Re: [PATCH v3 2/2] prctl: add PR_[GS]ET_KILLABLE

On Fri,  3 Aug 2018 16:40:21 +0200 Jürg Billeter [off-list ref] 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:

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

There are other scenarios besides job control in a shell where
applications rely on the default actions as described in signal(7) and
PID isolation may be useful. This new prctl makes the signal protection
for "init" processes optional, without breaking backward compatibility.

This one is above my pay grade.  Eric & Oleg: could you please provide
input?