ebied...@xmission.com (Eric W. Biederman) writes: > Oleg Nesterov <o...@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.
In my testing I found something that concerns me. Because we wind up with SIGKILL in shard_pending we can not kill a process in do_exit that has stopped at PTRACE_EVENT_EXIT. That bug seems to go back a long ways. Other than that, it looks like we can do the following to fix the regression I introduced. Oleg any ideas on how to make PTRACE_EVENT_EXIT reliably killable? diff --git a/kernel/signal.c b/kernel/signal.c index 99fa8ff06fd9..a1f154dca73c 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -2544,6 +2544,9 @@ bool get_signal(struct ksignal *ksig) } fatal: + /* No more signals can be pending past this point */ + sigdelset(¤t->pending.signal, SIGKILL); + clear_tsk_thread_flag(current, TIF_SIGPENDING); spin_unlock_irq(&sighand->siglock); /* Eric