal(struct sigpending *pending, sigset_t *mask,
			siginfo_t *info)
{
	// (1)对于pending被设置的阻塞信号，信号处理时不予响应。
	int sig = next_signal(pending, mask);

	if (sig) {
		if (current->notifier) {
			if (sigismember(current->notifier_mask, sig)) {
				if (!(current->notifier)(current->notifier_data)) {
					clear_thread_flag(TIF_SIGPENDING);
					return 0;
				}
			}
		}

		collect_signal(sig, pending, info);
	}

	return sig;
}

4.信号的处理

  • 系统对信号的处理有三种方式：
信号响应 描述
忽略 ignore
调用用户态注册的处理函数 如果用户有注册信号处理函数，调用 sighand->action[signr-1] 中对应的注册函数
调用默认的内核态处理函数 如果用户没有注册对应的处理函数，调用默认的内核处理
  • 默认的内核态处理，进一步可以细分为几种：
信号默认内核处理类型 描述
Terminate 进程被中止(杀死)。
Dump 进程被中止(杀死)，并且输出 dump 文件。
Ignore 信号被忽略。
Stop 进程被停止，把进程设置为 TASK_STOPPED 状态。
Continue 如果进程被停止（TASK_STOPPED），把它设置成 TASK_RUNNING 状态。

4.1 do_signal()

信号处理的核心函数就是 do_signal()，下面我们来详细分析一下具体实现。

  • arch/arm64/kernel/signal.c:
  • -> ret_to_user() -> do_notify_resume() -> do_signal() -> get_signal()/handle_signal()
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static void do_signal(struct pt_regs *regs)
{
	unsigned long continue_addr = 0, restart_addr = 0;
	int retval = 0;
	int syscall = (int)regs->syscallno;
	struct ksignal ksig;

	/*
	 * If we were from a system call, check for system call restarting...
	 */
	// (1)如果是 system call 被信号中断，判断是否需要重启 system call
	if (syscall >= 0) {
		continue_addr = regs->pc;
		restart_addr = continue_addr - (compat_thumb_mode(regs) ? 2 : 4);
		retval = regs->regs[0];

		/*
		 * Avoid additional syscall restarting via ret_to_user.
		 */
		regs->syscallno = ~0UL;

		/*
		 * Prepare for system call restart. We do this here so that a
		 * debugger will see the already changed PC.
		 */
		switch (retval) {
		case -ERESTARTNOHAND:
		case -ERESTARTSYS:
		case -ERESTARTNOINTR:
		case -ERESTART_RESTARTBLOCK:
			regs->regs[0] = regs->orig_x0;
			regs->pc = restart_addr;
			break;
		}
	}

	/*
	 * Get the signal to deliver. When running under ptrace, at this point
	 * the debugger may change all of our registers.
	 */
	// (2) 从线程的信号 pending 队列中取出信号，
	// 如果没有对应的用户自定义处理函数，则执行默认的内核态处理函数
	if (get_signal(&ksig)) {
		/*
		 * Depending on the signal settings, we may need to revert the
		 * decision to restart the system call, but skip this if a
		 * debugger has chosen to restart at a different PC.
		 */
		if (regs->pc == restart_addr &&
		    (retval == -ERESTARTNOHAND ||
		     retval == -ERESTART_RESTARTBLOCK ||
		     (retval == -ERESTARTSYS &&
		      !(ksig.ka.sa.sa_flags & SA_RESTART)))) {
			regs->regs[0] = -EINTR;
			regs->pc = continue_addr;
		}

		// (3)如果有对应的用户自定义处理函数，则执行用户态处理函数
		handle_signal(&ksig, regs);
		return;
	}

	/*
	 * Handle restarting a different system call. As above, if a debugger
	 * has chosen to restart at a different PC, ignore the restart.
	 */
	// (4)重启被中断的system call
	if (syscall >= 0 && regs->pc == restart_addr) {
		if (retval == -ERESTART_RESTARTBLOCK)
			setup_restart_syscall(regs);
		user_rewind_single_step(current);
	}

	restore_saved_sigmask();
}
| →
int get_signal(struct ksignal *ksig)
{
	struct sighand_struct *sighand = current->sighand;
	struct signal_struct *signal = current->signal;
	int signr;

	// (2.1)执行task work机制中的work
	// 这是和信号无关的机制，属于搭便车在ret_to_user时刻去执行的机制
	if (unlikely(current->task_works))
		task_work_run();

	if (unlikely(uprobe_deny_signal()))
		return 0;

	/*
	 * Do this once, we can't return to user-mode if freezing() == T.
	 * do_signal_stop() and ptrace_stop() do freezable_schedule() and
	 * thus do not need another check after return.
	 */
	// (2.2)第二个搭便车的机制freeze，
	// 系统在suspend时会调用suspend_freeze_processes()来freeze线程
	// 实际上也是唤醒线程，让线程在ret_to_user时刻去freeze自己
	try_to_freeze();

relock:
	spin_lock_irq(&sighand->siglock);
	/*
	 * Every stopped thread goes here after wakeup. Check to see if
	 * we should notify the parent, prepare_signal(SIGCONT) encodes
	 * the CLD_ si_code into SIGNAL_CLD_MASK bits.
	 */
	// (2.3)在子进程状态变化的情况下，发送SIGCHLD信号通知父进程
	if (unlikely(signal->flags & SIGNAL_CLD_MASK)) {
		int why;

		if (signal->flags & SIGNAL_CLD_CONTINUED)
			why = CLD_CONTINUED;
		else
			why = CLD_STOPPED;

		signal->flags &= ~SIGNAL_CLD_MASK;

		spin_unlock_irq(&sighand->siglock);

		/*
		 * Notify the parent that we're continuing.  This event is
		 * always per-process and doesn't make whole lot of sense
		 * for ptracers, who shouldn't consume the state via
		 * wait(2) either, but, for backward compatibility, notify
		 * the ptracer of the group leader too unless it's gonna be
		 * a duplicate.
		 */
		read_lock(&tasklist_lock);
		do_notify_parent_cldstop(current, false, why);

		if (ptrace_reparented(current->group_leader))
			do_notify_parent_cldstop(current->group_leader,
						true, why);
		read_unlock(&tasklist_lock);

		goto relock;
	}

	for (;;) {
		struct k_sigaction *ka;

		if (unlikely(current->jobctl & JOBCTL_STOP_PENDING) &&
		    do_signal_stop(0))
			goto relock;

		if (unlikely(current->jobctl & JOBCTL_TRAP_MASK)) {
			do_jobctl_trap();
			spin_unlock_irq(&sighand->siglock);
			goto relock;
		}

		// (2.4)从信号pending队列中，取出优先级最好的信号
		signr = dequeue_signal(current, &current->blocked, &ksig->info);

		if (!signr)
			break; /* will return 0 */

		if (unlikely(current->ptrace) && signr != SIGKILL) {
			signr = ptrace_signal(signr, &ksig->info);
			if (!signr)
				continue;
		}

		// (2.5)从信号处理数组sighand中，取出信号对应的处理函数
		ka = &sighand->action[signr-1];

		/* Trace actually delivered signals. */
		trace_signal_deliver(signr, &ksig->info, ka);

		// (2.6.1)信号处理的第1种方法：忽略
		if (ka->sa.sa_handler == SIG_IGN) /* Do nothing.  */
			continue;
		// (2.6.2)信号处理的第2种方法：调用用户态注册的处理函数
		// 获取到用户态的处理函数指针，返回调用handle_signal()来执行
		if (ka->sa.sa_handler != SIG_DFL) {
			/* Run the handler.  */
			ksig->ka = *ka;

			if (ka->sa.sa_flags & SA_ONESHOT)
				ka->sa.sa_handler = SIG_DFL;

			break; /* will return non-zero "signr" value */
		}

		// (2.6.3)信号处理的第3种方法：调用默认的内核态处理函数
		/*
		 * Now we are doing the default action for this signal.
		 */
		// (2.6.3.1)SIG_KERNEL_IGNORE_MASK信号的默认处理方法Ignore：忽略
		// #define SIG_KERNEL_IGNORE_MASK (\
		//        rt_sigmask(SIGCONT)   |  rt_sigmask(SIGCHLD)   | \
		//         rt_sigmask(SIGWINCH)  |  rt_sigmask(SIGURG)    )
		if (sig_kernel_ignore(signr)) /* Default is nothing. */
			continue;

		/*
		 * Global init gets no signals it doesn't want.
		 * Container-init gets no signals it doesn't want from same
		 * container.
		 *
		 * Note that if global/container-init sees a sig_kernel_only()
		 * signal here, the signal must have been generated internally
		 * or must have come from an ancestor namespace. In either
		 * case, the signal cannot be dropped.
		 */
		if (unlikely(signal->flags & SIGNAL_UNKILLABLE) &&
				!sig_kernel_only(signr))
			continue;

		// (2.6.3.2)SIG_KERNEL_STOP_MASK信号的默认处理方法Stop：do_signal_stop()
		// #define SIG_KERNEL_STOP_MASK (\
		// rt_sigmask(SIGSTOP)   |  rt_sigmask(SIGTSTP)   | \
		// rt_sigmask(SIGTTIN)   |  rt_sigmask(SIGTTOU)   )
		if (sig_kernel_stop(signr)) {
			/*
			 * The default action is to stop all threads in
			 * the thread group.  The job control signals
			 * do nothing in an orphaned pgrp, but SIGSTOP
			 * always works.  Note that siglock needs to be
			 * dropped during the call to is_orphaned_pgrp()
			 * because of lock ordering with tasklist_lock.
			 * This allows an intervening SIGCONT to be posted.
			 * We need to check for that and bail out if necessary.
			 */
			if (signr != SIGSTOP) {
				spin_unlock_irq(&sighand->siglock);

				/* signals can be posted during this window */
				// 不是SIGSTOP信号，且是孤儿进程组
				if (is_current_pgrp_orphaned())
					goto relock;

				spin_lock_irq(&sighand->siglock);
			}

			if (likely(do_signal_stop(ksig->info.si_signo))) {
				/* It released the siglock.  */
				goto relock;
			}

			/*
			 * We didn't actually stop, due to a race
			 * with SIGCONT or something like that.
			 */
			continue;
		}

		spin_unlock_irq(&sighand->siglock);

		/*
		 * Anything else is fatal, maybe with a core dump.
		 */
		current->flags |= PF_SIGNALED;

		// (2.6.3.3)SIG_KERNEL_COREDUMP_MASK信号的默认处理方法Dump：do_coredump() & do_group_exit()
		// #define SIG_KERNEL_COREDUMP_MASK (\
		//         rt_sigmask(SIGQUIT)   |  rt_sigmask(SIGILL)    | \
		// 	rt_sigmask(SIGTRAP)   |  rt_sigmask(SIGABRT)   | \
		//         rt_sigmask(SIGFPE)    |  rt_sigmask(SIGSEGV)   | \
		// 	rt_sigmask(SIGBUS)    |  rt_sigmask(SIGSYS)    | \
		//         rt_sigmask(SIGXCPU)   |  rt_sigmask(SIGXFSZ)   | \
		// 	SIGEMT_MASK				       )
		if (sig_kernel_coredump(signr)) {
			if (print_fatal_signals)
				print_fatal_signal(ksig->info.si_signo);
			proc_coredump_connector(current);
			/*
			 * If it was able to dump core, this kills all
			 * other threads in the group and synchronizes with
			 * their demise.  If we lost the race with another
			 * thread getting here, it set group_exit_code
			 * first and our do_group_exit call below will use
			 * that value and ignore the one we pass it.
			 */
			do_coredump(&ksig->info);
		}

		/*
		 * Death signals, no core dump.
		 */
		// (2.6.3.4)Death signals信号的默认处理方法Terminate：do_group_exit()
		do_group_exit(ksig->info.si_signo);
		/* NOTREACHED */
	}
	spin_unlock_irq(&sighand->siglock);

	ksig->sig = signr;
	return ksig->sig > 0;
}

如果用户没有注册信号处理函数，默认的内核处理函数在 get_signal() 函数中执行完了。对于用户有注册处理函数的信号，但是因为这些处理函数都是用户态的，所以内核使用了一个技巧：先构造堆栈，返回用户态去执行自定义信号处理函数，再返回内核态继续被信号打断的返回用户态的动作。

信号处理

我们来看 handle_signal() 函数中的具体实现。

  • arch/arm64/kernel/signal.c:
  • -> ret_to_user() -> do_notify_resume() -> do_signal() -> handle_signal()
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static void handle_signal(struct ksignal *ksig, struct pt_regs *regs)
{
	struct thread_info *thread = current_thread_info();
	struct task_struct *tsk = current;
	sigset_t *oldset = sigmask_to_save();
	int usig = ksig->sig;
	int ret;

	/*
	 * translate the signal
	 */
	if (usig < 32 && thread->exec_domain && thread->exec_domain->signal_invmap)
		usig = thread->exec_domain->signal_invmap[usig];

	/*
	 * Set up the stack frame
	 */
	// (1)构造返回堆栈，将用户态返回地址替换成用户注册的信号处理函数&ksig->ka
	if (is_compat_task()) {
		if (ksig->ka.sa.sa_flags & SA_SIGINFO)
			ret = compat_setup_rt_frame(usig, ksig, oldset, regs);
		else
			ret = compat_setup_frame(usig, ksig, oldset, regs);
	} else {
		ret = setup_rt_frame(usig, ksig, oldset, regs);
	}

	/*
	 * Check that the resulting registers are actually sane.
	 */
	ret |= !valid_user_regs(&regs->user_regs);

	/*
	 * Fast forward the stepping logic so we step into the signal
	 * handler.
	 */
	if (!ret)
		user_fastforward_single_step(tsk);

	signal_setup_done(ret, ksig, 0);
}

参考资料

ULK. Understanding the Linux Kernel ↩

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