* Christian Brauner: > On Mon, Dec 03, 2018 at 05:57:51PM +0100, Florian Weimer wrote: >> * Christian Brauner: >> >> > Ok, I finally have access to source code again. Scratch what I said above! >> > I looked at the code and tested it. If the process has exited but not >> > yet waited upon aka is a zombie procfd_send_signal() will return 0. This >> > is identical to kill(2) behavior. It should've been sort-of obvious >> > since when a process is in zombie state /proc/<pid> will still be around >> > which means that struct pid must still be around. >> >> Should we make this state more accessible, by providing a different >> error code? > > No, I don't think we want that. Imho, It's not really helpful. Signals > are still delivered to zombies. If zombie state were to always mean that > no-one is going to wait on this thread anymore then it would make sense > to me. But given that zombie can also mean that someone put a > sleep(1000) right before their wait() call in the parent it seems odd to > report back that it is a zombie. It allows for error checking that the recipient of a signal is still running. It's obviously not reliable, but I think it could be helpful in the context of closely cooperating processes. >> Will the system call ever return ESRCH, given that you have a handle for >> the process? > > Yes, whenever you signal a process that has already been waited upon: > - get procfd handle referring to <proc> > - <proc> exits and is waited upon > - procfd_send_signal(procfd, ...) returns -1 with errno == ESRCH I see, thanks. >> Do you want to land all this in one kernel release? I wonder how >> applications are supposed to discover kernel support if functionality is >> split across several kernel releases. If you get EINVAL or EBADF, it >> may not be obvious what is going on. > > Sigh, I get that but I really don't want to have to land this in one big > chunk. I want this syscall to go in in a as soon as we can to fulfill > the most basic need: having a way that guarantees us that we signal the > process that we intended to signal. > > The thread case is easy to implement on top of it. But I suspect we will > quibble about the exact semantics for a long time. Even now we have been > on multiple - justified - detrous. That's all pefectly fine and > expected. But if we have the basic functionality in we have time to do > all of that. We might even land it in the same kernel release still. I > really don't want to come of as tea-party-kernel-conservative here but I > have time-and-time again seen that making something fancy and cover ever > interesting feature in one patchset takes a very very long time. > > If you care about userspace being able to detect that case I can return > EOPNOTSUPP when a tid descriptor is passed. I suppose that's fine. Or alternatively, when thread group support is added, introduce a flag that applications have to use to enable it, so that they can probe for support by checking support for the flag. I wouldn't be opposed to a new system call like this either: int procfd_open (pid_t thread_group, pid_t thread_id, unsigned flags); But I think this is frowned upon on the kernel side. >> What happens if you use the new interface with an O_PATH descriptor? > > You get EINVAL. When an O_PATH file descriptor is created the kernel > will set file->f_op = &empty_fops at which point the check I added > if (!proc_is_tgid_procfd(f.file)) > goto err; > will fail. Imho this is correct behavior since technically signaling a > struct pid is the equivalent of writing to a file and hence doesn't > purely operate on the file descriptor level. Yes, that's quite reasonable. Thanks. Florian
