Re: [PATCH] fcntl: make F_GETOWN(EX) return 0 on dead owner task

[Jeff Layton <jlayton@kernel.org>]

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@kernel.org>