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Properly respond to job control signals

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This commit is contained in:
Chris Kuehl 2015-09-18 21:59:31 -07:00
parent f3aa9cc93c
commit b39643dcbc
1 changed files with 52 additions and 1 deletions
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51
dumb-init.c
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@ -37,11 +37,62 @@ void forward_signal(int signum) {
}
}
/*
* The dumb-init signal handler.
*
* The main job of this signal handler is to forward signals along to our child
* process(es). In setsid mode, this means signaling the entire process group
* rooted at our child. In non-setsid mode, this is just signaling the primary
* child.
*
* In most cases, simply proxying the received signal is sufficient. If we
* receive a job control signal, however, we should not only forward it, but
* also sleep dumb-init itself.
*
* This allows users to run foreground processes using dumb-init and to
* control them using normal shell job control features (e.g. Ctrl-Z to
* generate a SIGTSTP and suspend the process).
*
* The libc manual is useful:
* https://www.gnu.org/software/libc/manual/html_node/Job-Control-Signals.html
*
* When running in setsid mode, however, it is not sufficient to forward
* SIGTSTP/SIGTTIN/SIGTTOU in most cases. If the process has not added a custom
* signal handler for these signals, then the kernel will not apply default
* signal handling behavior (which would be suspending the process) since it is
* a member of an orphaned process group.
*
* Sadly this doesn't appear to be well documented except in the kernel itself:
* https://github.com/torvalds/linux/blob/v4.2/kernel/signal.c#L2296-L2299
*
* Forwarding SIGSTOP instead is effective, though not ideal; unlike SIGTSTP,
* SIGSTOP cannot be caught, and so it doesn't allow processes a change to
* clean up before suspending. In non-setsid mode, we proxy the original signal
* instead of SIGSTOP for this reason.
*/
void handle_signal(int signum) {
DEBUG("Received signal %d.\n", signum);
if (
signum == SIGTSTP || // tty: background yourself
signum == SIGTTIN || // tty: stop reading
signum == SIGTTOU // tty: stop writing
) {
if (use_setsid) {
DEBUG("Running in setsid mode, so forwarding SIGSTOP instead.\n");
forward_signal(SIGSTOP);
} else {
DEBUG("Not running in setsid mode, so forwarding the original signal (%d).\n", signum);
forward_signal(signum);
}
DEBUG("Suspending self due to TTY signal.\n");
kill(getpid(), SIGSTOP);
} else {
forward_signal(signum);
}
}
void print_help(char *argv[]) {
fprintf(stderr,
"Usage: %s COMMAND [[ARG] ...]\n"