On Tue, 30 May 2017, Linus Torvalds wrote:
> On Tue, May 30, 2017 at 10:04 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> > Obviously this is a user-visible change and it can break something. Say, an
> > application does sigwaitinfo(SIGCHLD) and SIGCHLD is ignored (SIG_IGN), this
> > will no longer work.
>
> That's an interesting special case. Yes, SIG_IGN actually has magical
> properties wrt SIGCHLD. It basically means the opposite of ignoring
> it, it's an "implicit signal handler". So I could imagine people
> using SIG_IGN to avoid the signal handler, but then block SIG_CHLD and
> using sigwait() for it.
>
> That sounds nonportable as hell, but I could imagine people doing it
> because it happens to work.
Just that it does not work. See do_notify_parent()
if (!tsk->ptrace && sig == SIGCHLD &&
(psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN ||
(psig->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT))) {
/*
* We are exiting and our parent doesn't care. POSIX.1
* defines special semantics for setting SIGCHLD to SIG_IGN
* or setting the SA_NOCLDWAIT flag: we should be reaped
* automatically and not left for our parent's wait4 call.
* Rather than having the parent do it as a magic kind of
* signal handler, we just set this to tell do_exit that we
* can be cleaned up without becoming a zombie. Note that
* we still call __wake_up_parent in this case, because a
* blocked sys_wait4 might now return -ECHILD.
*
* Whether we send SIGCHLD or not for SA_NOCLDWAIT
* is implementation-defined: we do (if you don't want
* it, just use SIG_IGN instead).
*/
autoreap = true;
if (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN)
sig = 0;
}
if (valid_signal(sig) && sig)
__group_send_sig_info(sig, &info, tsk->parent);
So if the oarent has SIG_IGN we do not send a signal at all. So it's not a
really interesting special case and the magic properties are not that magic
either. Test case below. The parent waits forever.
Thanks,
tglx
---
#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
int main(void)
{
struct sigaction action;
sigset_t set;
int signum;
sigemptyset(&set);
sigaddset (&set, SIGCHLD);
memset(&action, 0, sizeof(action));
action.sa_handler = SIG_IGN;
sigaction(SIGCHLD, &action, NULL);
sigprocmask(SIG_BLOCK, &set, NULL);
if (fork() == 0) {
sleep(1);
printf("Child exiting\n");
exit(0);
}
sigwait(&set, &signum);
printf("Parent exiting\n");
return 0;
}
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