From mboxrd@z Thu Jan 1 00:00:00 1970 From: Florian Weimer Subject: Re: [PATCH v4] signal: add taskfd_send_signal() syscall Date: Thu, 06 Dec 2018 14:17:09 +0100 Message-ID: <87lg526bhm.fsf@oldenburg2.str.redhat.com> References: <20181206121858.12215-1-christian@brauner.io> <87h8fq7s84.fsf@oldenburg2.str.redhat.com> <20181206125354.ef3zlg3o75w32ymx@brauner.io> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: <20181206125354.ef3zlg3o75w32ymx@brauner.io> (Christian Brauner's message of "Thu, 6 Dec 2018 13:53:57 +0100") Sender: linux-kernel-owner@vger.kernel.org To: Christian Brauner Cc: linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, luto@kernel.org, arnd@arndb.de, ebiederm@xmission.com, serge@hallyn.com, jannh@google.com, akpm@linux-foundation.org, oleg@redhat.com, cyphar@cyphar.com, viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org, dancol@google.com, timmurray@google.com, linux-man@vger.kernel.org, keescook@chromium.org, tglx@linutronix.de, x86@kernel.org List-Id: linux-api@vger.kernel.org * Christian Brauner: > On Thu, Dec 06, 2018 at 01:30:19PM +0100, Florian Weimer wrote: >> * Christian Brauner: >> >> > /* zombies */ >> > Zombies can be signaled just as any other process. No special error will be >> > reported since a zombie state is an unreliable state (cf. [3]). >> >> I still disagree with this analysis. If I know that the target process >> is still alive, and it is not, this is a persistent error condition >> which can be reliably reported. Given that someone might send SIGKILL >> to the process behind my back, detecting this error condition could be >> useful. > > Apart from my objection that this is not actually a reliable state > because of timing issues between e.g. calling wait and a process > exiting The point is that if you are in an error state, the error state does not go away, *especially* if you do not expect the process to terminate and have not arranged for something calling waitpid on the PID. > I have two more concerns and one helpful suggestion. > First, this is hooking pretty deep into kernel internals. So far > EXIT_ZOMBIE is only exposed in kernel/exit.c and I don't see enough > value to drag all of this into kernel/signal.c > Second, all other signal syscalls don't do report errors when signaling > to zombies as well. They cannot do this reliably because the error state is not persistent: the PID can be reused. So for the legacy interface, a difference in error signaling would just have encouraged a bad programming model. > It would be odd if this one suddenly did. I don't think so. My point is that the FD-based mechanism finally allows to cope with this in a reasonable way. > Third, if this really becomes such a big issue for userspace in the > future that we want to do that work then we can add a flag like > TASKFD_DETECT_ZOMBIE (or some such name) that will allow userspace to > get an error back when signaling a zombie. I can live with that. Thanks, Florian