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

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Date:   Wed, 3 Feb 2021 22:32:01 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
Cc:     Jeff Layton <jlayton@...chiereds.net>,
        Jeff Layton <jlayton@...nel.org>,
        "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 Wed, Feb 03, 2021 at 03:41:56PM +0300, Pavel Tikhomirov wrote:
> Currently there is no way to differentiate the file with alive owner
> from the file with dead owner but pid of the owner reused. That's why
> CRIU can't actually know if it needs to restore file owner or not,
> because if it restores owner but actual owner was dead, this can
> introduce unexpected signals to the "false"-owner (which reused the
> pid).

Hi! Thanks for the patch. You know I manage to forget the fowner internals.
Could you please enlighten me -- when owner is set with some pid we do

f_setown_ex
  __f_setown
    f_modown
      filp->f_owner.pid = get_pid(pid);

Thus pid get refcount incremented. Then the owner exits but refcounter
should be still up and running and pid should not be reused, no? Or
I miss something obvious?

The patch itself looks ok on a first glance.