Messages in this thread | | | From | (Eric W. Biederman) | Date | Mon, 11 Feb 2019 18:42:39 -0600 | Subject | Re: [PATCH 1/2] signal: Always notice exiting tasks |
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Oleg Nesterov <oleg@redhat.com> writes:
> sorry again for delay... > > On 02/07, Eric W. Biederman wrote: >> >> --- a/kernel/signal.c >> +++ b/kernel/signal.c >> @@ -2393,6 +2393,11 @@ bool get_signal(struct ksignal *ksig) >> goto relock; >> } >> >> + /* Has this task already been marked for death? */ >> + ksig->info.si_signo = signr = SIGKILL; >> + if (signal_group_exit(signal)) >> + goto fatal; >> + >> for (;;) { >> struct k_sigaction *ka; >> >> @@ -2488,6 +2493,7 @@ bool get_signal(struct ksignal *ksig) >> continue; >> } >> >> + fatal: >> spin_unlock_irq(&sighand->siglock); > > Eric, but this is wrong. At least this is the serious user-visible > change. > > Afaics, with this patch the tracee will never stop in PTRACE_EVENT_EXIT in case > of group_exit/exec, because schedule() in TASK_TRACED state won't block due to > __fatal_signal_pending(). > > Yes, yes, as I said many times the semantics of PTRACE_EVENT_EXIT was never really > defined, it depends on /dev/random, but still I don't think we should break it even > more.
Well it changes PTRACE_EVENT_EXIT I grant that. It looks like that changes makes PTRACE_EVENT_EXIT is less than useful.
The only way to perfectly preserve the previous semantics is probably to do something like my JOBCTL_TASK_EXIT proposal.
That said I don't think even adding a JOBCTL_TASK_EXIT is enough to have a reliable stop of ptrace_event_exit after a process has exited. As any other pending signal can cause problems there as well.
I have received a report that strace -f in some cases is not noticing children before they die and it looks like a stop in PTRACE_EVENT_EXIT would fix that strace behavior.
Sigh.
Here I was trying for the simple minimal change and I hit this landmine. Which leaves me with the question of what should be semantics of signal handling after exit.
I think from dim memory of previous conversations the desired semantics look like: a) Ignore all signal state except for SIGKILL. b) Letting SIGKILL wake up the process should be sufficient.
I will see if I can reproduce the strace failure and see if I can cook up something minimal that addresses just that. If you have suggestions I would love to hear them.
As this was a minimal fix for SIGKILL being broken I have already sent the fix to Linus. So we are looking at an incremental fix at this point.
Eric
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