174 lines
6.8 KiB
C++
174 lines
6.8 KiB
C++
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// Copyright (c) 2013 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#include "base/process/kill.h"
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#include <signal.h>
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#include <sys/event.h>
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#include <sys/types.h>
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#include <sys/wait.h>
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#include "base/file_util.h"
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#include "base/logging.h"
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#include "base/posix/eintr_wrapper.h"
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namespace base {
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namespace {
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const int kWaitBeforeKillSeconds = 2;
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// Reap |child| process. This call blocks until completion.
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void BlockingReap(pid_t child) {
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const pid_t result = HANDLE_EINTR(waitpid(child, NULL, 0));
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if (result == -1) {
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DPLOG(ERROR) << "waitpid(" << child << ", NULL, 0)";
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}
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}
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// Waits for |timeout| seconds for the given |child| to exit and reap it. If
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// the child doesn't exit within the time specified, kills it.
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//
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// This function takes two approaches: first, it tries to use kqueue to
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// observe when the process exits. kevent can monitor a kqueue with a
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// timeout, so this method is preferred to wait for a specified period of
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// time. Once the kqueue indicates the process has exited, waitpid will reap
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// the exited child. If the kqueue doesn't provide an exit event notification,
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// before the timeout expires, or if the kqueue fails or misbehaves, the
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// process will be mercilessly killed and reaped.
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//
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// A child process passed to this function may be in one of several states:
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// running, terminated and not yet reaped, and (apparently, and unfortunately)
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// terminated and already reaped. Normally, a process will at least have been
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// asked to exit before this function is called, but this is not required.
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// If a process is terminating and unreaped, there may be a window between the
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// time that kqueue will no longer recognize it and when it becomes an actual
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// zombie that a non-blocking (WNOHANG) waitpid can reap. This condition is
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// detected when kqueue indicates that the process is not running and a
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// non-blocking waitpid fails to reap the process but indicates that it is
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// still running. In this event, a blocking attempt to reap the process
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// collects the known-dying child, preventing zombies from congregating.
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//
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// In the event that the kqueue misbehaves entirely, as it might under a
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// EMFILE condition ("too many open files", or out of file descriptors), this
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// function will forcibly kill and reap the child without delay. This
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// eliminates another potential zombie vector. (If you're out of file
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// descriptors, you're probably deep into something else, but that doesn't
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// mean that zombies be allowed to kick you while you're down.)
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//
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// The fact that this function seemingly can be called to wait on a child
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// that's not only already terminated but already reaped is a bit of a
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// problem: a reaped child's pid can be reclaimed and may refer to a distinct
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// process in that case. The fact that this function can seemingly be called
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// to wait on a process that's not even a child is also a problem: kqueue will
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// work in that case, but waitpid won't, and killing a non-child might not be
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// the best approach.
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void WaitForChildToDie(pid_t child, int timeout) {
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DCHECK(child > 0);
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DCHECK(timeout > 0);
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// DON'T ADD ANY EARLY RETURNS TO THIS FUNCTION without ensuring that
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// |child| has been reaped. Specifically, even if a kqueue, kevent, or other
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// call fails, this function should fall back to the last resort of trying
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// to kill and reap the process. Not observing this rule will resurrect
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// zombies.
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int result;
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int kq = HANDLE_EINTR(kqueue());
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if (kq == -1) {
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DPLOG(ERROR) << "kqueue()";
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} else {
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file_util::ScopedFD auto_close_kq(&kq);
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struct kevent change = {0};
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EV_SET(&change, child, EVFILT_PROC, EV_ADD, NOTE_EXIT, 0, NULL);
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result = HANDLE_EINTR(kevent(kq, &change, 1, NULL, 0, NULL));
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if (result == -1) {
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if (errno != ESRCH) {
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DPLOG(ERROR) << "kevent (setup " << child << ")";
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} else {
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// At this point, one of the following has occurred:
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// 1. The process has died but has not yet been reaped.
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// 2. The process has died and has already been reaped.
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// 3. The process is in the process of dying. It's no longer
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// kqueueable, but it may not be waitable yet either. Mark calls
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// this case the "zombie death race".
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result = HANDLE_EINTR(waitpid(child, NULL, WNOHANG));
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if (result != 0) {
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// A positive result indicates case 1. waitpid succeeded and reaped
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// the child. A result of -1 indicates case 2. The child has already
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// been reaped. In both of these cases, no further action is
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// necessary.
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return;
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}
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// |result| is 0, indicating case 3. The process will be waitable in
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// short order. Fall back out of the kqueue code to kill it (for good
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// measure) and reap it.
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}
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} else {
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// Keep track of the elapsed time to be able to restart kevent if it's
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// interrupted.
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TimeDelta remaining_delta = TimeDelta::FromSeconds(timeout);
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TimeTicks deadline = TimeTicks::Now() + remaining_delta;
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result = -1;
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struct kevent event = {0};
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while (remaining_delta.InMilliseconds() > 0) {
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const struct timespec remaining_timespec = remaining_delta.ToTimeSpec();
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result = kevent(kq, NULL, 0, &event, 1, &remaining_timespec);
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if (result == -1 && errno == EINTR) {
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remaining_delta = deadline - TimeTicks::Now();
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result = 0;
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} else {
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break;
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}
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}
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if (result == -1) {
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DPLOG(ERROR) << "kevent (wait " << child << ")";
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} else if (result > 1) {
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DLOG(ERROR) << "kevent (wait " << child << "): unexpected result "
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<< result;
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} else if (result == 1) {
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if ((event.fflags & NOTE_EXIT) &&
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(event.ident == static_cast<uintptr_t>(child))) {
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// The process is dead or dying. This won't block for long, if at
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// all.
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BlockingReap(child);
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return;
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} else {
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DLOG(ERROR) << "kevent (wait " << child
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<< "): unexpected event: fflags=" << event.fflags
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<< ", ident=" << event.ident;
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}
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}
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}
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}
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// The child is still alive, or is very freshly dead. Be sure by sending it
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// a signal. This is safe even if it's freshly dead, because it will be a
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// zombie (or on the way to zombiedom) and kill will return 0 even if the
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// signal is not delivered to a live process.
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result = kill(child, SIGKILL);
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if (result == -1) {
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DPLOG(ERROR) << "kill(" << child << ", SIGKILL)";
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} else {
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// The child is definitely on the way out now. BlockingReap won't need to
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// wait for long, if at all.
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BlockingReap(child);
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}
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}
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} // namespace
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void EnsureProcessTerminated(ProcessHandle process) {
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WaitForChildToDie(process, kWaitBeforeKillSeconds);
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}
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} // namespace base
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