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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     David Laight <David.Laight@ACULAB.COM>
Cc:     'Daniel Colascione' <dancol@google.com>,
        Tycho Andersen <tycho@tycho.ws>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        Joel Fernandes <joelaf@google.com>,
        "Suren Baghdasaryan" <surenb@google.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        "Christian Brauner" <christian.brauner@canonical.com>,
        Kees Cook <keescook@chromium.org>,
        Oleg Nesterov <oleg@redhat.com>
References: <20181029221037.87724-1-dancol@google.com>
        <20181031155912.45088-1-dancol@google.com>
        <20181031175448.GC2180@cisco>
        <CAKOZuevrFxWLY1J1DVPNGaEy8UbkD1r_M9T9FQTOtSrqp-G0qw@mail.gmail.com>
        <20181031181717.GD2180@cisco>
        <CAKOZuetdSK8jhD1snja8p4AQTNPrLOugQrVn642RCH8S4QBGFg@mail.gmail.com>
        <c49f552e34784e8dbb40a43015bed642@AcuMS.aculab.com>
Date:   Sun, 11 Nov 2018 19:19:30 -0600
In-Reply-To: <c49f552e34784e8dbb40a43015bed642@AcuMS.aculab.com> (David
        Laight's message of "Thu, 1 Nov 2018 11:33:29 +0000")
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Subject: Re: [PATCH v3] Implement /proc/pid/kill
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David Laight <David.Laight@ACULAB.COM> writes:

> From: Daniel Colascione
>> Sent: 31 October 2018 19:33
> ...
>> You can't do it today with kill. The idea that keeping a open file
>> descriptor to a /proc/pid or a file within it prevents PID reuse is
>> widespread, but incorrect.
>
> Is there a real good reason why that shouldn't be the case?
> ie Holding a reference on the 'struct pid' being enough to stop reuse.
>
> A patch to do that would be more generally useful.

Holding an open file descriptor to /proc/pid is enough to prevent pid
reuse problems.  If a pid number is reused a new 'struct pid' is
generated.  There is not 'struct pid' reuse.

So in solving this the kernel data structure you would need to hold onto
is a 'struct pid'.  It is just necessary to add an interface to sending
signals to that 'struct pid' and not looking up the 'struct pid' again
by number.

Eric