Re: [PATCH v2] prctl: add PR_[GS]ET_KILLABLE

On 07/31, Jürg Billeter wrote:

PR_SET_KILLABLE clears the SIGNAL_UNKILLABLE flag. This allows
CLONE_NEWPID tasks to restore normal signal behavior, opting out of the
special signal protection for init processes. This prctl does not allow
setting the SIGNAL_UNKILLABLE flag, only clearing.

The SIGNAL_UNKILLABLE flag, which is implicitly set for tasks cloned
with CLONE_NEWPID, has the effect of ignoring all signals (from
userspace) if the corresponding handler is set to SIG_DFL. The only
exceptions are SIGKILL and SIGSTOP and they are only accepted if raised
from an ancestor namespace.

SIGINT, SIGQUIT and SIGTSTP are used in job control for ^C, ^\, ^Z.
While a task with the SIGNAL_UNKILLABLE flag could install handlers for
these signals, this is not sufficient to implement a shell that uses
CLONE_NEWPID for child processes:

Ah. My question wasn't clear, sorry.

Could you explain your use-case? Why a shell wants to use CLONE_NEWPID?
And what do we actually want in, say, ^Z case? Just stop the child reaper
or may be it would be better to stop the whole pid namespace?

 * As SIGSTOP is ignored when raised from the SIGNAL_UNKILLABLE process
   itself, it's not possible to implement the stop action in a custom
   SIGTSTP handler.

Yes. So may be we actually want to change __isig() paths to use
SEND_SIG_FORCED (this is not that simple), or perhaps we can change
__send_signal() to not drop SIGSTOP sent to itself, or may be we can even
introduce SIG_DFL_EVEN_IF_INIT, I dunno.

 * Many applications do not install handlers for these signals and
   thus, job control won't work properly with unmodified applications.

I can't understand this. An application should be changed anyway to do
PR_SET_KILLABLE?


Let me clarify. I am not arguing with this patch, probably it makes sense in
any case. I am just trying to understand your real motivation for this change.

+	case PR_SET_KILLABLE:
+		if (arg2 != 1 || arg3 || arg4 || arg5)
+			return -EINVAL;
+		spin_lock_irq(&me->sighand->siglock);
+		me->signal->flags &= ~SIGNAL_UNKILLABLE;
+		spin_unlock_irq(&me->sighand->siglock);

OK, but then you need to change the CLONE_PARENT/SIGNAL_UNKILLABLE check
in copy_process().

Oleg.