On Fri, 3 Aug 2018 16:40:21 +0200 Jürg Billeter <j@xxxxxxxxx> 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?
