On Tue, Dec 04, 2018 at 01:55:10PM +0100, Florian Weimer wrote: > * 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.
If this is purely about getting a procfd then I think this isn't really necessary since you can get it from /proc/<pid> and /proc/<pid>/task/<tid> so a syscall just for that is likely overkill. However, I started to pick up the CLONE_FD patchset but ideally I would like it to be way simpler to what was proposed back in the day (which is not a critique, I just don't feel comfortable with bringing massive patches to the table that I can barely judge wrt to their correctness. :)). I have toyed around with this a little and I'm tempted to simply have the syscall always return an fd for the process and not require a separate flag for this. But I need to work through the details and this is really far out into the (kernel) future. > > >> 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
