Re: [PATCH 1/2] signal: Always notice exiting tasks

From: Oleg Nesterov
Date: Tue Feb 12 2019 - 11:50:31 EST

On 02/12, Eric W. Biederman wrote:
> > 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.

Yes, currently it is undefined. Even signal_pending() is random.

> > 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.

signal_wake_up(true) to make fatal_signal_pending() == T, I think.

> Oleg any ideas on how to make PTRACE_EVENT_EXIT reliably killable?

My answer is very simple: PTRACE_EVENT_EXIT must not stop if the tracee was
killed by the "real" SIGKILL (not by group_exit/etc), that is all. But this
is another user-visible change, it can equally confuse, say, strace (albeit
not too much iiuc).

But this needs another discussion.

> 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(&current->pending.signal, SIGKILL);

Well, this is very confusing. In fact, this is not really correct. Say, we should
not remove the pending SIGKILL if we are going to call do_coredump(). This is
possible if ptrace_signal() was called, or after is_current_pgrp_orphaned() returns

> + clear_tsk_thread_flag(current, TIF_SIGPENDING);

I don't understand this change, it looks irrelevant. Possibly makes sense, but
this connects to "semantics of signal handling after exit".

OK, we need a minimal incremental fix for now. I'd suggest to replace

ksig->info.si_signo = signr = SIGKILL;
if (signal_group_exit(signal))
goto fatal;

added by this patch with

if (__fatal_signal_pending(current)) {
ksig->info.si_signo = signr = SIGKILL;
sigdelset(&current->pending.signal, SIGKILL);
goto fatal;

__fatal_signal_pending() is cheaper and looks more understandable.