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

[Pavel Tikhomirov]

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.

> 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!

--
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.