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.
