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

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Date:   Mon, 8 Feb 2021 15:57:15 +0300
From:   Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
To:     Jeff Layton <jlayton@...nel.org>,
        Cyrill Gorcunov <gorcunov@...il.com>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andrei Vagin <avagin@...il.com>
Subject: Re: [PATCH] fcntl: make F_GETOWN(EX) return 0 on dead owner task



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.