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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Christian Brauner <christian@brauner.io>
Cc:     Daniel Colascione <dancol@google.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "Serge E. Hallyn" <serge@hallyn.com>, Jann Horn <jannh@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        linux-man <linux-man@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>
References: <20181119103241.5229-1-christian@brauner.io>
        <20181119103241.5229-3-christian@brauner.io>
        <CAKOZueshQ2DAwUYp+dLpmfYG6FtvCqeRQxH41SjNS+WqY3e0Fw@mail.gmail.com>
        <20181119182902.fadw6qiu3eepndm3@brauner.io>
Date:   Mon, 19 Nov 2018 13:02:06 -0600
In-Reply-To: <20181119182902.fadw6qiu3eepndm3@brauner.io> (Christian Brauner's
        message of "Mon, 19 Nov 2018 19:29:05 +0100")
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Subject: Re: [PATCH v1 2/2] signal: add procfd_signal() syscall
Sender: linux-kernel-owner@vger.kernel.org
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Christian Brauner <christian@brauner.io> writes:

> On Mon, Nov 19, 2018 at 07:59:24AM -0800, Daniel Colascione wrote:
>> You never addressed my comment on the previous patch about your use of
>
> Sorry, that thread exploded so quickly that I might have missed it.
>
>> private_data here. Why can't you use the struct pid reference that's
>> already in the inode?
>
> If that's what people prefer we can probably use that. There was
> precedent for stashing away such data in fs/proc/base.c already for
> various other things including user namespaces and struct mm so I
> followed this model. A prior version of my patch (I didn't send out) did
> retrive the inode via proc_pid() in .open() took an additional reference
> via get_pid() and dropped it in .release(). Do we prefer that?

If you are using proc/<pid>/ directories as your file descriptors, you
don't need to add an open or a release method at all.  The existing file
descriptors hold a reference to the inode which holds a reference the
the struct pid.

The only time you need to get a reference is when you need a task
and kill_pid_info already performs that work for you.

So using proc_pid is all you need to do to get the pid from the existing
file descriptors.

Eric