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// Copyright 2017 The Crashpad Authors. All rights reserved.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include "util/posix/double_fork_and_exec.h"
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>
#include "base/logging.h"
#include "base/posix/eintr_wrapper.h"
#include "base/strings/stringprintf.h"
#include "util/posix/close_multiple.h"
namespace crashpad {
bool DoubleForkAndExec(const std::vector<std::string>& argv,
const std::vector<std::string>* envp,
int preserve_fd,
bool use_path,
void (*child_function)()) {
DCHECK(!envp || !use_path);
// argv_c contains const char* pointers and is terminated by nullptr. This is
// suitable for passing to execv(). Although argv_c is not used in the parent
// process, it must be built in the parent process because it’s unsafe to do
// so in the child or grandchild process.
std::vector<const char*> argv_c;
argv_c.reserve(argv.size() + 1);
for (const std::string& argument : argv) {
std::vector<const char*> envp_c;
if (envp) {
envp_c.reserve(envp->size() + 1);
for (const std::string& variable : *envp) {
// Double-fork(). The three processes involved are parent, child, and
// grandchild. The grandchild will call execv(). The child exits immediately
// after spawning the grandchild, so the grandchild becomes an orphan and its
// parent process ID becomes 1. This relieves the parent and child of the
// responsibility to reap the grandchild with waitpid() or similar. The
// grandchild is expected to outlive the parent process, so the parent
// shouldn’t be concerned with reaping it. This approach means that accidental
// early termination of the handler process will not result in a zombie
// process.
pid_t pid = fork();
if (pid < 0) {
PLOG(ERROR) << "fork";
return false;
if (pid == 0) {
// Child process.
if (child_function) {
// Call setsid(), creating a new process group and a new session, both led
// by this process. The new process group has no controlling terminal. This
// disconnects it from signals generated by the parent process’ terminal.
// setsid() is done in the child instead of the grandchild so that the
// grandchild will not be a session leader. If it were a session leader, an
// accidental open() of a terminal device without O_NOCTTY would make that
// terminal the controlling terminal.
// It’s not desirable for the grandchild to have a controlling terminal. The
// grandchild manages its own lifetime, such as by monitoring clients on its
// own and exiting when it loses all clients and when it deems it
// appropraite to do so. It may serve clients in different process groups or
// sessions than its original client, and receiving signals intended for its
// original client’s process group could be harmful in that case.
PCHECK(setsid() != -1) << "setsid";
pid = fork();
if (pid < 0) {
PLOG(FATAL) << "fork";
if (pid > 0) {
// Child process.
// _exit() instead of exit(), because fork() was called.
// Grandchild process.
CloseMultipleNowOrOnExec(STDERR_FILENO + 1, preserve_fd);
// &argv_c[0] is a pointer to a pointer to const char data, but because of
// how C (not C++) works, execvp() wants a pointer to a const pointer to
// char data. It modifies neither the data nor the pointers, so the
// const_cast is safe.
char* const* argv_for_execv = const_cast<char* const*>(&argv_c[0]);
if (envp) {
// This cast is safe for the same reason that the argv_for_execv cast is.
char* const* envp_for_execv = const_cast<char* const*>(&envp_c[0]);
execve(argv_for_execv[0], argv_for_execv, envp_for_execv);
PLOG(FATAL) << "execve " << argv_for_execv[0];
if (use_path) {
execvp(argv_for_execv[0], argv_for_execv);
PLOG(FATAL) << "execvp " << argv_for_execv[0];
execv(argv_for_execv[0], argv_for_execv);
PLOG(FATAL) << "execv " << argv_for_execv[0];
// waitpid() for the child, so that it does not become a zombie process. The
// child normally exits quickly.
// Failures from this point on may result in the accumulation of a zombie, but
// should not be considered fatal. Log only warnings, but don’t treat these
// failures as a failure of the function overall.
int status;
pid_t wait_pid = HANDLE_EINTR(waitpid(pid, &status, 0));
if (wait_pid == -1) {
PLOG(WARNING) << "waitpid";
return true;
DCHECK_EQ(wait_pid, pid);
if (WIFSIGNALED(status)) {
int sig = WTERMSIG(status);
LOG(WARNING) << base::StringPrintf(
"intermediate process terminated by signal %d (%s)%s",
WCOREDUMP(status) ? " (core dumped)" : "");
} else if (!WIFEXITED(status)) {
LOG(WARNING) << base::StringPrintf(
"intermediate process: unknown termination 0x%x", status);
} else if (WEXITSTATUS(status) != EXIT_SUCCESS) {
LOG(WARNING) << "intermediate process exited with code "
<< WEXITSTATUS(status);
return true;
} // namespace crashpad