linux-kernel - Re: [PATCH v3] Implement /proc/pid/kill

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Date:   Sun, 11 Nov 2018 19:19:30 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Daniel Colascione' <dancol@...gle.com>,
        Tycho Andersen <tycho@...ho.ws>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        "Suren Baghdasaryan" <surenb@...gle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        "Christian Brauner" <christian.brauner@...onical.com>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v3] Implement /proc/pid/kill

David Laight <David.Laight@...LAB.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