On Mon, 2021-02-08 at 15:57 +0300, Pavel Tikhomirov wrote: > > On 2/8/21 3:31 PM, Jeff Layton wrote: > > On Thu, 2021-02-04 at 01:17 +0300, Cyrill Gorcunov wrote: > > > On Thu, Feb 04, 2021 at 12:35:42AM +0300, Pavel Tikhomirov wrote: > > > > > > > > AFAICS if pid is held only by 1) fowner refcount and by 2) single process > > > > (without threads, group and session for simplicity), on process exit we go > > > > through: > > > > > > > > do_exit > > > > exit_notify > > > > release_task > > > > __exit_signal > > > > __unhash_process > > > > detach_pid > > > > __change_pid > > > > free_pid > > > > idr_remove > > > > > > > > So pid is removed from idr, and after that alloc_pid can reuse pid numbers > > > > even if old pid structure is still alive and is still held by fowner. > > > ... > > > > Hope this answers your question, Thanks! > > > > > > Yeah, indeed, thanks! So the change is sane still I'm > > > a bit worried about backward compatibility, gimme some > > > time I'll try to refresh my memory first, in a couple > > > of days or weekend (though here are a number of experienced > > > developers CC'ed maybe they reply even faster). > > > > I always find it helpful to refer to the POSIX spec [1] for this sort of > > thing. In this case, it says: > > > > F_GETOWN > > If fildes refers to a socket, get the process ID or process group ID > > specified to receive SIGURG signals when out-of-band data is available. > > Positive values shall indicate a process ID; negative values, other than > > -1, shall indicate a process group ID; the value zero shall indicate > > that no SIGURG signals are to be sent. If fildes does not refer to a > > socket, the results are unspecified. > > > > In the event that the PID is reused, the kernel won't send signals to > > the replacement task, correct? > > Correct. Looks like only places to send signal to owner are send_sigio() > and send_sigurg() (at least nobody else dereferences fown->pid_type). > And in both places we lookup for task to send signal to with pid_task() > or do_each_pid_task() (similar to what I do in patch) and will not find > any task if pid was reused. Thus no signal would be sent. > Thanks for confirming it. I queued it up for linux-next (with a small amendment to the changelog), and it should be there later today or tomorrow. It might not hurt to roll up a manpage patch too if you have the cycles. It'd be nice to spell out what a 0 return means there. > > Assuming that's the case, then this patch > > looks fine to me too. I'll plan to pick it for linux-next later today, > > and we can hopefully get this into v5.12. > > > > [1]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/fcntl.html#tag_16_122 > > > > Thanks for finding it! > No problem. That site is worth bookmarking for this sort of thing... ;) -- Jeff Layton <jlayton@xxxxxxxxxx>
