在 Linux 的世界里，進程就像生活在城市中的人，它們需要相互溝通來協(xié)調(diào)行動。而信號機制呢，就像是一種神奇的 “信號彈”，用于進程之間的交流。當一個進程有重要消息要傳達給另一個進程時，就會發(fā)射出這樣的 “信號彈”。這就是 Linux 信號機制，它是管理進程間通信的一把 “金鑰匙”，讓我們一起深入了解它是如何發(fā)揮作用的吧。
一、概述
Linux 的信號機制作為進程間通信的重要方式，發(fā)揮著關鍵作用。它本質(zhì)上是一種軟件中斷，能夠異步地通知進程發(fā)生了特定事件。信號的全稱為軟中斷信號，簡稱軟中斷，在頭文件<signal.h>中定義了 64 種信號，這些信號的名字都以SIG開頭，且都被定義為正整數(shù)，稱為信號編號。可以用 “kill -l” 命令查看信號的具體名稱。
其中，編號為 1~31 的信號為早期 Linux 所支持的信號，是不可靠信號（非實時的），編號為 34~63 的信號時后來擴充的，稱為可靠信號（實時信號）。不可靠信號與可靠信號的區(qū)別在于前者不支持排隊，可能會造成信號丟失，而后者的注冊機制是每收到一個可靠信號就會去注冊這個信號，不會丟失。
軟件條件：主要介紹alarm函數(shù)和SIGALRM信號。調(diào)用alarm(unsigned int seconds)函數(shù)可以設定一個鬧鐘，也就是告訴內(nèi)核在seconds秒后給當前進程發(fā)送SIGALRM信號，該信號的默認處理動作是終止當前進程。這個函數(shù)的返回值是 0 或者是以前設定的鬧鐘時間還余下的秒數(shù)。
if (unlikely((error_code & (X86_PF_USER | X86_PF_INSTR)) == X86_PF_INSTR)) { /* * Whoops, this is kernel mode code trying to execute from * user memory. Unless this is AMD erratum #93, which * corrupts RIP such that it looks like a user address, * this is unrecoverable. Don't even try to look up the * VMA or look for extable entries. */ if (is_errata93(regs, address)) return;
/* kprobes don't want to hook the spurious faults: */ if (WARN_ON_ONCE(kprobe_page_fault(regs, X86_TRAP_PF))) return;
/* * Reserved bits are never expected to be set on * entries in the user portion of the page tables. */ if (unlikely(error_code & X86_PF_RSVD)) pgtable_bad(regs, error_code, address);
/* * If SMAP is on, check for invalid kernel (supervisor) access to user * pages in the user address space. The odd case here is WRUSS, * which, according to the preliminary documentation, does not respect * SMAP and will have the USER bit set so, in all cases, SMAP * enforcement appears to be consistent with the USER bit. */ if (unlikely(cpu_feature_enabled(X86_FEATURE_SMAP) && !(error_code & X86_PF_USER) && !(regs->flags & X86_EFLAGS_AC))) { /* * No extable entry here. This was a kernel access to an * invalid pointer. get_kernel_nofault() will not get here. */ page_fault_oops(regs, error_code, address); return; }
/* * If we're in an interrupt, have no user context or are running * in a region with pagefaults disabled then we must not take the fault */ if (unlikely(faulthandler_disabled() || !mm)) { bad_area_nosemaphore(regs, error_code, address); return; }
/* * It's safe to allow irq's after cr2 has been saved and the * vmalloc fault has been handled. * * User-mode registers count as a user access even for any * potential system fault or CPU buglet: */ if (user_mode(regs)) { local_irq_enable(); flags |= FAULT_FLAG_USER; } else { if (regs->flags & X86_EFLAGS_IF) local_irq_enable(); }
if (error_code & X86_PF_WRITE) flags |= FAULT_FLAG_WRITE; if (error_code & X86_PF_INSTR) flags |= FAULT_FLAG_INSTRUCTION;
#ifdef CONFIG_X86_64 /* * Faults in the vsyscall page might need emulation. The * vsyscall page is at a high address (>PAGE_OFFSET), but is * considered to be part of the user address space. * * The vsyscall page does not have a "real" VMA, so do this * emulation before we go searching for VMAs. * * PKRU never rejects instruction fetches, so we don't need * to consider the PF_PK bit. */ if (is_vsyscall_vaddr(address)) { if (emulate_vsyscall(error_code, regs, address)) return; } #endif
/* * Kernel-mode access to the user address space should only occur * on well-defined single instructions listed in the exception * tables. But, an erroneous kernel fault occurring outside one of * those areas which also holds mmap_lock might deadlock attempting * to validate the fault against the address space. * * Only do the expensive exception table search when we might be at * risk of a deadlock. This happens if we * 1. Failed to acquire mmap_lock, and * 2. The access did not originate in userspace. */ if (unlikely(!mmap_read_trylock(mm))) { if (!user_mode(regs) && !search_exception_tables(regs->ip)) { /* * Fault from code in kernel from * which we do not expect faults. */ bad_area_nosemaphore(regs, error_code, address); return; } retry: mmap_read_lock(mm); } else { /* * The above down_read_trylock() might have succeeded in * which case we'll have missed the might_sleep() from * down_read(): */ might_sleep(); }
vma = find_vma(mm, address); if (unlikely(!vma)) { bad_area(regs, error_code, address); return; } if (likely(vma->vm_start <= address)) goto good_area; if (unlikely(!(vma->vm_flags & VM_GROWSDOWN))) { bad_area(regs, error_code, address); return; } if (unlikely(expand_stack(vma, address))) { bad_area(regs, error_code, address); return; }
/* * Ok, we have a good vm_area for this memory access, so * we can handle it.. */ good_area: if (unlikely(access_error(error_code, vma))) { bad_area_access_error(regs, error_code, address, vma); return; }
/* * If for any reason at all we couldn't handle the fault, * make sure we exit gracefully rather than endlessly redo * the fault. Since we never set FAULT_FLAG_RETRY_NOWAIT, if * we get VM_FAULT_RETRY back, the mmap_lock has been unlocked. * * Note that handle_userfault() may also release and reacquire mmap_lock * (and not return with VM_FAULT_RETRY), when returning to userland to * repeat the page fault later with a VM_FAULT_NOPAGE retval * (potentially after handling any pending signal during the return to * userland). The return to userland is identified whenever * FAULT_FLAG_USER|FAULT_FLAG_KILLABLE are both set in flags. */ fault = handle_mm_fault(vma, address, flags, regs);
if (fault_signal_pending(fault, regs)) { /* * Quick path to respond to signals. The core mm code * has unlocked the mm for us if we get here. */ if (!user_mode(regs)) kernelmode_fixup_or_oops(regs, error_code, address, SIGBUS, BUS_ADRERR, ARCH_DEFAULT_PKEY); return; }
/* * If we need to retry the mmap_lock has already been released, * and if there is a fatal signal pending there is no guarantee * that we made any progress. Handle this case first. */ if (unlikely((fault & VM_FAULT_RETRY) && (flags & FAULT_FLAG_ALLOW_RETRY))) { flags |= FAULT_FLAG_TRIED; goto retry; }
mmap_read_unlock(mm); if (likely(!(fault & VM_FAULT_ERROR))) return;
if (fault & VM_FAULT_OOM) { /* Kernel mode? Handle exceptions or die: */ if (!user_mode(regs)) { kernelmode_fixup_or_oops(regs, error_code, address, SIGSEGV, SEGV_MAPERR, ARCH_DEFAULT_PKEY); return; }
/* * We ran out of memory, call the OOM killer, and return the * userspace (which will retry the fault, or kill us if we got * oom-killed): */ pagefault_out_of_memory(); } else { if (fault & (VM_FAULT_SIGBUS|VM_FAULT_HWPOISON| VM_FAULT_HWPOISON_LARGE)) do_sigbus(regs, error_code, address, fault); else if (fault & VM_FAULT_SIGSEGV) bad_area_nosemaphore(regs, error_code, address); else BUG(); } } static void __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code, unsigned long address, u32 pkey, int si_code) { struct task_struct *tsk = current; if (likely(show_unhandled_signals)) show_signal_msg(regs, error_code, address, tsk);
static int send_signal(int sig, struct kernel_siginfo *info, struct task_struct *t, enum pid_type type) { /* Should SIGKILL or SIGSTOP be received by a pid namespace init? */ bool force = false;
if (info == SEND_SIG_NOINFO) { /* Force if sent from an ancestor pid namespace */ force = !task_pid_nr_ns(current, task_active_pid_ns(t)); } else if (info == SEND_SIG_PRIV) { /* Don't ignore kernel generated signals */ force = true; } else if (has_si_pid_and_uid(info)) { /* SIGKILL and SIGSTOP is special or has ids */ struct user_namespace *t_user_ns;
/* A kernel generated signal? */ force = (info->si_code == SI_KERNEL);
/* From an ancestor pid namespace? */ if (!task_pid_nr_ns(current, task_active_pid_ns(t))) { info->si_pid = 0; force = true; } } return __send_signal(sig, info, t, type, force); }
static int __send_signal(int sig, struct kernel_siginfo *info, struct task_struct *t, enum pid_type type, bool force) { struct sigpending *pending; struct sigqueue *q; int override_rlimit; int ret = 0, result;
assert_spin_locked(&t->sighand->siglock);
result = TRACE_SIGNAL_IGNORED; if (!prepare_signal(sig, t, force)) goto ret;
pending = (type != PIDTYPE_PID) ? &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; if (legacy_queue(pending, sig)) goto ret;
result = TRACE_SIGNAL_DELIVERED; /* * Skip useless siginfo allocation for SIGKILL and kernel threads. */ if ((sig == SIGKILL) || (t->flags & PF_KTHREAD)) 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. */ if (sig < SIGRTMIN) override_rlimit = (is_si_special(info) || info->si_code >= 0); else override_rlimit = 0;
if (q) { list_add_tail(&q->list, &pending->list); switch ((unsigned long) info) { case (unsigned long) SEND_SIG_NOINFO: clear_siginfo(&q->info); 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)); rcu_read_lock(); q->info.si_uid = from_kuid_munged(task_cred_xxx(t, user_ns), current_uid()); rcu_read_unlock(); break; case (unsigned long) SEND_SIG_PRIV: clear_siginfo(&q->info); 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); break; } } else if (!is_si_special(info) && 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; }
static int force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, enum sig_handler handler) { unsigned long int flags; int ret, blocked, ignored; struct k_sigaction *action; int sig = info->si_signo;
spin_lock_irqsave(&t->sighand->siglock, flags); action = &t->sighand->action[sig-1]; ignored = action->sa.sa_handler == SIG_IGN; blocked = sigismember(&t->blocked, sig); if (blocked || ignored || (handler != HANDLER_CURRENT)) { action->sa.sa_handler = SIG_DFL; if (handler == HANDLER_EXIT) action->sa.sa_flags |= SA_IMMUTABLE; if (blocked) { sigdelset(&t->blocked, sig); recalc_sigpending_and_wake(t); } } /* * Don't clear SIGNAL_UNKILLABLE for traced tasks, users won't expect * debugging to leave init killable. But HANDLER_EXIT is always fatal. */ if (action->sa.sa_handler == SIG_DFL && (!t->ptrace || (handler == HANDLER_EXIT))) t->signal->flags &= ~SIGNAL_UNKILLABLE; ret = send_signal(sig, info, t, PIDTYPE_PID); spin_unlock_irqrestore(&t->sighand->siglock, flags);
int kill_pid_info(int sig, struct kernel_siginfo *info, struct pid *pid) { int error = -ESRCH; struct task_struct *p;
for (;;) { rcu_read_lock(); p = pid_task(pid, PIDTYPE_PID); if (p) error = group_send_sig_info(sig, info, p, PIDTYPE_TGID); rcu_read_unlock(); if (likely(!p || error != -ESRCH)) return error;
/* * The task was unhashed in between, try again. If it * is dead, pid_task() will return NULL, if we race with * de_thread() it will find the new leader. */ } }
下面再來看一下tgkill函數(shù)的流程：linux-src/kernel/signal.c
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); }
static int do_tkill(pid_t tgid, pid_t pid, int sig) { struct kernel_siginfo info;
信號的阻塞、未決和遞達是理解 Linux 信號機制的重要概念。執(zhí)行信號的處理動作稱為信號遞達（Delivery），信號從產(chǎn)生到遞達之間的狀態(tài)，稱為信號未決（Pending）。進程可以選擇阻塞（Block）某個信號。被阻塞的信號產(chǎn)生時將保持在未決狀態(tài)，直到進程解除對此信號的阻塞，才執(zhí)行遞達的動作。注意，阻塞和忽略是不同，只要信號被阻塞就不會遞達，而忽略是在遞達之后可選的一種處理動作。
if (v8086_mode(regs)) save_v86_state((struct kernel_vm86_regs *) regs, VM86_SIGNAL);
/* Are we from a system call? */ if (syscall_get_nr(current, regs) != -1) { /* If so, check system call restarting.. */ switch (syscall_get_error(current, regs)) { case -ERESTART_RESTARTBLOCK: case -ERESTARTNOHAND: regs->ax = -EINTR; break;
case -ERESTARTSYS: if (!(ksig->ka.sa.sa_flags & SA_RESTART)) { regs->ax = -EINTR; break; } fallthrough; case -ERESTARTNOINTR: regs->ax = regs->orig_ax; regs->ip -= 2; break; } }
/* * If TF is set due to a debugger (TIF_FORCED_TF), clear TF now * so that register information in the sigcontext is correct and * then notify the tracer before entering the signal handler. */ stepping = test_thread_flag(TIF_SINGLESTEP); if (stepping) user_disable_single_step(current);
failed = (setup_rt_frame(ksig, regs) < 0); if (!failed) { /* * Clear the direction flag as per the ABI for function entry. * * Clear RF when entering the signal handler, because * it might disable possible debug exception from the * signal handler. * * Clear TF for the case when it wasn't set by debugger to * avoid the recursive send_sigtrap() in SIGTRAP handler. */ regs->flags &= ~(X86_EFLAGS_DF|X86_EFLAGS_RF|X86_EFLAGS_TF); /* * Ensure the signal handler starts with the new fpu state. */ fpu__clear_user_states(fpu); } signal_setup_done(failed, ksig, stepping); }
Linux 信號機制在編寫健壯程序中具有至關重要的意義。首先，它提供了一種靈活的方式來處理異步事件。在復雜的多進程或多線程環(huán)境中，程序可能會面臨各種不可預測的情況，如硬件故障、用戶輸入、系統(tǒng)資源變化等。通過信號機制，程序可以及時響應這些事件，采取適當?shù)拇胧?，避免出現(xiàn)不可預料的錯誤或崩潰。
